Session BP12: Poster Session I: DIII-D and Conventional Tokamaks 1 HBT-EP and TCV Space PlasmasICF1: Analytical and Computational Techniques; Machine learning and data science techniques in inertially confined plasmas; Z-pinch, X-pinch, exploding wire plasma, and dense plasma focus; Compression and burn; Magneto-inertial fusionHigh Energy Density Physics 9:30 AM - 12:30 PM

DIII-D to Close Critical Gaps to Fusion Energy

DIII-D is pursuing an ambitious plan to rapidly close gaps to a Fusion Pilot Plant and prepare for ITER, integrating performance, wall and exhaust solutions, addressing plasma interacting materials and technology, and engaging with the private sector. A volume and shaping rise has been implemented, which together with funded increases of electron cyclotron and neutral beam power (through RF sources), and new ‘helicon’ and HFS-LHCD systems, is expanding the limits of fusion performance. A reactor relevant wall is planned to assess innovative materials in the plasma environment. A novel ‘chimney’ divertor concept offers the potential for high dissipation with high performance, and to resolve critical divertor science for reactor projection, while a pumped NT divertor will test an alternative path. A runaway electron mitigation coil and new pellet injectors will meet the disruption challenge, while spin polarized fusion tests could transform multiple fusion concepts. DIII-D is being established as a testbed for private industry, with machine learning, materials, plasma research, diagnostics and other technology tests. The facility is also being interfaced to U.S. Integrated Research Infrastructure (IRI) and supercomputer facilities. This work will accelerate the fusion path, providing unique and vitally needed insights in the development of fusion energy.   

Confinement and performance trends in DIII-D negative triangularity discharges

DIII-D negative triangularity (NT) plasmas exhibit H-mode-like confinement with an ELM-free boundary and a tolerance for high normalized beta operation (β_N ~3). Using fits to profiles and sampling of confinement parameters of the DIII-D NT database of ~600 discharges, trends in transport are investigated and compared with more traditional positive triangularity (PT) shots. From profile fits the electron temperature gradient scale length R/LT_e increases with stronger NT shaping, but the energy confinement does not vary strongly as the average triangularity δ_avg changes from -0.1 to -0.5. However, NT discharges in general have higher values of R/LT_e for radii 0.5 < r/a < 0.8 than PT H-mode discharges with similar parameters; typical values would be ~10 for NT compared to ~5 for PT. Also noted is that, with higher T_e gradients in the confinement zone and lower NT edge pedestals, both T_e and n_e, NT has on average 10-20% higher pressure peaking factors than typical H-modes, p_e0/<p_e> ~2. This can contribute the observed neutron rates, normalized to volume, which are of the same order as other DIII‑D high performance discharges.   

Edge turbulence in DIII-D plasmas with strong negative triangularity shaping

In the DIII-D 2023 experimental campaign, plasmas with strong negative triangularity shaping with an average triangularity ~-0.5 have demonstrated high performance similar to the conventional positive triangularity shape H-modes and in the absence of edge localized modes (ELMs). This work presents broadband fluctuations of electron temperature (T_e) from correlation electron cyclotron emission at the edge of these plasmas, as well as density (n_e) turbulence and turbulence poloidal flow velocities from Doppler backscattering in a radial range of ρ~0.85-1.0. It is observed that T_e turbulence has a much larger (generally more than 2-3 times) amplitude in negative triangularity than that in conventional H-mode plasmas. The radial correlation length in T_e turbulence is typically 5-10 times of the ion gyro-radius, similar to that in the conventional L-mode plasmas. The RMS fluctuation level of the T_e and n_e turbulence exhibits a peaking radial profile feature, and the peaks roughly coincide with the radial electric field well near ρ~0.95 inferred from the poloidal flow velocity of the n_e turbulence. Results of linear stability turbulence simulations will also be presented.   

Experimental and modeling studies of scrape-off-layer impurity velocity stagnation points in negative triangularity plasmas on DIII-D

Comparison between DIII-D measurements of impurity ion flow stagnation point (Carbon-2+ emitting at Te ~ 7- 10 eV) and modeling with drifts are made in positive (PT) and negative triangularity (NT) discharges in both ion directions. NT discharges provided high quality measurements of the C2+ SOL flows without edge localized modes and at high injected powers. Measurements of plasma flow stagnation points in the divertor and main-chamber SOL are needed to validate predictions of complex SOL physics models implemented in the SOLPS and UEDGE edge fluid codes. In NT discharges (Ip = 0.6-1 MA, ne/ng = 0.5-1.5, Pinj = 3-9 MW), with drift into the divertor, the main chamber C2+stagnation point is located on the high field side (HFS) and moves poloidally from the inner X-point up to the crown as the density is increased towards detachment. Changes of the main-chamber impurity stagnation point with density but before detachment are not observed in PT discharges. At detachment, the HFS C2+ radiation front sits stably above the X-point, below the flow stagnation point. These NT results are compared to UEDGE simulations with drifts, which are typically able to predict the location of the impurity stagnation point in PT L-mode discharges.   

Characterization of Turbulence in Negative Triangularity DIII-D Plasmas using Beam Emission Spectroscopy

Negative Triangularity (NT) shaped plasmas at DIII-D demonstrate high performance, ELM-free operation (H98 > 1, βN ≥ 2.5, fGW > 1) with improved turbulent characteristics which are not fully understood [1]. Turbulence is measured with localized (kyρs < 1), high speed (1 MS/s), multichannel (64 channels), 2D density fluctuation measurements using Beam Emission Spectroscopy (BES) [2]. In strong NT plasmas (δ∼-0.5), low amplitude fluctuations (n ̃/n<0.5%)  consistent with Ion Temperature Gradient (ITG) turbulence are observed propagating in the ion-diamagnetic direction for ρ∼0.65-0.85. In the edge (ρ>0.85), modes consistent with Trapped Electron Mode (TEM) turbulence are observed propagating in the electron-diamagnetic direction with reduced fluctuation amplitude at the separatrix. Upcoming experiments are planned to sweep triangularity at constant power from δ=+0.2→-0.2. First, low beam power below the L-H power threshold P_aux<P_LH will keep the plasma in L-mode in Positive Triangularity (PT). Then, high beam power P_aux>P_LH will be used to capture the transitions from H-mode in PT and weak NT to the ELM-free NT-edge. Finally, a mix of NBI and ECH power will be used to alter the balance between ion and electron turbulent modes. An overview of the experimental discharges and the preliminary turbulent results will be presented. BES fluctuation analysis can shed light on how negative triangularity influences turbulent transport, improving our understanding of NT as a core-edge integration solution.

[1] A. Marinoni et al 2021 Nucl. Fusion 61 116010

[2] GR. McKee et al 2010 Rev. Sci. Instrum. 81, 10D741   

Interplay of Turbulence, MARFE Dynamics, and Density Limit in Negative Triangularity Plasmas on DIII-D

This study investigates the microscopic physics of the density limit in negative triangularity plasmas in the DIII-D tokamak. By varying the input power up to 12 MW, we examined the relationship between maximum achievable density and power input, noting a modest positive correlation. Detailed measurements highlighted the behavior of turbulence and shear flows, particularly as the density limit is approached. Initially, turbulence at the low-field-side (LFS) increases prior to the emergence of the X-point multifaceted asymmetric radiation from the edge (MARFE). Subsequently, this MARFE migrates towards the high-field-side (HFS), intensifying HFS turbulence and precipitating significant edge cooling. The appearance of HFS MARFE correlates with the saturation of edge equilibrium density (ρ>0.95) at a level below the Greenwald limit, while the core density continues to increase, surpassing the Greenwald density. However, the growth of core density starts to cease when mid-plane line-averaged density fluctuations surpass a critical threshold, consistent with a collapse in the mean shear layer and reduced confinement time. Moreover, the adiabaticity parameter drops below unity when disruptive events are observed. These findings offer novel insights into the complex interplay between MARFE dynamics, turbulence, and the density limit in tokamak plasmas.   

Simulation of negative triangularity plasmas on DIII-D using SOLPS-ITER

Boundary plasma simulations constrained against experimental data of DIII-D negative triangularity (NT) plasmas with SOLPS-ITER are used to inform the design of dedicated divertor components. Across a range of plasma conditions and triangularities (delta_avg=-0.2/-0.5), the upstream kinetic profiles and heat flux widths are well matched. The measured trend in improved edge thermal confinement with increasing plasma current is reproduced from the inferred cross-field diffusivities. Divertor target profiles are reproduced withing a factor of ~2x, with further refinement likely requiring activation of cross-field drifts. It is found that the narrow heat flux width, open divertor shape, and short connection length make access to detachment without degradation of the core conditions challenging, consistent with experimental results. The dedicated NT divertor aims to increase the closure such that the beneficial properties of NT plasmas can be achieved along with detached divertor conditions. Using predictive simulations, it is found that a closed divertor reduces the upstream density at which rollover occurs by a factor of 2, with a significant reduction in the target electron temperatures. About half of the change comes from the increased connection length and the other half from closure, consistent with two-point model scalings.   

Design of a new closed divertor for negative triangularity operation in DIII-D

A new closed, pumped divertor is designed for the DIII-D tokamak to improve core-edge compatibility of Negative Triangularity (NT) scenarios. Discharges with strong NT shaping in the armor campaign required high density (f_Gw~1.3) to achieve detachment with intrinsic radiation due to limited divertor volume and short connection length. Confinement degradation was observed with increasing density, and MARFE formation affected confinement after detachment. A new divertor is being designed for installation in DIII-D, optimized for shapes with intermediate NT ( δ~-0.3) with longer parallel connection lengths (2.5x) and poloidal leg length (2x) compared to strong NT, and access to divertor pumping. The intermediate NT prevents access to second stability in simulations, ensuring ELM-free operation. Closed divertor plasma-facing components with private flux region pumping were optimized to reduce detachment onset density and improve particle control. SOLPS-ITER and UEDGE simulations show a reduction in detachment onset density due to the plasma geometry by about 30%, with further reduction enabled by the outer vertical target. Simulations show higher T_e at the X-point before the onset of detachment in the new geometry, reducing confinement degradation when approaching detachment.   

SOLPS-ITER modeling of a new divertor for negative triangularity plasmas in DIII-D tokamak

SOLPS-ITER modeling with full drifts was used to determine the shape of a dedicated divertor for Negative Triangularity (NT) plasmas in DIII-D, favoring a closed divertor to reduce the detachment threshold density. The DIII-D NT campaign in 2023 successfully demonstrated its potential to be a solution to core-edge integration with improved confinement in an ELM-free regime. However, it requires very high densities to detach the divertor with confinement degradation due to the small divertor volume and short connection length. Thus, a dedicated new divertor is needed. A variety of divertor concepts was tested with different outer baffle shapes. It is found that the effects of divertor closure are modest in reducing the detachment threshold density with ion BΧ▽B drift into the divertor. However, a more closed divertor reduces parallel heat flux to the target by a factor of 2 -3. With ion BΧ▽B drift out of the divertor, the EΧB drifts push plasma from the inner divertor to the outer divertor, leading to significantly lower detachment onset density. The divertor closure plays a more profound role in this drift direction. The detachment onset density is reduced by ~30% in a closed divertor, accompanied by significantly reduced heat flux to the target, compared to a semi-open divertor.   

Pursuit of high performance, small ELM, high-qmin plasmas with stronger shaping

Fusion pilot plant steady-state designs such as CAT-DEMO consider operation with q_min > 2 and q₉₅ ~ 5–6.5, but at higher β_N (~3.5–4.5) than has been achieved experimentally in DIII-D high q_min (q_min > 2) plasmas. These are not limited by ideal or resistive wall modes, and lower order rational q surfaces are excluded to prevent most deleterious instabilities; however, operation at high β_N (> 3.5) is still often stifled by tearing modes. Maintaining q_min > 2 is another challenge, requiring significant off-axis current drive. Recent experiments used upgraded electron cyclotron current drive (ECCD) capabilities and the new Stage 1 “Shape and Volume Rise (SVR)” divertor to improve both MHD stability and current drive. Prior to the SVR, ECCD was launched from the top of the machine into a typical double null DIII-D shape. Though EC current was more broadly deposited than ECCD injected from the low field side (LFS), it was often not absorbed. In subsequent experiments, the higher elongation and triangularity SVR shape was used with LFS ECCD only. We examine and compare these experimental results to IPS-FASTRAN predictions, which anticipated higher pedestal pressure and broader pressure profiles, stronger wall stabilization of kink modes, and higher q₉₅ for higher bootstrap fraction.   

DIII-D high beta hybrid with high frequency ELMs

Recent high beta (β_N ~ 3.5), high confinement (H_98y2 ~ 1.3) DIII-D hybrid experiments observed an increase in the ELM frequency at higher plasma density. These experiments showed increased confinement quality in low-rotation hybrid plasmas by increasing the plasma density (line-averaged density of ~7x10¹⁹ m^-3) and toroidal current (Ip = 1.2 MA). Discharges with higher scrape-off layer density showed an increase in the ELM frequency and a loss of type-I ELMs. Stability analysis with BOUT++ indicates that the plasma is unstable to a resistive ballooning mode near the separatrix. The modeling shows that this mode becomes unstable at a low pedestal pressure and high separatrix density/collisionality, leading to the high frequency ELMs. These high frequency ELMs prevent the pedestal pressure from increasing to the high levels needed for type-I ELMs to form. This operating regime appears to be an extension to higher beta and confinement of the quasi-continuous exhaust (QCE) mode discovered on ASDEX-Upgrade [1].

[1] Harrer, G.F., et al. Phys. Rev. Let. 129, 165001 (2022)   

Experimental studies of the interaction between fast ions, Alfvén eigenmodes and fishbones, microturbulence, and zonal flows in the DIII-D tokamak

Cross-scale couplings between fast ions, Alfvén eigenmodes (AE), and microturbulence are theoretically predicted to impact both saturated AE and drift-wave amplitudes and the fast- and thermal-ion transport they cause. An experimental “thrust” to study these interactions in the DIII-D tokamak is underway. Three dedicated experiments are scheduled for Summer 2024. The first focuses on the effect of fast-ion dilution on microturbulence in plasmas without AE activity. The second seeks detection of AE-induced zonal flows using beam-emission spectroscopy (BES) and Doppler backscattering (DBS) diagnostics. In the third experiment, BES and DBS diagnostics will search for the theoretically predicted fishbone-induced zonal flow that was correlated with triggering of an internal transport barrier [1].  Analysis of existing data hints at the existence of  zonal flows that are driven by either AEs or fast ions. Preliminary thrust highlights will be presented.

 

[1] G. Brochard et al., PRL 132 (2024) 075101.   

Progress in Developing Spin-Polarized Fusion Fuels for Enhanced Reactor Performance

Spin-polarized fusion can increase the fusion cross section by 50% and significantly enhance fusion power output through increased alpha heating. Experiments with polarized D and ³He in magnetic confinement devices can assess fusion reaction spin-physics, avoid hazardous tritium handling and reduce tritium consumption. Polarized ³He (65% polarization) can be prepared by permeating optically-pumped ³He into shell pellets, while deuterium can be polarized using dynamically polarized ⁷Li-D pellets (70% vector polarization) or frozen-spin H-D pellets (40% vector polarization). Prior work explored the feasibility of diagnosing polarization lifetimes in thermonuclear and beam-plasma scenarios at DIII-D by analyzing the energy, pitch, and poloidal distributions of fusion products, since spin polarization affects these signals. Recent simulations of D-D fusion reactions with ideal polarization show significant variations in the pitch and energy distributions of the fusion products, and a practical assessment of these results is presented. Despite the complex electromagnetic fields in fusion plasmas, this work suggests that polarization can survive, and the most important depolarization mechanisms can be experimentally tested. Potential issues with reactor implementation are discussed, and dedicated injectors and experimental scenarios for DIII-D and other facilities are proposed to study polarization lifetimes and reactor-relevant depolarization mechanisms.   

Spin Polarized LiD and 3He injector development at ORNL for a SPF test on the DIII-D National Fusion Facility

The goal of the spin polarized fusion (SPF) program is to measure the lifetime of spin polarized nuclei (D, 3He)  in the DIII-D tokamak.  Polarized D-T fuel for a fusion reactor is attractive because it could approximately double the fusion power (compared to ordinary, unpolarized nuclei in D-T power plants), while significantly reducing the required tritium inventory.  This project is a collaboration between Thomas Jefferson Laboratory (JLab), University of Virginia (UVa), ORNL, and the University of California Irvine (UCI). Producing polarized deuterium pellets is JLab’s responsibility, polarized 3He shell pellets are UVa’s responsibility, and injecting the polarized pellets is ORNL’s responsibility. UCI’s role is project coordination, plasma scenario development, and polarization detection.  Gas gun injectors for both LiD pellets, where the D is polarized, and polarized 3He in an inertial confinement fusion shell are being developed for this project.  The LiD injector and injection line must be maintained as close to 4 K temperature as possible and  in a 20 mT magnetic field.  The 3He injector must be maintained at 77K and similar magnetic field.  Here we describe the  injector development and the prototype injector designs thus far achieved and requirements for installation of the two injectors on DIII-D.

    

The Role of Island Bifurcation on Deconfinement of Energetic Electrons

Recent Frontier Science DIII-D experiments demonstrated that energetic electrons (EEs) of energies >10 MeV can be trapped within magnetic islands in the core plasma. Synchrotron emission camera data showed that the electrons remain mostly trapped inside the island. However, during island rotation, periodic bursts of EEs were detected by X-ray scintillators, suggesting that these particles can become deconfined and hit the wall. Here we argue that changing the direction of the I-coil current (needed for the rotation) changes the contribution of the dominant wave mode creating an island on the q=2 surface. As a result, the q=2 island chain bifurcates between a structure with 2 O-points and 2 X-points and a structure with 4 O-points and 4 X-points. To verify the role of island bifurcation on the deconfinement of EEs, the 3D magnetic field topology is reconstructed using the field line tracing code TRIP3D. Electron diffusion across the bifurcating q=2 island is then modeled by implementing a collisional operator in TRIP3D. Tracer electrons launched at different starting locations relative to the island allow for comparison of electron diffusion across the different island structures resulting from the bifurcation.   

The ECE radiation signature of runaway electrons in optically thick and thin plasmas

Dedicated forward modeling is applied to interpret Electron Cyclotron Emission (ECE) radiation from runaway electrons (>500 keV) on the DIII-D tokamak. ECE measures the radiation temperature profile T_e,rad (R) using the 2^nd harmonic cold resonance in a tokamak. It is found the runaway’s effect on  depends heavily on the plasma optical depth. For T_e,rad (R)  near the plasma axis where ECE is optically thick, only the runaways at the outboard side of the cold resonance contribute to the radiation. In this case, the increased T_e,rad (R)  by runaway is made through emission from the relativistic downshift of the 3^rd or 4^th harmonic ECE resonance, and a large pitch angle is needed for runaways to make noticeable change on T_e,rad (R). For T_e,rad (R)  near the plasma edge where the ECE is optically thin, the downshifted 2^nd harmonic radiation from runaways at the inboard side play the dominant role increasing the radiation. In this case, a large pitch angle is not required for runaways to make a noticeable change on T_e,rad (R) . The knowledge from the modeling successfully explains T_e,rad (R)  in a DIII-D runaway discharge, where  T_e,rad (R) close to the axis is unsymmetrical between the low and high field side, and the edge T_e,rad (R)  is significantly higher than the thermal value. Future application will involve interpreting T_e,rad (R) when whistler waves scatter the runaways.   

Characterization of Startup Runaway Electrons in DIII-D Plasmas

Startup runaway electrons (REs) reaching an energy of 25 MeV are produced in low density ohmic plasmas in the DIII-D tokamak. The generation and evolution of the observed startup REs are experimentally studied to support the ITPA analysis task which aims to understand the formation of startup REs and extrapolate to future devices. The evolution of RE quantity, maximum energy, and growth rate is determined using the Gamma Ray Imager measuring hard-x-ray (HXR) bremsstrahlung radiation with the key findings as follows: (1) the REs are observed from the early plasma current I_P ramp up phase and reach an energy of about 25 MeV in the current flattop; (2) the RE growth rate depends nonlinearly on the pre-fill gas pressure and consequently electron density during the I_P ramp, with 10% reduction in the line-averaged electron density resulting in approximately 4 times higher HXR signal at the end of the I_P ramp; (3) the RE quantity and energy continue to increase during the I_P flattop if not mitigated; (4) discharges with electron-cyclotron-assisted startup exhibit a reduced RE growth rate, but this does not always prevent the generation of sufficient RE seed as similar levels of the HXR signal can be observed in the discharges with and without assisted startup. These are among the first quantitative results obtained on tokamaks. They will be used for comparison with results from other devices, validation of startup models, and development of safe ITER discharge startup.   

The Role and Impacts of Isotope Mass on Burning Plasma Performance from DIII-D Similarity Experiments

The performance of burning plasmas is strongly influenced by the ion mass, M_I, which affects global energy and impurity particle confinement times, L-H power threshold, pedestal height, width, ELM characteristics, energetic particle modes, and RMP ELM-suppression. To understand the mechanisms behind these M_I dependencies and enable more accurate extrapolations towards D-T plasmas in ITER or an FPP, a set of systematic experiments and related modeling were organized and performed at DIII-D in similar hydrogen and deuterium plasmas. Several techniques for reducing P_LH in hydrogen plasmas were found: helium gas puffing, initiating L-H transitions at lower I_p and neoclassical toroidal viscosity rotation driven by non-resonant magnetic perturbations [1]. The H-mode pedestal width and pressure height increase while f_ELM decreases with M_I in dimensionally similar plasmas, and RMP ELM-suppression has so far not been achieved in hydrogen within the known deuterium access criteria [2]. Core turbulence amplitude surprisingly increases with M_I, counter to the lower observed transport, but the radial correlation length decreases [3]; CGYRO simulations indicate a shift to higher wavenumber instabilities at lower M_I. For a given injected power, Alfven eigenmode amplitudes and fast ion transport are generally higher for higher M_fast and M_I, consistent with TGLF-EP calculations and of potential concern for D-T plasmas [4].   

Radial Propagation of Turbulence Signals in DIII-D

Turbulent density fluctuations measured via Doppler back-scattering (DBS) on the DIII-D tokamak show strong signs of radial propagation of RMS signals across the plasma. The DBS system on DIII-D [1] uses injected microwave beams at 8 frequencies (55 - 75 GHz) to simultaneously probe wavenumber-resolved fluctuations in plasma density near the cutoff layers for each frequency. Measurements in the plasma edge can see overlapping signals, leading to strong correlation between channels for turbulent modes with radial extent; however, core measurements are radially separated by several centimeters, and correlations between channels have a time lag, suggesting the turbulent modes are propagating. Poloidal propagation of sheared eddies can lead to radial propagation of RMS signals. Comparisons of RMS signals over a range of perpendicular wavenumbers in L-mode plasmas are made in both quiescent plasmas and those with MHD activity. Vertically displaced plasmas allow for comparing radial drift of turbulent modes with the DBS diagnostic beam aimed at various degrees of tangency to the plasma flux surfaces. A comparison to data from H-mode discharges is also included.

[1] W A Peebles et al 2010 Rev Sci Instrum 81, 10D902   

Gyrokinetic simulations of high-performance wide pedestal quiescent H-mode at DIII-D

Wide pedestal quiescent H-mode at DIII-D exhibits insensitivity of the energy confinement time to the heating power (P_NBI). The lack of power degradation of the confinement is due to ion internal transport barriers (ITB) formation and the increased stored energy as P_NBI increases [Houshmandyar et al NF (2022)]. Flux-matched TGLF/TGYRO analysis predicts that the ion temperature gradient (ITG) mode is the dominant turbulence mechanism and it is stabilized by the Shafranov shift within the ITB. GENE gyrokinetic simulations of the flux-matched profiles show that within the ITB, the underlying turbulence mechanism changes from trapped electron mode -TEM- to ITG (for k_θρ_s < 1) as the heating power increased, while for k_θρ_s ~ 0.1-0.2 evidence for microtearing modes exists. As P_NBI is increased, several seemingly disparate elements undergo significant changes, prompting a working hypothesis: inward particle transport contributes to the lack of power degradation in energy confinement by stabilizing ITG modes. Key observations include: 1) large coherent turbulent structures measured by BES - commonly known as blob-void pair - have their birth-place location moved into mid-pedestal; 2) the density gradient increased within the ITB; 3) density fluctuations which originate from mid-pedestal regions propagate inward. Within the pedestal of these discharges, the preliminary results from global GENE simulations are consistent with TEM.   

Isotope dependence of the radial electric field in the DIII-D tokamak

DIII-D experiments in hydrogen and deuterium plasmas have identified differences in the L-mode radial electric field profiles between isotopes just before the L-H transition, which are observed by four independent radial electric field diagnostics: impurity charge exchange spectroscopy, Doppler BackScattering, Beam Emission Spectroscopy, and Langmuir probes. The electric field differences between isotope plasmas are investigated from the perspective of both the open and closed field line regions. In the open field line region, a hotter outer strike point is found to generate a more positive radial electric field at the separatrix in hydrogen plasmas compared to deuterium. In the closed field line region, the higher radial electric field is attributed to changes in poloidal rotation and turbulent Reynolds stress between isotopes, with Reynolds stress profiles consistent with predictions from E×B shear eddy tilting. The measured differences in L-mode Er and E×B shear are modeled to quantitatively assess their impact on the observed L-H transition power threshold.   

Advancements in full-wave synthetic diagnostic modeling of Doppler back-scattering

Advancements in modeling the Doppler back-scattering (DBS) fusion plasma diagnostic are presented. DBS measures localized, wavenumber-resolved density fluctuations through plasma scattering of electromagnetic waves. High physics-fidelity full-wave simulations of DBS using the commercial software COMSOL are presented. The full-wave model of DBS wave propagation in the background plasma is compared with ray and beam tracing models¹. Simulations of the DBS beam use realistic diagnostic and plasma parameters from experiments on the DIII-D tokamak. The reciprocity framework² is used to calculate the DBS weighting function in the linear scattering regime. The weighting function is studied to quantify the wavenumber resolution and spatial localization of the diagnostic. Full-wave results are combined with nonlinear gyrokinetics simulations to produce synthetic DBS frequency spectra. The simulated frequency spectra are compared with experimental DBS measurements from DIII-D discharges. The nonlinear scattering regime is also investigated using the full-wave COMSOL model.

¹V. Hall-Chen et al., Plasma Phys. Control. Fusion 64 095002 (2022)

²A. Piliya, A. Popov, Plasma Phys. Control. Fusion 44 467 (2002)   

Density Fluctuation Statistics and Turbulence Spreading Dynamics at the Edge-SOL Interface on DIII-D

Turbulence properties of long-wavelength ion gyro-scale ( kρ_i< 1) density fluctuations are investigated at the edge of DIII-D L-mode plasmas (ρ=0.88-1.1) using 2D Beam Emission Spectroscopy (BES) and advanced imaging analyses. Turbulence intensity flux〈~V_R~n〉was calculated using BES velocimetry in the region 0.9<ρ<1.1 for the first time at DIII-D to characterize turbulence spreading. Radial profiles of turbulence intensity flux 〈~V_R~n〉show that it is directed inward inside the separatrix, which demonstrates inward spreading of turbulence from the edge plasma region towards the core. The observation of positive and negative skewness of δn/n supports a theory of ‘blob-void’ pair formation as the origin of turbulence spreading: inward spreading is associated with the inward movement of negative density fluctuations, or ‘voids’.

Turbulence spreading from the plasma edge to the SOL is of great interest because this can increase the SOL width, hence lowering the heat flux to the divertor. Handling divertor heat loads is crucial for the safe operation of plasma-facing components in future devices like ITER.

In L-mode plasmas significantly weaker inward spreading is observed with co-I_p neutral beam torque. During the torque scan, a strong correlation is observed between the power scrape-off length λ_q, injected torque T_inj and turbulence amplitude δn/n(ρ= 0.97), showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.   

Multi-scale Interaction for Edge-Localized-Mode Suppression in Turbulent Pedestal in DIII-D plasmas

The multi-scale pedestal turbulence interaction shows a viable means for suppression of Edge Localized Modes (ELMs). Specifically, how interactions between large-scale MHD and small-scale drift wave turbulence modulate particle flux is studied in DIII-D wide pedestal quiescent H-mode (WPQH). The large-scale, low-frequency MHD (10-60 kHz) rotates in the ion-diamagnetic direction and is identified as weakly excited Peeling-Ballooning (PB) mode; the small-scale, high-frequency turbulence (60kHz-2MHz) rotates in the electron-diamagnetic direction and is comprised of electron drift waves. Alternating evolution of PB mode fluctuations, electron drift waves, and background density/temperature gradients are observed in WPQH mode pedestals. BES velocimetry analysis reveals that strong bicoherence, negative inward turbulent particle flux, and scatter of the cross phase between density and radial velocity perturbation of MHD during an electron drift wave burst. Such results demonstrate that the interplay between scale-separated modes plays a crucial role in determining ELM dynamics. Synergistic numerical modeling demonstrates that small-scale electron drift waves scatter the cross phase of the pressure and radial velocity perturbation of PB mode, resulting in decoherence of the PB-driven flux. This scattering interaction prevents PB growth and suppresses the ELM. A theoretical model to quantify the impact of electron drift wave scattering on PB modes has also been developed. This work yields a novel nonlinear prediction of the shift of the ELM onset boundary induced by the ambient electron drift waves, thereby indicating when a turbulent pedestal can be maintained in a quiescent state in this scenario. In addition, this work showcases a new type of multi-scale interaction physics, which can play a role in a wide range of physical systems.   

DIII-D High Field Side Lower Hybrid Current Drive Experiment Status

High field side lower hybrid current drive (HFS LHCD) is potentially an efficient off-axis current drive tool, r/a~0.6-0.8, for advanced tokamak DIII-D discharges. LH waves launched from HFS are expected to bridge the spectral gap through mode transformation from slow to fast back to slow wave resulting in improved accessibility and single pass absorption due to favorable wavenumber upshift. A compact coupler has been designed utilizing a traveling wave, 4-way splitter and a multi-junction to distribute power poloidally and toroidally, respectively, and has imbedded matching structures to maximize performance. The imbedded matching elements required additive manufacturing and post processing to achieve the desired RF voltage and loss characteristics. The HFS LHCD coupler and waveguides have been installed in the DIII-D tokamak. Prior to operation, vacuum issues arose after machine bake and have prevented RF operation into plasma. A new design to accommodate differential expansion between the waveguides and the machine has been analyzed and is planned to be implemented at the first opportunity. Klystron commissioning has progressed well and the klystrons have been operated up to 400 kW, 1 s into dummy load. The latest analysis, results and system status will be presented.   

Predictions of HFS LHCD Absorption in DIII-D LH Commissioning Targets

High field side (HFS) lower hybrid current drive (LHCD) is predicted to improve accessibility and penetration over low field side launch, permitting efficient off-axis (⍴ ≈ 0.6-0.8) current drive. The first-ever HFS LHCD launcher has been installed on DIII-D and will be commissioned during the 2025 campaign. A set of target plasmas for commissioning has been identified for which high (50-100%) single pass absorption (SPA) is predicted, reducing the risk of undamped LH power damaging device components. These targets include a variety of plasmas, including L-mode and both toroidal field directions. Many are cleaning plasmas, allowing the bulk of commissioning to occur during startup, thereby making the best use of experimental run days. Due to mode conversion being the expected mechanism of N|| upshift, the optimal launched N|| for these plasmas can be counter-intuitive. This set of commissioning plasmas, together with a sensitivity study of the SPA on the launched N|| throughout the duration of each shot, is presented.   

DIII-D Electron Cyclotron Heating and Current Drive Systems Status and Plans for Expansion to 10 Gyrotrons

The ECH/ECCD systems on DIII-D currently comprise five non-depressed and one depressed collector 110 GHz CPI gyrotrons. The combined total injected power is 3.4 MW into the DIII-D plasma via four dual-launchers with steerable mirrors that are mounted above the mid-plane at 240°, 255°, 270° and 285° in DIII-D port coordinates.  A seventh CPI gyrotron is expected later this year to bring the total injected power to approximately 4 MW. An 8-gyrotron (8-G) project is ongoing to install two new Thales TH1512 depressed collector 117.5 GHz gyrotrons (replacing one) that will use existing waveguides and launchers to establish eight systems with injected power of approximately 5.2 MWs by 2026. Another 10-gyrotron (10-G) project is ongoing to install two additional ITER-like 1 MW gyrotrons from Kyoto Fusioneering. These operate at 104, 137 and 170 GHz, though this variant will be tuned for optimal power at the 104 and 137 GHz frequencies, closer to the optimum frequencies for DIII-D rather than the ITER 170 GHz need. Two new transmission lines and dual-launcher will inject power from below the mid-plane at 255° symmetrically with the existing launchers. The 8-G and 10-G projects will bring the DIII-D ECH/ECCD capability to 10 systems with injected power approaching 7 MW by the beginning of the FY2026 plasma experimental campaign. We describe the latest status of operational systems and power calibration method; along with the plans and progress of the expansion projects.   

Experimental study of coupling 476 MHz helicon power to DIII-D plasmas

Up to ~0.5 MW of power at 476 MHz has been launched from a 30-element traveling wave antenna (TWA) into DIII-D plasmas and to date no rf-specific impurities have been detected when the antenna is powered. A fraction of the launched power is observed in the core as electron heating, motivating detailed study of the coupling process. The fraction of the power applied to the input of the antenna system emitted into the plasma is determined by the plasma conditions in the scrape-off layer, where the electron density is <~ 1 × 10¹⁹ m^-3 , and is strongly affected by the distance from the antenna face and the separatrix (TWAGAP), ELMs in H-mode discharges and by the level and location of gas puffing used to fuel the discharge. The coupling is measured with an array of rf current monitors embedded in the TWA and with directional couplers near the input of the antenna, at a time resolution of about 10 microseconds. The fraction of the coupled power in the unwanted slow mode polarization as well as the level of nonlinear activity such as parametric decay instability also depend on the plasma parameters in the SOL. Profile reflectometry, Thomson scattering, and Langmuir probes are used to constrain the electron density near the TWA.

    

Parametric instabilities associated with helicon wave injection in DIII-D*

Helicon (whistler) waves at 0.476 GHz have been launched on DIII-D with powers up to 0.6 MW for heating and current drive. Previously we have shown that under typical experimental conditions parametric decay instability (PDI) is expected in the edge plasma region. Measurements with magnetic pickup loops at the outboard edge of the plasma showed evidence of PDI corresponding to the local ion cyclotron frequency and its harmonics (ion cyclotron quasi-modes) and corresponding lower hybrid and/or ion Bernstein wave sidebands. Here we present the latest experimental observations of PDI under different plasma discharge conditions, and extend our numerical calculations of growth rates and frequencies to different values of the edge plasma parameters, including PDI associated with the E_|| of parasitically excited slow waves. For the first time we have also calculated excitation of PDI into ion sound quasi-modes and lower hybrid waves. The resulting broadening of the pump wave frequency spectrum due to this process may be indistinguishable from scattering by low frequency edge turbulence. Convective thresholds are also calculated to assess the possibility of pump wave depletion.

 

^*Work supported by US DOE under DE-FC02-04ER54698, DE-SC0016154, DE-SC0020337

 and DE-SC0014264.

    

Thermal Helium Beam Spectroscopy at the DIII-D Helicon Antenna

A thermal helium beam diagnostic (He-Beam) has been commissioned at the DIII-D tokamak. The diagnostic aims to measure the radial profiles of plasma density and temperature in the scrape-off layer surrounding the helicon antenna. These measurements are essential for evaluating plasma heating methods and for the general characterization of DIII-D boundary plasmas. The diagnostic is composed of two spectrometers with eighteen lines of sight each. A piezoelectric gas control system allows for precise puffs of helium gas above and below the helicon antenna, and using a collisional radiative model (CRM), the radial density and temperature profiles can be inferred via a line ratio technique. This method compares the intensity of lines at 667.8 nm, 706.5 nm, and 728.1 nm, which have shown good agreement with Thomson and Langmuir data in previous implementations at both Wendelstein-7X and ASDEX Upgrade.  Measurable signals for this diagnostic range from 1x10¹⁸ m^-3 < n_e < 5x10¹⁹ m^-3 for density and 15eV < T_e < 400 eV for temperature when making use of a new, time-dependent CRM. This contribution presents the details of the diagnostic setup, and first results from shots with and without helicon antenna operation. Along with first results, future goals will be presented and discussed.   

Conditioning Process of the Helicon Radiofrequency Traveling Wave Antenna

The helicon system has been operational in the DIII-D tokamak since 2021 with a 476 MHz klystron injecting up to 1.2 MW of RF power into a comb-line traveling wave antenna through co-axial transmission lines. Each time the DIII-D vacuum vessel is vented, the antenna needs to undergo a conditioning phase to restore it to its full operational level. An unconditioned antenna is subject to nonlinear dissipative processes such as multipactor-induced plasmas within the stripline, vacuum feedthrough, in-vessel co-axial line, or antenna components that can absorb and/or reflect incident RF power, causing an RF pulse train to run short due to a perceived arc and a loss in RF power before it reaches the antenna. Progress in conditioning is quantified with three metrics: an increase in the total RF on-time; an increase in the total power reaching the antenna; and a decrease in the fraction of reflected power. Previous conditioning phases have demonstrated an exponential rise in RF on-time with # of plasma shots and a corresponding increase in the fraction of applied power reaching the feed module of the antenna. Improvements to the system such as pressurized transmission lines and anti-multipactor coating of components have led to an improvement in conditioning recovery following a vent.   

High-Power Helicon System Upgrades and Repairs at DIII-D

Helicon plasma waves offer a promising route for plasma heating and current drive in fusion reactors. The DIII-D tokamak hosts a helicon system operating at 476 MHz, capable of outputting up to 1.2 MW.

In FY23, operational issues emerged, including damage to in-vessel thermocouples and RF probes around the antenna. Damaged thermocouples were removed and routed behind tiles was ensured for those remaining. All damaged RF probes were replaced and calibrated during the FY24 vent. Loose screws on the 150° inner conductor caused contact with the septum, rendering the 150° port inoperable. The bent bellows at the 150° port in-vessel inner conductor was removed, and the inner conductor was replaced during the FY24 maintenance vent.

During FY23, overheating incidents occurred in the elbow of the 210° port during prolonged high-power shots, though no visible damage was noted. To address this, elbows were copper-plated and diamond-coated to avoid multipactor effects in the future.

In the FY24 maintenance vent, the team removed the non-pressurized waveguide switch and implemented a patch panel to restore functionality of the Helicon feeding from both the 150° co-current and 210° counter-current directions.   

Spatial Distribution of Helicon Wave Amplitude Using Turbulence-induced Doppler Backscattering Measurements at DIII-D

A novel, frequency-stepped Doppler Backscattering System (DBS) has been employed at the DIII-D tokamak to investigate high-frequency broadband fluctuations observed near the frequency (476MHz) of externally launched helicon waves. These fluctuations are hypothesized to result via direct backscatter from plasma turbulence modulated by the externally injected helicon waves. By analyzing and comparing simultaneously measured low-frequency turbulence with the above high-frequency broadband helicon signal, we can infer the electric field amplitude of the helicon wave (E_hel) together with its spatial profile. Preliminary estimations of E_hel, as obtained through DBS, will be presented. The findings reveal specific spatial distribution patterns of E_hel within the plasma, highlighting the system's capability to measure these distributions. These measurements represent a crucial step toward utilizing helicon waves for current drive applications in DIII-D. Understanding the amplitude and spatial distribution of helicon waves will aid in optimizing their use for efficient current drive, a key component for the future of controlled fusion energy.   

High-fidelity simulations of helicon wave coupling in DIII-D H-mode plasmas

Helicon injection is being tested in DIII-D and KSTAR as a current drive source for steady-state tokamaks. While evidence of coupling and heat deposition was reported, a demonstration of effective core heating and current drive is still missing. The latter strongly depends on the ability to minimize and/or suppress the coupling with the slow mode, which might result in significant power losses in the scrape-off layer (SOL). We used Petra-M simulations in DIII-D conditions to quantify coupling effects with the slow mode. Here, we focused on two effects: (a) the misalignment between the antenna and background magnetic field and (b) the role of turbulent edge density on the helicon and slow waves. Parametric scans indicate that the misalignment of the Faraday screen is the most effective knob to reduce the slow mode and that a misalignment angle < 5 degrees minimizes the slow wave excitation. The predicted threshold for the misalignment angle closely matches the typical misalignment angle of 4-5 degrees in DIII-D. Therefore, the simulation findings suggested that even though the antenna can directly produce slow modes, the SOL power losses resulting from the slow mode might be insignificant. To study the effect of edge turbulence, we also carried out a full-wave simulation on a realistic background plasma obtained from the XGC code, including spatial density fluctuations in the edge and SOL, focusing on a DIII-D scenario with prominent edge turbulence (wide pedestal QH-mode). The results show that, in this scenario, edge density fluctuations strongly affect coupling by the effect of wave scattering in the core and wave trapping in the pedestal. The insights from the simulations discussed here inform the upcoming DIII-D experiments of helicon antenna coupling for long pulse scenarios.   

Real-time Charge Exchange Recombination Spectroscopy at DIII-D

At the DIII-D tokamak, the charge exchange recombination spectroscopy (CER) diagnostic routinely measures impurity temperature, rotation, and density. The ability to make these measurements in real-time within the Plasma Control System (PCS) has been expanded and improved upon in recent years. High speed network connections send raw data from 24 tangential CER views that cover the entire plasma radius to the PCS where spectral analysis is performed. Analysis is completed quickly by a simplified version of the least-squares fitting used for the highest quality CER analysis. Results are typically available within a millisecond of when the raw data becomes available and new raw data is typically sent to the PCS every 5 ms (for the most common CER sampling frequency). The results are shown to be quite close to more complex automatic analysis that becomes available minutes after a discharge is complete. Fully ionized carbon is the most common impurity used for analysis, and analysis can also be done for other impurities with relatively simple spectra such as neon and argon. The real-time analysis can be used for control in the PCS but it is most commonly used as a method for gaining very rapid assessment of ion conditions for the last discharge during an experiment.   

Overview: RF ICP Ion Source Developments for the DIII-D NBI System

Our collaboration between NC State University and the DIII-D develops radio-frequency (RF) inductively coupled plasma (ICP) positive ion sources to enable a heating power increase of the DIII-D neutral beam injection (NBI) system. We will present an overview of the NC State project scope and progress. Our goal is to design and demonstrate an ion source that is fully compatible with the present NBI accelerator and that operates in hydrogen, deuterium and helium. To inform the prototype ion source design, we perform RF ICP and finite element electromagnetic modeling. We are building two experimental setups at NC State, LUPIN and AMAROK. LUPIN is a reduced scale (cylindrical, 20 cm diameter) ICP source to test a novel, multi-strap RF antenna concept, to characterize plasma uniformity and to validate hybrid kinetic-fluid plasma simulations under NBI-relevant RF power and plasma densities. LUPIN has an internal Faraday screen and up to 20 kW RF power at 2 MHz frequency. AMAROK will be a full-scale pre-prototype ion source that will demonstrate a homogeneous plasma density across the future ion extraction area of 48 by 12 cm, which would translate into an 85 A positive ion beam. It will have at least 150 kW of installed RF power and also match the NBI injector gas flow rate of 15 Torr-L/s at operating pressures 1 to 10 Pa, which is required to study power losses through neutral gas heating. In conjunction with experimental setups that are based at DIII-D, we plan to deliver a prototype ion source design to be installed on a DIII-D NBI.   

First Plasmas and Initial Characterization with the RISE and SupRISE Test Devices for the DIII-D Neutral Beam System Upgrade

As part of the recent initiative to develop inductively coupled plasma (ICP) ion sources for the DIII-D neutral beam injection system, two ICP test devices have been constructed at DIII-D to tackle several engineering challenges and optimize design parameters for the prototype that will be installed in the near-future. A small-scale device called RISE (Radiofrequency Ion Source Experiment) has been retrofitted with additional plasma diagnostics and RF power handling capabilities to investigate various Faraday grid geometries and run comparative studies with the LUPIN ion source developed by our partners at NC State. In parallel, a full-scale test device called SupRISE (Superior Radiofrequency Ion Source Experiment) aims to determine the optimal RF frequency and ICP chamber dimensions for high power coupling as well as explore high voltage isolation capabilities for DIII-D facility integration. Both test devices are also being used to characterize a 50 kW solid state RF generator manufactured by Aethera Technologies^TM. SupRISE couples 50 kW of RF power (between 4 – 8 MHz) through a ~70 x 30 x 30 cm quartz vessel to achieve a plasma density of ~10¹⁸ m^-3 with a 10s ON, 3.5m OFF duty cycle. Results from initial experiments on plasmas generated with RISE and SupRISE will be presented.

This work is supported by the US DOE under DE-SC0024523 and DE-FC02-04ER54698.   

Development and Testing of LUPIN: A High-Density RF Ion Source for Enhanced NBI on DIII-D

The Large, Uniform Plasma for Ionizing Neutrals (LUPIN) is an RF inductively coupled plasma (ICP) ion source for the DIII-D Neutral Beam Injection (NBI) system. LUPIN drives 20 kW RF power at 2 MHz in a 20 cm long, 10 cm radius quartz vessel to create hydrogen plasmas with electron densities of 10¹⁸ m^-3 over 10 s, yielding an extracted ion current of 85 A which matches power density requirements for a full-scale ion source. Vacuum conductance and gas flow calculations predict a maximum neutral flow rate of 7.5 Torr L/s at 2.5 Pa. Designs and thermomechanical simulations have been developed for an internal Faraday shield to mitigate heat flux and ion sputtering on the dielectric vessel. LUPIN will investigate ion source physics, including neutral gas dynamics, plasma density uniformity, interactions with magnetic cusp fields, Faraday shielding, and power coupling to novel RF antenna designs. Development of an RF ICP ion source for DIII-D will help increase both power and reliability currently limited by electrical failures of existing arc-and-filament ion sources at higher power. Results of experimental investigations on LUPIN will guide the design of a full-scale prototype for DIII-D integration.   

Development of AMAROK: A prototype RF ICP source for the DIII-D NBI System

The Advanced Multi-turn Adaptive Radio-frequency source on Kinetic Neutrals (AMAROK) is currently under development at NC State University to help inform the design of an improved DIII-D Neutral Beam Injection (NBI) system. AMAROK is an RF-Inductively Coupled Plasma (ICP) source featuring a racetrack-shaped dielectric window measuring 40 cm in length, with a turn diameter of 28 cm at the curved ends, and a height of 20 cm. AMAROK will operate with hydrogen, deuterium, and helium at a flow rate of 15 Torr-L/s to match the conditions in the DIII- D NBI injector, with an operating pressure ranging from 1 to 10 Pa. This configuration aims to create a homogeneous plasma source across a potential extraction area of 48 cm by 12 cm. The system also includes a racetrack-shaped, water-cooled Faraday shield designed to minimize the erosion of the dielectric window. The setup incorporates commercially available solid-state RF generators capable of providing at least 150 kW of power. The optimal frequency for efficient power coupling is currently being explored, with the goal of achieving an ion density of approximately 1018 m−3 that is needed to sustain the required ion density for extraction of an 85 A positive ion beam. AMAROK is expected to explore innovations in RF generator technology to help deliver a design for upgrading the NBI ion sources of the DIII-D tokamak.   

Design & Characterization of the RF Matching Network for the LUPIN Ion Source

This study presents an impedance matching network (MN) design and characterization procedure

for the Large, Uniform Plasma for Ionizing Neutrals (LUPIN), an RF inductively coupled plasma

using a single strap, four turn (SSFT) coil and a multi-strap, single turn (MSST) coil. To match

power densities of the full-scale ion source, LUPIN will drive 10 kW RF power at 2 MHz

through a tunable L-type MN composed of 20 - 2000 pF capacitors placed in series and shunt.

LUPIN reaches electron densities (ne) ranging between 1014 and 1017 m-3 between 1 and 2 Pa.

Transformer equivalent circuit modeling is used to determine plasma inductance and resistance

for the SSFT coil. The SSFT coil has a nominal impedance of 105 Ω. A minimum 20% increase

in the load impedance is observed as ne increased from 1014 to 1017 m-3. Moreover, frequency

tuning across the generator range 1.80 - 2.17 MHz enabled an additional 15% change in the load

impedance with fixed electron density. Future research will use a vector network analyzer to

experimentally validate the MN design and build a matching network for the MSST coil.   

Investigating core transport and confinement discrepancies between two high density H-mode DIII-D discharges

Operation at high pedestal pressure and density is a promising route to high performance in future fusion reactors. In this work, we investigate why a recent hybrid scenario experiment carried out on the DIII-D tokamak was unable to access the “peeling-limited” pedestal branch at high density, despite matching operational parameters and using more heating power than an earlier reference discharge that entered this regime. As a key difference between the experiments was a slightly different combination of electron cyclotron heating and co/counter-beam injection, a sensitivity analysis for confinement dependence on rotation and heating power was conducted. Transport modeling using TGYRO and TGLF predicts negligible impact from changes in heating power, or rotation on the electron temperature or density. However, ion temperature is found to increase as rotation is increased. These results are consistent with suppression of stiff long wavelength turbulence by increased shear flow. Computational analysis with the CGYRO and EPED codes will be used to analyze the impact of rotation-induced changes in core pressure and pedestal stability and interpreted towards future experiments and fusion reactors.   

Exploring trends in Faraday-effect polarimetric measurements of the DIII-D tokamak core magnetic fields for future diagnostic and control applications

Faraday-effect polarimetry can provide fast time resolution (<1 μs) measurements of tokamak core magnetic fields without dependence on neutral beams. Using measurements from the three chords of the DIII-D tokamak’s Radial Interferometer-Polarimeter (RIP) diagnostic along with simple analytic models, one can obtain real-time information of the on-axis current density (J₀) and position of the magnetic axis (Z₀), in addition to line-integrated density (all important parameters for device control).  Additional equilibrium parameters (such as on-axis safety factor q₀) can be obtained by constraining the Grad-Shafranov equilibrium fitting code EFIT with RIP measurements. To explore the bounds of validity of these models, and to gain a foundation for further refinement of the models’ accuracy and speed, a survey of >500 DIII-D plasma discharges has been conducted, chosen based on data availability for Faraday rotation, electron density profiles, and Motional Stark Effect (MSE), so as to make comparisons between analytic models, Faraday-constrained EFIT, and MSE-constrained EFIT, and EFIT constrained only with external coil measurements. The ITER baseline scenario on DIII-D is an example scenario found to benefit from internal magnetic constraints (Faraday or MSE) for accurate calculation of J₀ and q₀, to show q₀ < 1 during periods of sawtooth crash activity. Plasma parameters are explored for their impact on Faraday constraints’ accuracy, such as the impact of electron density, toroidal magnetic field, and plasma shape on the influence of the Cotton-Mouton effect, in order to show that real-time measurement of the position of the magnetic axis and J₀ are possible, and to next steps for potential application of Faraday-effect polarimetry to real-time diagnosis and control.   

HBT-EP program: MHD dynamics and active control through 3D fields and currents

The HBT-EP research program aims to: (i) develop new machine learning and real-time data reduction methods for active control, (ii) explore the connections between edge transport physics, non-symmetric magnetic fields, and rotation, and (iii) study the role of large-scale passively-driven magnetic perturbations on disruption dynamics, relativistic electron loss, and electromagnetic loads. A convolution neural network (CNN) was developed to predict the amplitude and phase of an n=1 MHD mode using solely optical measurements from the upgraded HBT-EP high-speed videography system[1]. A real-time application of this algorithm using an FPGA has achieved a latency of 17.6 μs, on par with the magnetic sensor GPU-based control system [2]. A two-color multi-energy EUV/SXR tangential array has been used to observe the suppression of sawtooth MHD activity correlated with the coupling of an internal 2/1 mode and an external 3/1 kink mode consistent with observations of flux pumping as has been seen in hybrid plasmas in DIII-D and JET [3]. A Runaway Electron Mitigation Coil (REMC) driven by the disruption induced loop voltage to provide a “fail‐safe” prevention of high‐energy runaway electrons has been installed inside the HBT-EP vacuum vessel to carry out the first experimental test of the REMC concept.

[1] Y. Wei, et al., PPCF 65 (2023) 074002 

[2] Y. Wei, et al., RSI accepted for publication: https://arxiv.org/html/2312.00128v2

[3] Boting Li, et al., Nuclear Fusion 64 (2024) 046020.   

Disruption current observations in HBT-EP with expanded tile sensor array

Using the newly installed toroidally complete set of four High Field Side Scrape-Off Layer (HFS SOL) tile arrays on the HBT-EP experiment, this poster presents direct observations of currents flowing around a tokamak during a disrupting plasma, building on previous HBT studies of disruption currents using jumpers between insulated sections [1]. During disruptions, currents several percent of plasma current flow in helical filaments around the inwardly crashing  plasma column in the same direction as the plasma current. These filaments produce a periodic signal of increasing frequency on the tile arrays [2]. The remaining two tile arrays were installed with better poloidal resolution, providing data on currents flowing along the HFS at four locations separated by roughly 90˚ and each covering the same poloidal angle of about 80˚ around the HFS midplane. Using the fine poloidal resolution of the sensors, the spatial structure and rotation of the current filaments is detailed with comparison to the magnetic and fast camera data. Finally, the temporal structure of the currents is presented, with information on the frequency evolution of the signal as the disruption progresses. Changes to disruption currents due to the operation of the upcoming Runaway Electron Mitigation Coil (REMC), as well as the operation of control coils are discussed. Plans for biasing the tiles for use as actuators are discussed.

[1] J.P. Levesque et al 2017 Nucl. Fusion 57 086035

[2] A.R. Saperstein et al 2022 Nucl. Fusion 62 026044   

Experimental study of the toroidal distribution of energetic electron loss in the presence of a Runaway Electron Mitigation Coil (REMC)

The Runaway Electron Mitigation Coil (REMC) concept first proposed in [1] utilizes a passive, toroidally non-axisymmetric wire that couples to the plasma current during disruptions via the induced loop voltage. The coil’s magnetic fields break the flux surfaces throughout the plasma, helping to inhibit the production and avalanche of runaway electrons. Following the first-ever installation of an REMC, this poster presents studies of the Hard X-Ray (HXR) signal generated by runaway electrons on the HBT-EP tokamak. The runaway electrons generated during low-density plasma operation produce HXRs when they collide with plasma-facing material. HXR detectors have been previously utilized to observe the loss of energetic electrons in HBT-EP, and will be located at anticipated impact regions to measure the spatial emission distribution of HXRs with and without an REMC. We plan to present the differences in HXR signals between three different cases: normal plasma disruptions, disruptions where the high-field side REMC is activated, and disruptions where the low-field side REMC is activated. Cases where the coil is externally driven by a high-power amplifier during plasma operation (i.e. before the current quench) will also be discussed.

[1] Allen H Boozer 2011 Plasma Phys. Control. Fusion 53 084002   

Electromagnetic modeling of the HBT-EP runaway electron mitigation coil

Electromagnetic modeling is presented from the design of a Runaway Electron Mitigation Coil (REMC) in the HBT-EP tokamak – the first coil of this kind. ThinCurr, a thin-wall, 3D electromagnetic modeling code, predicts currents and fields in the coupled REMC, plasma, vacuum vessel, and ohmic heating coil system [Battey NF 2024]. These predictions will be compared to experimental data as available to validate numerical models, starting with initial tests performed under vacuum conditions. The magnetic field of the REMC driven by high-power amplifiers will be measured by Mirnov coil arrays. The signals are predicted to exhibit complex time dependencies due to varying 3D eddy current decay time scales. Then, the REMC current will be measured as it is inductively driven by the ohmic heating (OH) coils. More than 3 kA is expected to be driven by a maximum OH coil ramp. Following these tests, plasmas will be allowed to disrupt and inductively drive the REMC. A maximum of 15% of the plasma current is expected to be converted. An analogous workflow has been used for the upcoming DIII-D [Weisberg NF 2021] and SPARC [Tinguely NF 2021] REMCs with similar mid-disruption normalized field perturbations for each device. The HBT-EP REMC research program seeks to experimentally validate the workflow predicting these fields and currents.   

Development of an n = 1 optical mode tracking feedback control system on HBT-EP using a deep learning neural network

Active feedback control is critical to tokamak operation for mitigating plasma instabilities. Feedback latency is a key challenge for effective control systems and must be significantly lower than the time scale of instability growth. In this work we describe the development of an optical based mode control system on HBT-EP as well as the development and training of the mode tracking machine learning algorithm.  Using a fast camera diagnostic and convolutional neural network (CNN) deployed on a field programmable gate array (FPGA), we predict sine and cosine components of n=1 modes [1]. We analyze the feedback outputs and predictions from the CNN compared to GPU driven magnetic control signals [2]. The fast camera feedback system achieves a trigger to output latency of 17.6us making it sufficient for MHD mode control and competitive to the GPU system.  We also discuss the implementation of the feedback system on HBT-EP.

[1] Y. Wei et al 2023 Plasma Phys. Control. Fusion 65 074002

[2] Q Peng et al 2016 Plasma Phys. Control. Fusion 58 045001   

Design and operation of the first Runaway Electron Mitigation Coil (REMC)

A Runaway Electron Mitigation Coil (REMC) is a non-axisymmetric passive conducting structure designed to reduce the potential damage from runaway electrons (REs) by spoiling their confinement before they accelerate to dangerously high energies and currents during disruptions. Two in-vessel REMCs have been installed and plan to be operated this year in the High Beta Tokamak – Extended Pulse (HBT-EP) facility, marking the first implementation of such a purpose-built passive coil in any device. Two coils were built rather than one in order to explore coupling for different configurations, and also to mitigate experimental risk in case one of them suffers from a technical failure. Coils remain open-circuited during plasma startup, then one coil is switched closed prior to the disruption to be passively driven by the disruption loop voltage. Details of coil designs are presented, along with results from expected initial operations starting in summer/fall 2024. This study explores the effect of the coils on the progression of HBT-EP disruptions, including the influence on MHD behavior and halo current rotation as measured by magnetic sensors and current-collecting tiles. Upcoming plans for utilizing the coil are also presented, including extensive campaigns to measure disruption forces, RE energies, and RE deposition locations.   

Development of non-axisymmetric resistive wall models for MHD simulations of HBT-EP and other tokamaks

The NIMROD [1] code is used to validate multiphysics models (MHD + resistive wall) for the prediction of mode structures and scrape-off-layer (SOL) currents in tokamaks using high-resolution current, magnetic, and optical diagnostics of HBT-EP [2]. NIMROD’s existing thin resistive wall boundary condition is extended to include non-axisymmetric wall resistivity, capturing effects of ports and other wall structures. Simulations of HBT-EP with a resistive wall observe non-disruptive, saturated mode activity consistent with experimental data. Effects of varying thermal transport and varying wall resistivity with toroidal mode number are investigated in the context of a saturated external kink resistive wall mode with magnetic islands. Simulations of sawtoothing activity are established with varied macroscopic transport and current ramping rates for comparison to experiment. Work on improving the resistive wall boundary conditions to capture bulk n=0 equilibrium evolution during disruptions is shown. Applications toward better understanding the 3D structure of wall-connected currents and the effects of runaway electron mitigation coil (REMC) fields will be presented. Initial validation studies of numerical models for SOL currents are conducted by analyzing synthetic and experimental phase differences between diagnostic signals on HBT-EP with the goal of improving SOL and wall models for ITER and next-step devices.

[1] C. Sovinec et al., J. Comput. Phys. (2004)

[2] J. Levesque et al., Nucl. Fusion (2017)

[3] Boting Li et al., Nucl. Fusion (2024)   

Dependence of wall stabilization and qa on sawtooth triggering of saturated 2/1 tearing modes from a coupled kink-tearing mode in HBT-EP

Accurate modeling of tearing modes will be crucial to ensuring the reliability of future fusion reactor designs. This poster presents experimental data on the interactions of 2/1 tearing modes with edge kink modes, sawteeth, and a conducting wall, on the HBT-EP tokamak. Using a 4-fan 64 cord poloidal Extreme Ultraviolet (pEUV) diagnostic and tomographic inversion, the internal structures of modes in the plasma are reliably uncovered. Many diverse diagnostic signals are combined in a multidiagnostic Singular Value Decomposition (mdSVD), showing the dominance in typical perturbed signals of a 2/1 tearing mode rotating in phase with an edge 3/1 kink mode. This coupled 2/1-3/1 tearing kink system is observed to rapidly transition from low amplitude into a large saturated 2/1 tearing mode, at a specific sawtooth crash. Leveraging HBT-EP's unique radially adjustable outboard resistive walls, it is observed that as the wall stabilization is increased, the q=2 rational surface must be progressively closer to the surface of the plasma for the tearing mode to be triggered and remain saturated. It is observed that this dependency is not captured by Δ' in the absence of additional driving terms, nor explained by changes to the equilibrium profile.   

Parameterizing Wall Effects for Coupled Tearing Modes in HBT-EP

Otto Klüber’s now classic paper [1] reported coupling of multiple tearing modes and the detection of coupled helical scrape-off-layer currents. With newly installed arrays of EUV detectors and scape-off-layer detectors, HBT-EP also detects coupled mode activity. Boting Li [2] observed the coupling of rotating (m, n) = (1, 1), (2, 1), and (3, 1) modes that suppress sawteeth whenever the adjustable resistive wall was retracted from the plasma boundary. When the wall is moved inward, the amplitude of the (3, 1) mode and the other coupled modes decreased, and strong sawtooth oscillations reappeared. Similarly, Rian Chandra [3] observed the stabilizing influence of the resistive wall on sawtooth triggering of tearing modes. When the wall is positioned close to the plasma, sawteeth do not trigger tearing modes, unless the plasma current is sufficiently large. For all coupled modes, the position of the wall is important and coherent helical currents are observed in the SOL. To explore coupled tearing modes in HBT-EP experiments, we combine these with the now classical paper by Richard Fitzpatrick [4] and discuss the predicted wall effects and observations.

[1] Klüber, et al., NucFusion, 31, 907 (1991).

[2] Boting Li, et al., NucFusion, 64, 046020 (2024).

[3] Rian Chandra, Dissertation in prep, Columbia University, (2024).

[4] Fitzpatrick, et al. NucFusion, 33, 1533 (1993); and 33 1049 (1993).   

Edge MHD Mode Regulation by Plasma-Wall Coupling and Sawtooth Suppression by Flux-Pumping

The tangential multi-energy extreme ultraviolet and soft x-ray (ME-EUV/SXR) diagnostic system has been installed on the High Beta Tokamak-Extended Pulse (HBT-EP) device for measurements of electron temperature and study of mode dynamics [1]. Using this system, this study examines the mechanisms underlying sawtooth suppression on HBT-EP [2]. It is observed that strong-intensity sawtooth activities correlate with reduced-amplitude m/n=3/1 external kink modes, while sawtooth suppression correlates with larger and saturated MHD activities. By manipulating the plasma-wall coupling via adjusting the positions of the conducting walls in HBT-EP, it was found that strong sawtooth events occur when the normalized wall radius b/a is within a critical value. Even slight variations in the wall location result in significantly different discharge styles, categorized as “sawtoothing discharges” and “sawtooth-suppressed discharges” respectively. Through a series of mode structure analyses, we confirm the coexistence and coupling of the m/n=1/1 helical core, m/n=2/1 tearing mode, and m/n=3/1 external kink mode during sawtooth-suppression, and that this coupling induces anomalous current broadening. Based on these findings, we conclude that sawtooth suppression in the HBT-EP tokamak is consistent with the process of magnetic flux pumping.

[1] Boting Li et al Rev. Sci. Instrum. 1 October 2023; 94 (10): 103503.

[2] Boting Li et al 2024 Nucl. Fusion 64 046020.   

Characterization of Fast Ion losses in various ELMy regimes on the TCV tokamak

The large gradient region at the plasma edge of H-mode plasmas can suffer from cyclic instabilities, the so-called Edge Localized Modes (ELMs), that reduce the overall confinement for H-mode operation and provoke large heat loads towards the wall during the ELM crash. These can damage the Plasma Facing Components (PFCs) critically. The ELMs also lead to the undesired localised expulsion of fast ions (FI) further damaging the PFCs. Thus, the interplay between the FI, the ELMs and the present MHD activity has been extensively investigated on the TCV tokamak. Various H-mode scenarios displaying many ELM frequencies and amplitudes have been explored. First, in the presence of large type-I ELMs, MHD modes precursing the ELMs identified as ballooning modes [1], are observed in the magnetic pick-up coils and the Soft X-Ray (SXR). For the first time coherent diffusive transport of confined FIs preceding the ELM crash correlated with these modes has been observed. The novel TCV Fast Ion Loss Detector (FILD) measures a non-negligible amount of FI losses before the ELM crash, consistent with the decrease in the neutron level possibly indicating a significant FI transport. The characteristic frequency of these FI losses is consistent with the one observed with the SXR and the magnetic pick-up coils.

The TRACERS twin-spacecraft mission on magnetic reconnection and cusp dynamics

The overarching science goal of the TRACERS mission is to discover how spatial or temporal variations in magnetic reconnection drive cusp dynamics. Under this goal, TRACERS will answer the fundamental question of whether reconnection is dominantly spatially or temporally variable, and determine the associated reconnection rate. TRACERS will achieve its science goal with a simple mission design comprising two identical, small spacecraft in identical low-Earth orbits in a follow-the-leader configuration. TRACERS will make thousands of crossings in the northern cusp for a twelve-month primary mission using plasma and field instruments. These data will be analyzed using established dual-spacecraft techniques and supported by modeling that ensures science closure on the objectives. Our TRACERS team leverages hardware collaborations between the University of Iowa, Southwest Research Institute, University of California Los Angles, University of California Berkeley, and Millennium Space Systems. The science team consists of experts in reconnection, cusp physics, and modeling. TRACERS is dedicated to its proposer, and original Principal Investigator, Professor Craig Kletzing.   

Direct Radiation Resistance Measurement on a Loop Dipole Antenna from Excitation of Whistler Waves

High energy electrons from either solar wind or from human activity may become trapped inside the Van Allen radiation belts or create an artificial radiation belt that can persist for long periods of time. Spacecraft flying through these belts may be susceptible to damage from these trapped electrons. Whistler waves are known to precipitate electrons into the atmosphere, so a proposed solution is using spacecraft to carry compact electron beams or antennas to remediate these trapped electrons. Results of a laboratory plasma experiment investigating the efficiency of exciting whistler waves by loop antenna are presented here. For the first time, the complex impedance on a loop antenna has been directly measured by measuring the voltage and current directly on the antenna loop. A significant decrease in the real part of the impedance is measured as the plasma density is decreased. Indicating a successful measurement of the radiation resistance, because as the density goes to zero, the whistler wave can no longer be excited, and coupling of the long wavelength light wave is extremely poor. Characterization of the loop’s complex impedance will help further understand the coupling efficiency for whistler waves in a magnetized plasma and allow us to test theories that have long waited on the shelf. (I.G Kondrat’ev et al, Radiation of whistler waves in magnetoactive plasma, 1992). The results from this experiment are pertinent to active space-based experiments on artificial whistler wave excitation because the results of the loop will be compared to that of an electric dipole in our future work to help determine the more efficient radiator of the purposes of radiation belt remediation.   

Laboratory Experiments of Alfvén Wave Interactions through the Transition from the MHD to the Kinetic Range

Turbulence is ubiquitous throughout different space plasma environments, facilitating the cascade of energy down to smaller and smaller length scales.  That said, the different parameter regimes at which these plasmas exist have a significant effect on the way the cascade develops.  Though in-situ measurements can provide a wealth of knowledge about the properties of space turbulence, they are limited by their spatial extent relative to the plasma environment and their reproducibility.  Laboratory plasma experiments like those run on the LArge Plasma Device (LAPD) at the University of California-Los Angeles can provide insight complementary to satellite data.  The space plasma turbulence group at Queen Mary University of London (QMUL) has run Alfvén wave experiments on LAPD studying weak and strong interactions at a range of k_⊥ρ_s values, from very small (MHD limit) up to order unity (kinetic limit).  The change in the properties of the drive waves and their interaction products between these limits has been quantified via detailed measurements of magnetic and electric field fluctuations in multiple different counter-propagating wave configurations.  Further data runs allowed for an analysis of the residual energy- and cross helicity-dependent properties of the interactions.  With this experimental setup, the fundamental physics of the three-wave interaction can be studied in detail while minimizing the impact of other solar wind phenomena.    

A novel discretization method of a hybrid parallel-kinetic-perpendicular-moment model

A recent innovation in modeling collisionless, magnetized plasmas, dubbed the parallel-kinetic-perpendicular-moment, or PKPM, model employs a hybrid discretization of the Vlasov-Maxwell system of equations via a spectral expansion in only the perpendicular degrees of freedom, with perpendicular defined with respect to the local magnetic field. This approach reduces the six-dimensional Vlasov equation to a set of four-dimensional equations, with the exact number of four-dimensional equations encoding the amount of perpendicular resolution of the simulated plasma. The specific spectral expansion: Laguerre polynomials in the perpendicular velocity and Fourier harmonics in the gyrophase, is highly efficient. A number of kinetic plasma problems utilizing only a few spectral coefficients while still obtaining good agreement with fully kinetic simulations have been performed. 

In this presentation, we give an overview of how the remaining four-dimensional equations are discretized for maximum efficiency and robustness with a discontinuous Galerkin finite element method. We showcase a number of magnetized plasma problems where retaining the full local parallel velocity kinetics has manifest benefit, as fine features in phase space form in the parallel degree of freedom even as the plasma remains mostly gyrotropic.    

Benchmarking a Novel Hybrid Kinetic Model for Magnetized Plasmas

Simulating plasma devices is crucial to improving our understanding of how plasmas behave in different systems. While the robustness and speed of fluid simulations lend themselves to being a prevalent method of simulating plasmas, the technique has important drawbacks. Fluid simulations aren't able to capture Landau damping and other forms of plasma damping. One solution is to turn to kinetic simulations, but PIC codes are susceptible to noise and Vlasov codes can be extremely expensive to run for large systems. The Gkeyll simulation framework's novel Parallel Kinetic Perpendicular Moment (PKPM) model is a new model for weakly-collisional, magnetized plasmas. The model is derived from the distinct dynamics that occur parallel and perpendicular to the local magnetic field. By approximating plasma dynamics perpendicular to the magnetic field with a spectral expansion, this hybrid approach is much more computationally efficient. By simulating 1D Euler shocks and comparing the results to existing 5-moment fluid simulations, we can test the robustness of this scheme. We also simulate the Large Plasma Device at UCLA. Compared to existing fluid simulations, we aim to capture the plasma damping and its effect on the dynamics over time.   

Reduced Kinetic Gkeyll Simulations of Alfven Wave Reflection From an Alfven Speed Gradient in LAPD

The heating of the solar corona and acceleration of the solar wind is likely driven by Alfvenic turbulence, which requires counter-propagating Alfvenic fluctuations. Alfven waves are observed to be driven from the base of the corona, but the source of inward propagating waves is not yet established. The leading candidate is reflection from an Alfven speed gradient in the solar atmosphere, and recent experiments on the LArge Plasma Device (LAPD) at UCLA have for the first time observed such gradient driven Alfven wave reflection [Bose et al ApJ 2024]. The reflection was also successfully modeled in two-fluid simulations using the Gkeyll simulation framework, finding good qualitative agreement with the experimental data. However, the two-fluid simulations are unable to capture electron Landau damping and collisional damping, both of which are important in LAPD and in the solar atmosphere. In this presentation, we employ a novel reduced kinetic model with Gkeyll called the parallel-kinetic-perpendicular-moment (PKPM) model to study Alfven wave reflection from an Alfven speed gradient in a system that includes all forms of damping, collisionless and collisional, that occur in LAPD and the solar atmosphere.   

The Kinetic Analog of the Pressure-Strain Interaction

Energy conversion in weakly collisional plasma systems is often studied with fluid models and diagnostics. However, the applicability of fluid models is necessarily limited when collisions are weak or absent, and using a fluid approach can obscure kinetic processes that provide key insights into the physics of energy conversion. A kinetic technique that retains all of the information in 3D-3V phase-space for the study of energy transfer between electromagnetic fields and kinetic energy (quantified by the rate of electromagnetic work in fluid models) is the field-particle correlation technique (Klein et al. JPP, 2017). This technique has demonstrated that leveraging the full information contained in phase-space via kinetic diagnostics can elucidate the mechanisms of collisionless energy transfer. A different channel of energy conversion—between fluid flow energy and particle internal energy— is quantified in fluid models via the pressure-strain interaction (Yang et al. PoP, 2017). Using a similar approach to that of the field-particle correlation technique, in this work we derive a kinetic analog to the pressure-strain interaction and use it alongside the field-particle correlation to analyze the flow of energy from electromagnetc fields into particle internal energy in two case studies of electron Landau damping.    

10-Moment, Multi-Fluid Simulations of Proton Firehose Instabilities with Electron Dynamics

In weakly collisional solar and astrophysical plasmas, pressure anisotropy-driven instabilities such as the firehose instabilities are common. Simulating proton-electron plasmas to study pressure anisotropy-driven instabilities is computationally expensive for fully kinetic models, and hybrid models typically simplify the electron species to an isothermal, massless neutralizing fluid. Using the plasma simulation framework Gkeyll, we run high-resolution simulations of firehose instabilities with a 10-moment, multi-fluid model. The higher order moments and gradient relaxation closure contained in this model permit pressure anisotropy to develop in all species. Allowing a finite anisotropy enables the electrons to play a significant role in the saturation of the parallel proton firehose instability. We present an expanding box extension to the 10-moment model to investigate how the additional free energy present in expanding plasma systems affects the evolution and saturation of firehose instabilities.   

Ten-moment multifluid simulations of magnetic reconnection and current sheet instabilities

Fluid simulations are important tools for the simulation of large-scale problems in magnetospheric physics, but are often hindered by their need to capture kinetic effects. The two-fluid ten-moment model is advantageous due to kinetic features such as electron inertia and pressure gradients being self-consistently embedded without requiring an explicit solving of a generalized Ohm's law. In the past this model has been used with a local heat flux closure for simulations of the lower hybrid drift instability (LHDI) and the Magnetospheric Multiscale (MMS) mission Burch reconnection event. An improved gradient-driven closure for the heat flux is added, allowing the model to capture previously neglected agyrotropic effects. Results from full 3D simulations of the LHDI and Burch event are presented here with the new closure.   

A Conservative Discontinuous Galerkin Algorithm for Particle Kinetics on Smooth Manifolds

We have developed a novel formulation for modeling species in a kinetic continuum scheme with complex geometries. The advantage of our method lies in choosing canonical coordinates to evolve our system, which allows for a simplified evolution equation using the Canonical Poisson bracket. The resulting scheme has no explicit appearance of Christoffel-symbols, and the Poisson bracket is in its simplest, canonical form. Discretizing the Canonical Poisson Bracket in a Discontinuous Galerkin representation results in a high-order scheme for the neutral species. Coupled with an implicit BGK collision term, we can simulate a wide range of collisionality from a collisionless kinetic limit to the fluid limit. We demonstrate this with a transition in collisionality in a sod shock problem. As well, we exemplify the geometric capabilities with Kelvin-Helmholtz Instability on the surface of a sphere. Future application may employ more complex geometries by specifying a metric inverse that encodes the desired geometry. Additionally, from an astrophysical perspective, this formulation provides a pathway towards a first of its kind numeric scheme that can model neutral flows with continuum kinetics around compact objects.   

Comparison of Particle-in-Cell and Spectral Plasma Solver codes on weak whistler wave instability driven by temperature anisotropy

During periods of increased magnetospheric convection, fresh plasma is carried into the inner magnetosphere, leading to the velocity distribution function of energetic electrons becoming anisotropic. The temperature anisotropy of tens of keV electrons generates whistler-mode chorus waves, which play a crucial role in the ring current and radiation belt dynamics through wave-particle interactions. Self-consistent simulations of plasma waves is one of the biggest problems in the inner magnetosphere modeling. The conventional approach to simulate self-consistent plasma dynamics is the particle-in-cell (PIC) method which is frequently used to simulate whistler waves in the magnetosphere. PIC is a simple and robust method, however it has a major limitation which is a statistical noise which can nontrivially affect weak plasma instabilities and wave-particle resonances making it challenging to apply to considered problem. In this work, we present extensive comparison of whistler wave simulations with VPIC code against simulations with another first-principles code, the Spectral Plasma Solver (SPS). SPS is a continuous Vlasov solver which is based on spectral expansion of the velocity space with Asymmetrically Weighted Hermite Polynomials (AWHP). The spectral AWHP expansion allows for a significant reduction in the number of degrees of freedom required to represent velocity space, while still retaining kinetic effects. The noiseless nature of SPS makes it particularly suitable to deal with weak instabilities considered in this work. We investigate various aspects of simulations of weak whistler instabilities with the two codes and demonstrate that while PIC compares well with SPS and linear theory for larger anisotropies (strong instability), important discrepancies become apparent as the temperature anisotropy becomes smaller. Due to the high computational cost of weak whistler instabilities simulations with PIC codes, convergence studies are often unfeasible, and we argue that in these situations alternative approaches, like SPS, might be preferred.   

Spacecraft surface charging as a function of material properties

Spacecraft material behavior plays a very important role in space missions. Spacecraft immersed in plasma get charged by absorbing plasma particles and by emitting electrons from spacecraft surfaces via photoelectron and secondary electron emission. Spacecraft charging depends heavily on material properties such as work function, secondary electron yield, dielectric constant, and electric conductivity among other. Material properties are typically assumed to be static in charging models. However, it is well known that this is not the case in space. This makes spacecraft charging predictions very challenging. Material properties are well characterized before the spacecraft is put in orbit through characterization in the lab under clean conditions. However, once in space, material properties change due to the harsh and very dynamic space environment. We present a new capability to predict material behavior in space from first-principles modeling. The ongoing effort seeks to couple material models, density functional theory (DFT) and molecular dynamic (MD) codes, with environment models, plasma kinetic codes. This preliminary study will show results of surface charging as a function of material work function, dielectric constant, and conductivity.   

Multiphysics simulations of E3 EMP using Topanga

We simulate the E3 electromagnetic pulse (EMP) from a high-altitude nuclear explosion (HANE) over the Ozarks, using the code Topanga. Topanga is a multiphysics hybrid particle in cell code that models plasma dynamics, ionospheric chemistry, X-ray ionization, and electromagnetic field evolution for a HANE on timescales of up to 100 seconds. The HANE scenario we model is based on the Starfish Prime nuclear test. We use experimentally measured magnetotelluric impedance tensors to calculate E3 electric fields in the ground.

The E3 electric field amplitude in our simulations for low ground conductivity values of approximately 1E-4 S/m is in the range of 10-15 V/km. This is a factor of 2-3 lower than the electric field amplitudes reported by Love et al. (2021), using the same magnetotelluric impedance tensors. Our analysis reveals that this discrepancy is due to differences in the E3 magnetic field signal used in the two studies. Whereas Love et al. (2021) used a generic E3 magnetic field signal based on an IEC report (1996), Topanga calculates electromagnetic fields ab-initio for a specific HANE scenario.

Our results show that accurate E3 electromagnetic waveforms in addition to realistic impedance tensors are required to model E3 EMP and quantitatively predict the hazard it poses to the power grid.   

Measurements of Ionospherically Relevant Ion-Ion and Ion-Neutral Cross Sections

We present experimentally determined ion-ion and ion-neutral collisional cross-sections for various ionospheric compositions at different altitudes, conducted in a laboratory environment. Cross-section measurements were performed in the Space and Beam Experiment (SABER) at West Virginia University using an ion gun to accelerate H2, O2 He, Ar, N, and N2, and NO ions. The ions are directed through a gas mixture and the transmitted current collected with a Faraday cup. To recreate the conditions and gas composition of the ionosphere at various altitudes, an array of mass flow controllers was employed controlling H2, O2 He, Ar, and N2, and NO composition. Additionally, a helicon source is used to produce a level of ionization corresponding to the altitude being simulated. Current models of ionospheric ion-neutral collision frequencies show significant variability compared to estimates derived from radar observations. Accurate measurements of these collision cross-sections are crucial for enhancing our understanding of Magnetosphere-Ionosphere-Thermosphere coupling.   

Using Hybrid Simulations to Investigate Nonlinear Processes in Electromagnetic Ion Cyclotron (EMIC) Wave Growth

Geomagnetic storms cause sharp changes in relativistic electron flux throughout the Van Allen Radiation belts. Over the course of a storm, the population of relativistic electrons spikes sharply followed by a fast partial loss and a slower decay down to normal levels. Ultra-low frequency (ULF) wave-particle interactions have been examined as a loss mechanism for relativistic electrons, mainly in relation to pitch angle scattering. EMIC waves are a type of ULF wave that contributes to electron scattering and are of particular interest as a candidate for the fast loss period. Simulations were carried out using a hybrid code, in which ions are considered as kinetic particles while electrons are treated as a massless fluid. We examined the formation of EMIC waves in a uniform plasma in 1- and 2D. Plasma with a temperature anisotropy and the accompanying instability led to the creation of EMIC waves. The results of the calculations were compared to theoretical descriptions of EMIC waves and agreed with previously determined results. Besides the primary mode, an electrostatic wave propagating parallel to the background field was generated in the nonlinear stage. EMIC wave formation in the presence of heavy ions was also investigated with multiple modes observed. Finally, it was found that heating of the background plasma during the wave generation process causes new behavior to emerge. Results showed evidence of nonlinear processes outside what can be described by traditional quasi-linear methods.   

Evidence for Bending Modes of the Heliosphere's Current Density Disk

The characteristic time dependence of “high energy” electron populations originating from the heliosphere has been identified by the Alpha Magnetic Spectrometer detectors on board the International Space Station [1]. This exhibits periodicities with frequencies equal to 3, 2, and 1 the Sun rotation frequency. The Current Density Disk [2], which is an important feature of the heliosphere and corotates with the Sun, is suggested to be at the origin of the periodicities. In fact, the disk was predicted [2] to be subject to the excitation of bending modes. The presented theory proposes that the first three harmonics of these modes are responsible for the detections of the periodicities of the relevant electron populations and for the observed large-scale sector structure of the heliospheric magnetic field configuration. Moreover, the energy gained by the detected electron populations is attributed to magnetic reconnection processes, of the kind introduced for the Earth’s magnetotail [3], that are appropriate for a collisionless plasma current density sheet.

1. M. Aguilar et al. (AMS Collaboration), Phys. Rev. Lett., 130, id.161001 (2023).

2. G. Bertin and B. Coppi, Astrophys. J., 298, 387 (1985).

3. B. Coppi, G. Laval, and R. Pellat, Phys. Rev. Lett., 16, 1207 (1966).   

Does the Coronal Heating Rate Depend on Microscopic Reconnection Physics?

The footpoints of coronal loops are constantly shuffled by convection on the solar surface, entangling the magnetic field lines. According to Parker's coronal heating model, the intertwining of magnetic field lines results in the formation of highly intense current sheets. These current sheets facilitate reconnection, thereby converting magnetic energy into plasma energy. Previously, Parker's model has been extensively studied using resistive magnetohydrodynamic models. However, in the coronal environment, collisionless reconnection is potentially vital as current sheets develop at kinetic scales. Collisionless reconnection, in turn, may affect the storage and release of magnetic energy and the overall heating rate. We investigate the impact of collisionless reconnection in Parker's model using a reduced two-field model that incorporates the electron skin depth and the ion sound Larmor radius as free parameters. We conduct a series of simulations, varying the ratios between the system size and kinetic scales, and compare the results with those obtained using the resistive reduced MHD model. The simulation results suggest that the heating rate may be insensitive to the details of the reconnection mechanism.   

Studying 3D reconnection heating in the solar corona via gyrokinetic simulation

Coronal heating has been known to exist since the start of the 20th century, however, its exact mechanism has not been fully understood. One promising candidate is reconnection turbulence, driven by tearing modes arising from current sheets. In order to capture the kinetic properties of reconnection turbulence, previous work used the gyrokinetic framework to model 2D reconnection in the corona. Here, we construct a 3D loop geometry and simulate with hydrogen mass ratio and realistic coronal β to verify earlier 2D extrapolations with both linear and nonlinear simulations. 

Linear studies show a difference of reconnection rates between 3D loop geometry, 3D slab geometry, and 2D slab geometry. The reconnection rates in these geometries are found to have γ_3D,loop<γ_2D<γ_3D,slab. While curvature stabilizes reconnection, parallel streaming can enhance it. Heating rates obtained from 3D nonlinear simulations with realistic field-line twist q=4 and q=3 are found to be consistent with the observed as well as 2D-extrapolated heating rates. In 3D, the parallel electric field is mostly electrostatic, unlike in the 2D case. Flux-rope merging events are also observed in the 3D nonlinear simulation; analysis of mergers are shown and compared with their counterpart in the 2D simulations.    

A study of Alfvén Wave and Proton Beam Interactions with Expanding Box Hybrid Simulations

Field aligned proton beams drifting with respect to the core proton population, slightly exceeding the local Alfvén speed, are commonly observed in the Alfvénic solar wind. Understanding the origin, evolution, and stability of proton beams in the expanding solar wind, and how they interact with waves and fields, is fundamental in understanding solar wind dynamics and heating. In this work, wave-particle interactions mediated by the collapse of an Alfvén wave are investigated by means of the hybrid expanding box model, a code that mimics dynamical effects introduced by the solar wind radial expansion within the hybrid-PIC framework. Starting with an amplitude-modulated Alfvén wave, I will show that the Alfvén wave undergoes a local collapse leading to the formation of a field-aligned beam drifting at the Alfvén speed. I will discuss the initial wave collapse and how the radial expansion of the wind affects onset of kinetic instabilities and wave-particle interactions over long time scales, by considering an ensemble of initial conditions matching solar wind observations at a radial distance of about 0.3 AU. I will compare simulation results with solar wind particle and magnetic field data at various radial distances and discuss the implications of this work to interpret solar wind observations.   

MMS observations of non-gyrotropic distribution functions in a compressed current sheet

Micro-scale features are now being resolved by NASA's Magnetospheric Multi-Scale (MMS) mission, which means for the first time, we are able to investigate gyro-scale current sheets in detail and assess their role in magnetic reconnection. Our analysis of kinetic-scale structures and dynamics associated with compressed current sheets in MMS data shows that a perpendicular ambipolar electric field is localized to the region of lower hybrid fluctuations and the pressure gradient is comparatively small, leading to the interpretation that E×B velocity shear is the underlying fluctuation driving mechanism. The presence and location of shear-driven waves at the center of current sheets is notable because laboratory experiments and PIC simulations have shown that shear-driven lower hybrid fluctuations are capable of producing significant anomalous cross-field transport and resistivity, which can trigger magnetic reconnection. We show that the electron distribution function is non-gyrotropic (generated by the quasi-static electric field), which theoretical arguments suggest is an indicator of the possibility for magnetic reconnection to occur. The relationship between ambipolar electric fields, non-gyrotropic distribution functions, and magnetic reconnection will be explored.   

Acceleration of Particles by a Moving Magnetic Mirror

Moving variations of the magnetic field magnitude, essentially moving magnetic mirror configurations, play a role in several different energizing processes for space plasmas such as transit-time damping and Fermi acceleration. Here we present a derivation of the single particle motion of charged particles within a moving magnetic mirror utilizing an asymptotic multiple time scale approach. The results of this derivation reflect the energizing processes present within such a configuration, with energy being contributed to the particle's perpendicular cyclotron motion through the electric field, which is then transferred to the particle's field parallel motion through the magnetic field as the particle travels through the mirror. We also perform a numerical integration of these results to track the trajectories of multiple different particles within such a configuration. Applying the field particle correlation (FPC) technique, we illustrate the transfer of energy from the fields to the particles for different populations of particles within velocity space, finding a distinct velocity space signature associated with the magnetic mirror energizing process.   

The First Phase Diagram for the Dissipation of Astrophysical Plasma Turbulence

A specific set of dimensionless plasma and turbulence parameters is introduced to characterize the nature of turbulence and its dissipation in weakly collisional space and astrophysical plasmas. To develop predictive models of the turbulent plasma heating that characterize the partitioning of dissipated turbulent energy between the ion and electron species and between the perpendicular and parallel degrees of freedom, it is essential to identify the kinetic physical mechanisms that govern the damping of the turbulent fluctuations. A set of ten general plasma and turbulence parameters are defined, and reasonable approximations, along with turbulence scaling theories, are used to reduce this general set to just three parameters in the isotropic temperature case: the ion plasma beta, the ion-to-electron temperature ratio, and the isotropic driving wavenumber. A critical step forward in this study is to identify the dependence of all of the proposed kinetic mechanisms for turbulent damping in terms of the same set of fundamental parameters. The scaling of each damping mechanism on these fundamental parameters is used to construct the first phase diagram for the turbulent damping mechanisms as a function of the ion plasma beta and isotropic driving wavenumber.   

Deflection of Solar Energetic Protons by an Ion Thruster

While Galactic Cosmic Rays (GCR) represent a long-term cancer risk to deep space human missions, Solar Energetic Particles (SEP) represent a significant acute radiation hazard to human spaceflight. This work investigates wave-particle interactions caused by the incidence of a beam of energetic protons, representing a notional SEP event, onto a cloud of xenon ions, representing a notional ion thruster plume. The resulting electromagnetic ion/ion instability is developed to investigate the degree to which energetic protons can be scattered out of the initial beam. The primary objective of this investigation is to develop a technique for active mitigation of SEP radiation threats to spacecraft and crew, by exploring the wave-particle interaction of a SEP event with the plasma plume expelled from an ion thruster.   

3-Dimensional Global Hybrid Simulation of the Magnetospheric Polar Cusp Region in Response to the Passage of a Solar Wind Rotational Discontinuity

Solar wind directional discontinuities, such as rotational discontinuities (RDs), have a significant influence on the energy and transport processes in the Earth's magnetosphere. This process is studied in a global hybrid simulation case of the magnetosphere in response to a solar wind RD. The simulation shows strong variances in the magnetopause, boundary layer, and cusp caused by the passing RD. After the RD arrives at the low latitude of the northern cusp region, ion precipitation intensifies, and a double cusp structure varying in latitude occurs near the noonside and lasts for several minutes. We find a connection between this double cusp region and ion injection from both high-latitude and low-latitude X-line regions. This suggests that the double cusp structure may be a signature of magnetic reconnection occurring at both high and low latitudes due to the dominant IMF By after RD arrival. Additionally, we observe other phenomena related to the passing RD, such as periodic stronger ion outflow from the southern cusp region related to pre-noon ion precipitation into the northern hemisphere. On the By-dominated interplanetary magnetic field condition, cold plasma would pile up in southern cusp region. As the invation of dense cold plasma into lower altitude (~4Re) of the cusp, the ions would suddenly be accelerated to energy~keV and propagate out to the equator along with strong Alfven variations in the route. The mechanism of acceleration remains to be analyzed.   

Multi-scale Analysis of Magnetopause Reconnection Using ANGIE3D: Structure and Reconnection Rate

 

This work investigates magnetopause reconnection and its associated reconnection rates using a global hybrid simulation model, the Auburn Global Hybrid Code in 3D (ANGIE3D). This hybrid model treats ions as fully kinetic particles while treating electrons as a massless fluid, enabling the resolution of ion kinetic-scale physics within global-scale dynamics. By simulating the TRICE-2 event, characterized by a strong dawn-dusk component of the interplanetary magnetic field (IMF), we examine magnetopause reconnection occurring at multiple sites in the northern-dawn, southern-dusk, and lower latitude regions. We first compare the simulated reconnection structures with the MMS observations at the magnetopause (with the magnetopause structure as observed by MMS, doi.org/10.1029/2023JA032263). Local reconnection rates are calculated from the electric field near the X-lines. Additionally, the global reconnection rate along the dayside magnetopause is calculated. We discuss the spatial and temporal variations of the reconnection rates, providing insights into the complex dynamics of solar wind-magnetosphere interactions.   

The Effects of Sub-Relativistic Magnetization on Reconnection X-Point Particle Acceleration

Magnetic reconnection is a ubiquitous plasma process responsible for rapid and substantial magnetic energy dissipation throughout the universe. It is also a major candidate to explain the generation of large populations of nonthermal particles observed in a wide variety of environments. While simulations of relativistic reconnection have convincingly demonstrated its potential to generate hard power-laws, non-relativistic simulations have faced difficulty doing the same. To elucidate the physics responsible for this discrepancy, we perform 2D particle-in-cell simulations of magnetic reconnection using the Tristan v2 code. We contrast the dynamics of the non-relativistic regime (magnetization < 1) with the fully relativistic regime (magnetization >> 1) for pair plasmas, focusing on the x-points, critical injection sites for subsequent acceleration. We find that in the sub-relativistic case the smaller spatial extent of the diffusion regions is primarily responsible for suppressed nonthermal particle energization.   

Simulating Shocks with Refined Semi-Implicit Kinetic Model FLEKS

Investigating plasma dynamics across shock structures and associated wave-particle interactions is fundamental for comprehending supercritical solar wind-magnetosphere interactions. This presentation details recent advancements in the semi-implicit energy conserving FLexible Exascale Kinetic Simulator (FLEKS) model and its applications in local and global magnetospheric shock simulations. FLEKS has already demonstrated its prowess in capturing kinetic processes related to magnetic reconnection, often coupled with large-scale magnetohydrodynamic (MHD) models to resolve kinetic effects within global magnetospheric configurations. However, the original semi-implicit algorithm presented challenges in shock simulations, leading to spurious plasma heating and numerical instabilities. The improved algorithm now enables FLEKS to successfully capture both quasi-perpendicular and quasi-parallel shock structures, generating waves absent in MHD models. We introduce the kinetic scaling factor in shock simulations and assess its applicability to global magnetospheric plasma transport. This development paves the way for integrating local shock physics into global magnetospheric plasma transport models, establishing the foundation for the next generation of magnetospheric modeling.   

Verification and Validation of Smooth Particle Hydrodynamics on Pulsed Plasma Accelerators

Smooth particle hydrodynamics (SPH) was initially developed for astrophysical problems in 1977 and has since been utilized in various fields of research. The Propulsion Research Center at the University of Alabama has developed their own SPH program, known as SPFMax, to simulate applications aimed at advancing space propulsion and fusion technologies. The objective of this project is to employ SPFMax using Ohm's law and a form of transmission line theory to validate and verify a known electric propulsion system, the coaxial plasma gun, focusing on convergence for further applications. Specifically, ordinary differential equations based on j x B physics will be compared to SPFMax results. In the long term, SPFMax aims to support laboratory magnetic reconnection experiments, such as torsional magnetic reconnection (TMR), which are intended for hypervelocity plasma acceleration and localized heating in magneto-inertial fusion and advanced propulsion applications.   

A Coupled Vlasov-Rosenbluth-Fokker-Planck and Radiation Modeling of a NIF Polar Direct Drive Exploding Pusher Capsule

Polar direct-drive exploding pusher (PDD-EP) capsules at NIF have been proposed as high-fluence neutron sources and a platform for neutron capture cross-section measurement for laboratory astrophysics experiments. Due to the low fill density and high-temperature conditions supported by PDD-EP capsules, however, the Knudsen number far exceeds unity, requiring a Vlasov-Fokker-Planck (VFP) kinetic description for the plasmas to correctly account for transport effects. The iFP spherical implosion VFP code [1] has recently been extended to include radiation capabilities that leverages the LANL TOPS LTE opacity library. We use iFP to model the Cutie PDD EP experiment [2] and report on observable metrics like nuclear yield and burn-averaged quantities. We present first-of-a-kind simulation results that faithfully account for kinetic plasma and radiation effects and compare our results with predictions from the xRAGE radiation hydrodynamic code and experiments.

[1] W.T. Taitano et al., Comp. Phys. Comm., 263 (2021) 107861

[2] C.B. Yeamans et al., Nucl. Fusion, 61 (2021) 046031   

Fully Electro-magnetic Relativistic Drift-Kinetic Formulation in Particle-in-Cell Code

The drift-kinetic treatment is a reduced-physics model for magnetized plasmas in which particles are advanced along their guiding center and cyclotron oscillations are effectively removed by time-averaging the actual particle orbit over a cyclotron period. The drift-kinetic model applied to magnetically insulated transmission lines (MITLs) was initially described in a recent paper by Evstatiev and Hess*, in which displacement current was neglected. We have implemented a fully electromagnetic version, including diamagnetic current corrections for dense plasmas, of the original model into the hybrid-PIC code Chicago. The model was originally limited to 2D rz cylindrical coordinates with the magnetic field in the azimuthal direction, but the model has been extended to 2D and 3D Cartesian coordinates. The model also allows for relativistic particles. We briefly describe the model, its implementation into Chicago, and present results from the test problems in the Evstatiev and Hess paper. We then demonstrate results for a realistic inner MITL geometry of the Sandia National Laboratories’ Z machine.  There is reasonable agreement between fully-kinetic and drift kinetic treatments. There is also significant increase in permissible timestep when using the drift-kinetic treatment as the requirement to resolve to the cyclotron period is lifted.

The roadmap towards scalable cross-beam energy transfer simulations

When multiple high-intensity laser beams overlap in a plasma, they can exchange energy via the cross-beam energy transfer (CBET) process. CBET is one of the dominating laser-plasma interactions that influences the efficiency of an inertial confinement fusion (ICF) implosion. However, modeling the CBET interaction within an ICF simulation poses a computational challenge at high resolution in 3D. Existing models either use simplifying assumptions to reduce the problem complexity at the expense of accuracy, or fail to harness modern hardware capabilities. In this work, we report on the testing and implementation of a 3D CBET package that follows best programming practices so it is able to scale efficiently on many processors and GPUs. We present our progress on development and validation, and share the roadmap for MPI and GPU implementations.   

Utility of differentiable simulators for innovative target design in ICF/IFE

ICF/IFE target design is a complex optimisation task, requiring both predictive simulations and many input parameters. Gradient-based optimisation methods are highly efficient when there are many design variables. However, for IFE target design this gradient information needs to propagate through the simulation, which is not possible with current state-of-the-art IFE simulation codes. Automatic differentiation (AD) is a key enabling technology for machine learning applications. It allows for differentiable programming, where accurate gradient information can be computed for any computer program (roughly) automatically. In this work, we discuss the ongoing development of a 1D Lagrangian radiation-hydrodynamics code for laser direct drive simulations, lagr-ADEPT. Because it is written in JAX, it is Pythonic, GPU-native, AD-enabled, and machine learning ready. We discuss its capabilities for prediction, inverse design and uncertainty quantification for implosion targets. We examine how machine learning technologies can be coupled with this code and what this means for future high-gain design studies.   

Machine Learning-based 3D Reconstruction of ICF Capsules via Self-Emission Images

In Inertial Confinement Fusion (ICF) experiments, pusher shots are composed of shell/ablator and filled with a mix of D-T and typically a noble gas for diagnostic purposes. Asymmetries during capsule compression can inaccurately determine parameters like neutron yield with 1D and 2D models. Even with experimental techniques, the lack of available multiple views fails to encompass all the 3-d effects of the implosion, necessitating theoretical assumptions. A key quantity for judging the accuracy of these models is amount of compression of the capsule, which can be gained from self-emission images. We propose a synthetic data for 3D reconstruction through Machine Learning techniques for the purpose of determining the convergence ratio for comparison with the simulation codes. Previous work done by Wolfe et al.  used convolutional neural networks (CNN) to produce 3D reconstructions of ICF models using simulated back-lighter images [1]. We adapted this work, we generated plasma profiles with Python, processed by Prism’s SPECT-3D [2] for accurate simulated self-emission images. This approach resulted in a training set consisting of thousands of profile-image pairs. Using our dataset, we will train a CNN to generate 3D reconstructions from self-emission images to improve feature distinction in imploding ICF capsules.

References:

[1] Wolfe, Bradley T. et al. “Machine Learning for Detection of 3D Features Using Sparse X-Ray Tomographic Reconstruction.” Review of scientific instruments 94.2 (2023): 023504. Print.

[2] . MacFarlane, J.J. et al. “SPECT3D – A Multi-Dimensional Collisional-Radiative Code for Generating Diagnostic Signatures Based on Hydrodynamics and PIC Simulation Output.” High energy density physics 3.1 (2007): 181–190. Print.   

Application of physics-informed neural networks to modeling discontinuities in hydrodynamics and MHD*

Simulating discontinuities in magnetohydrodynamics and fluid physics has long been challenging, particularly in phenomena with shock waves that exhibit strong nonlinearities. Previous simulations of the magnetized Noh test problem1 have frequently exhibited grid-induced errors. In comparison to traditional numerical methods, the recently developed physics-informed neural networks (PINNs) are mesh-free and thus can potentially handle such irregular and moving-domain problems. In this study, we perform simulations using the PINNs algorithm for suite of standard exact solutions for problems consisting of a strong shock forming as a compressible gas moves at a constant velocity towards a rigid wall both in the presence and absence of a magnetic field. These problems include the classic the classic Sod shock problem, Noh problem, the magnetized Noh test problem. Comparison to direct numerical simulations to results from the Arbitrary Lagrangian-Eulerian Finite Volume magnetohydrodynamic code MACH2 and significance for Z-pinch simulations with a moving mesh will also be discussed.

1.    Velikovich et. al., Phys. Plasmas 19, 012707 (2012).   

Multi-modal radiographic imaging and tomography (MM-RadIT) through data fusion and deep neural networks

Deep neural networks (DNNs) offer a generic platform for data fusion (DF) [1], which includes multi-instrument data fusion (MIDF), multi-experiment data fusion (MXDF), and simulation-experiment data fusion (SXDF). These features make DNNs attractive to nuclear fusion power plant applications, as well as multimodal (MM) radiographic imaging and tomography (RadIT) for non-destructive testing and plasma diagnostics, leveraging accelerated workflows through machine learning and artificial intelligence (AI). Here we first highlight recent advances in neutron-X-ray (NeuX) instrumentation and analysis [2-5]. We then summarize several possible new directions in MM-RadIT using the AI-enhanced framework of Physics-informed Meta-instrument for eXperiments (PiMiX) [1] including 1.) Advancing ‘mini-RadIT’ and their applications, addressing the gaps in spatial resolution, temporal resolution, high-repetition-rate, high-confidence (probability) and high-fidelity three-dimensional (3D), 4D (time-dependent 3D), and 4D+ tomographic and hyperspace reconstructions; 2.) Integrated RadIT data and information science approach, or data-driven RadIT towards AI-RadIT, through the state-of-the-art modeling, imaging hardware optimization, innovative RadIT modalities, advanced data algorithms, and uncertainty quantification (UQ). AI-RadIT may automatically harvest more information from experiments for carbon-neutral energy (e.g. nuclear fusion energy), and plasma processing of materials; 3.) Integrated ‘mini-RadIT’ and ‘AI-RadIT’ for mAI-RadIT; and 4.) Time-resolved positron imaging and tomography, as an emerging modality of time-resolved antiparticle RadIT, enabled by ultra-short-pulse high-power lasers and compact accelerators. Los Alamos Report number LA-UR-24-25903.   

Bringing non-destructive isotopic assay into the field with Dense Plasma Focus Nuclear Resonance Transmission Analysis

Numerous global security nuclear threat reduction (NTR) missions require non-destructive, unambiguous, isotopic assay of shielded materials in the field, including safeguards, waste remediation, emergency response, and arms control. We will investigate using a miniaturized, short-pulse neutron source called a dense plasma focus (DPF) in a field-portable isotopic assay platform to detect shielded nuclear materials using a compact form of nuclear resonance transmission analysis (NRTA). The DPF's neutrons are moderated, passed through a sample, and measured as a neutron absorption energy spectrum "fingerprint" unique to each isotope. Nuclear materials can be identified and quantified using this measurement. We will use the radiation transport software MCNP to determine the limitations and number of neutrons required for the measurement. We will validate the result with experiments on the existing facility sized DPF NRTA setup to establish NRTA as a viable candidate for isotopic assay. The eventual system will require tritium for a 50x boost to neutron output, so we will explore a system with 1/50th the output needed with a deuterium-only setup for this project which could establish the DPF NRTA system as capable of meeting the requirements. Such a field deployable system could provide a major tool in tracking and accounting for nuclear materials and play a role in numerous missions.   

Investigating Stagnation Degradation Mechanisms in a Dense Plasma Focus Pinch

The MJOLNIR dense plasma focus (DPF) at LLNL can deliver up to 4.4 MA of current to two coaxial electrodes, which generate a plasma sheath between them by ionizing deuterium gas. Driven by the JxB force, the sheath lifts off from the insulator and travels the length of the electrodes, converges on-axis, and forms a z-pinch geometry, producing neutrons in the stagnation and break-up phases. Experiments indicate that the sheath may not always sweep up all of the gas, leaving mass behind it; the large transient voltage induced during the stagnation and z-pinch breakup can turn this mass into an alternate current pathway, diverting current from reaching the z pinch. We have designed interferometry and Faraday rotation diagnostics to determine the electron density and the location of the current at stagnation. Faraday rotation, in combination with electron density, allows reconstruction of the azimuthal magnetic field, from which we can determine the plasma current. The current in the sheath region can be compared with the current measurements from a Rogowski coil at the base of the electrodes and B-dot probes mounted on the cathode rods along the pathway to the pinch region to determine if current is being lost between these locations during the discharge. Tracking the spatial current evolution will provide insight into the limitations that cause the neutron yield in MJ-class DPFs to stop increasing above a certain current threshold and help develop mitigation techniques.

LLNL-ABS-866149   

The MJOLNIR platform: a versatile MA pulsed power facility

The MegaJOuLe Neutron Imaging Radiography (MJOLNIR) platform at LLNL was commissioned in 2018 to investigate the viability of a dense plasma focus (DPF) as a source for dynamic radiography applications. In this work, we described recent experiments in which different anode shapes were fielded along with the introduction of inert dopant gases. Optical, x-ray, and neutron measurements from an extended suite of diagnostics will be presented. Furthermore, we present new capabilities to be added on the MJOLNIR platform, increasing experimental flexibility and allowing the investigation and mitigation of parasitic currents. Namely, a new cathode mount and a gas jet will be implemented to allow easy swapping of cathodes with different shapes and increase local gas density on-axis. The MJOLNIR platform can provide unique experimental capabilities to broader scientific communities that are interested in fundamental plasmas science and its applications outside of radiography. The MJOLNIR team is actively seeking potential external users.   

Sub-nanosecond Time-resolved X-ray Measurements of Multi-material Hybrid X-pinches

The Hybrid X-Pinch (HXP) has been shown to be an excellent point source of X-ray emission on the XP pulsed-power machine for radiography, producing X-ray radiation with photon energies up to 4keV ¹. Spectroscopic analysis of x-pinch dynamics has shown L-shell line emission during the compression phase, followed by a ~10ps continuum burst, with many emission lines in the expansion phase². Current studies are exploring the use of multi-material wire HXPs utilizing a sub-nanosecond time-resolved radiation technique³. Its goal is to determine if the multi-material wire HXP load configuration can predicably produce temporally spaced x-ray sources within the desired soft X-ray energy bands through material selection. This study is utilizing the 450 kA peak current XP pulsed power generator with a rise time of 60 ns (10 to 90%).   

Stability Analysis of Dynamic Screw-Pinch Driven Thin-Foil Liner Implosions

The dynamic screw pinch (DSP) has been shown to reduce instability growth in metal liner implosions relevant to magnetized liner inertial fusion (MagLIF) [1].  To experimentally investigate the effects and potential limits of instability mitigation in DSP implosions, a suite of return-current structures was designed and tested on the MAIZE facility at the University of Michigan. The suite allows a large range of initial drive field ratios to be tested, with initial axial to azimuthal magnetic field ratios of ~0.3 to >1.0, while minimally impacting the load inductance of the system. This was accomplished using various pitch angles, radii, and number of intertwined helices in the return-current structures.  The experiments were performed using 400-nm-thick metal foil liners. The implosion dynamics were captured using a 12-frame optical self-emission camera. Results from this study will be presented, along with considerations for future experiments.

[1] P. C. Campbell et al., PRL 125, 035001 (2020)   

Direct comparison of pinch dynamics of noble gases (Neon, Argon, Krypton and Xenon)On Linear Transformer Driver CESZAR

Radiation plays an important role in the implosion dynamics of pinches. In order to understand the effect of radiation, we conducted a comparative study of annular liner gas puff Z-pinches using Ne, Ar, Kr, and Xe on the CESZAR Linear Transformer Driver (LTD)  capable of producing ~500 kA current with a ~160 ns rise time. Diagnostic techniques implemented included a four-frame time-gated XUV pinhole camera and a two-frame schlieren imaging system enabling detailed visualization of the implosion dynamics and stability characteristics. Filtered X-ray pin diodes were also included to capture precise timing of the pinch and relative X-ray yield. The results show significant variations in stability and pinch dynamics as a function of atomic number of a gas. Detailed analysis will be presented at the meeting.   

Evidence of shock-reflected ions and azimuthal rotation in gas-puff z-pinch implosions on COBRA

Gas-puff z-pinch implosions are characterized by the formation of a dense annular plasma shell that is driven to the axis by a magnetic piston. The thickness of the shell and the velocity, temperature, and density profiles within have been shown to depend on the initial puff density, gas species, and axial magnetic field strength [1, 2]. However, in all cases tested on the 1-MA, 220 ns COBRA generator, Thomson scattering measurements indicate the presence of non-thermal spectral broadening that is consistent with dissipative turbulence driven by unstable plasma waves in a collisionless shock [2]. This observation aligns with the fact that the calculated collisional stopping length of upstream ions exceeds the shell thickness in these cases [2]. Here, we present Thomson scattering measurements of the shock layer for triple nozzle gas-puff implosions with varying levels of imposed axial magnetic field. The results show evidence of shock-reflected ions (i.e., a shock precursor) as well as azimuthal rotation of both precursor ions and the plasma in the imploding shell.  These observations corroborate previous reports of self-induced azimuthal rotation in gas-puff z-pinch implosions [3], offering new insights into the complex dynamics of these systems.

[1] E. S. Lavine, et al., Phys. Plasmas 28, 022703 (2021)

[2] E. S. Lavine, et al., Phys. Plasmas 29, 062702 (2022)

[3] M. Cvejić, et al. Phys. Rev. Lett. 128, 015001 (2022)   

An Improved Analytical Model of the Dynamic Z-Pinch

We present an analytical 1D axisymmetric model describing the implosion of the dynamic Z-pinch in the strong-shock limit. This model is capable of predicting the trajectories of the sheath's inner and outer radii, the shock and piston, along with density, temperature, and velocity profiles within the sheath for any arbitrary current, initial density profile, and axial field. Its implementation consists of simultaneously solving a pair of coupled ordinary differential equations, derived from the ideal MHD equations and Rankine-Hugoniot Jump Conditions, whose forms evolve throughout the different stages of the pinch: initialization, run-in, and reflected-shock, to best reflect the underlying physics. Comparison with experimental data from the COBRA pulsed-power facility after an MSE fit to determine the adiabatic index and starting radius is quite promising, and implies this model could prove useful in designing and analyzing future pulsed-power experiments.   

High-adiabat CH integrated designs for a high energy NIF upgrade

Indirect drive implosions on the NIF have repeatedly achieved fusion gain > 1 with high density carbon (HDC) capsule ablators and laser energies ≤ 2.2 MJ [1]. Plans for the NIF’s Enhanced Yield Capability (EYC) will increase the peak laser energy available from 2.2 MJ to 2.6 MJ, potentially allowing fusion yields in excess of 30 megajoules and enabling new applications in stockpile stewardship. Plastic (CH) ablators are less susceptible to instabilities seeded by capsule microstructure and more efficient in ablation than HDC ablators under typical ICF conditions, and continue to be an area of active investigation on NIF. We present integrated hohlraum simulations in the radiation-hydrodynamic code LASNEX, scaling existing high-adiabat CH designs on the present-day NIF to the 2.6 MJ/450 TW laser option which are predicted to be less susceptible to the tent perturbation which limited previous CH designs. Robustness to low-mode asymmetry and shock de-timing will be considered, as well as possible future laser configurations.

[1] A. L. Kritcher et al. "Design of first experiment to achieve fusion target gain > 1," Proc. SPIE 12939, High-Power Laser Ablation VIII, 1293905 (11 April 2024)

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Studying the fill tube interaction in double shell targets for inertial confinement fusion

One barrier to achieving uniform and efficient compression in inertial confinement fusion capsules is the fill tube, which is the mechanism for delivering liquid fuel to the spherical capsule. The Los Alamos double shell campaign uses two concentric shells with the purpose of maximizing the energy transfer from the laser drive to the outer aluminum shell which then collides with the high-Z inner shell, subsequently compressing the fuel inside. The fill tube disrupts the symmetry of the concentric shells and leads to less compression of the fuel and less energy output. This work attempts to isolate and remediate the detrimental effects of the fill tube, the bore hole (a hole drilled in the capsule to insert the fill tube), and glue. Radiation-hydrodynamics simulations predict a larger bore hole will allow more aluminum jetting into the capsule, and conversely, thicker fill tube walls will close off the bore hole and block aluminum jetting from causing asymmetry and mix. The OMEGA 60 laser was used to compress a planar target and investigate the effects of the bore hole, glue, and fill tube using point-projection and area backlighter radiography. We present design calculations of these experiments and compare to experimental results.   

Measurements of the early time symmetry of delayed two shock pulse shapes relevant to double shell implosions

Recent double shell experiments at the National Ignition Facility have utilized a two shock laser pulse shape. In contrast to the single shock pulse shapes typically used for double shell implosions, two shock laser pulses can potentially offer improved symmetry as well as better mitigation of the joint where the two outer shell hemispheres are joined. In addition to pulse shape tuning, beam repointings and wavelength shifts between the four NIF laser cones will be needed to adequately control backscatter and symmetry during these implosions. Here, we discuss the first set of two shock laser pulse, outer shell only, two-axis keyhole experiments. These experiments are able to measure the first shock breakout timings and speed, as well as the shock merger time, along two axes. Outer cone beam repointings, quad splitting, and wavelength shifts between the four NIF laser cones were used to control backscatter and symmetry. We will discuss the measured shock symmetry and backscatter, as well as comparisons to simulation.   

Integrated Hohlraum Simulations of Double Shell Implosions with the 2-Shock Laser Pulse

We present integrated Hohlraum simulations of Double Shell capsule implosions [1,2] using the 2-shock laser pulse. Double Shell implosions explore volumetric ignition at the National Ignition Facility. The capsules are composed of a low-Z outer shell and a high-Z inner shell which contains the fuel. The outer shell functions as the ablator. After being accelerated inwards by the Hohlraum drive, it collides with the inner shell and transfers its energy and momentum to it. The inner shell then compresses the fuel to ignition conditions. For our simulations, we use the LANL multi-physics code xRAGE [3,4] with its new capability to model capsule implosions in integrated simulations that include the Hohlraum. Here, we focus on the 2-shock laser pulse which is a candidate to reduce the imprint of the ablator joint gap. The latter is a result of the outer shell being assembled from two hemispheres which are connected at the equator. If the corresponding joint is not fully closed, i.e. contains a gap with a large width, it can have detrimental effects on capsule performance. We discuss the outcomes of our xRAGE Hohlraum studies, sensitivities of simulation outcomes to physics and numerical settings, as well as comparisons to the LLNL Hydra code. 

 

[1] D. S. Montgomery et al., Phys. Plasmas 25 (2018)

[2] E. C. Merritt et al., Phys. Plasmas 26 (2019)

[3] M. Gittings et al. Comput. Sci. Disc. 1.1, 015005 (2008)

[4] B. M. Haines et al. Phys. Plasmas 29, 083901 (2022)

    

Evaluation of Non-Thermal Fusion Probability in Dense p¹¹B Plasmas Using Newly-Reported Fusion Cross Section

It has been reported that in p¹¹B fuel, energetic protons recoiled by alpha particles produced from p¹¹B fusion reactions cause non-thermal fusion reactions with ¹¹B in the bulk plasma during their slowing down, which results in chain (or avalanche) reactions [1], being expected to contribute to the realization of p¹¹B inertial fusion. The earlier numerical study [2] showed 5 ~ 10% enhancement in the fusion reaction rate due to the chain reactions at temperature of 150 – 350 keV and density of 10¹⁶ – 10²⁶ cm^-3. The recently-reported p¹¹B fusion cross section [3] is larger than the previously reported one [4], which will enhance the chain reaction probability. So, we have performed transport calculations of alpha particles generated by the thermal p¹¹B fusion reactions and protons recoiled by the alpha-particles in dense p¹¹B plasmas using newly reported fusion cross section in order to evaluate the probability of non-thermal fusion reactions relative to thermal fusion reactions. We perform the calculations by changing plasma temperature and concentrations of proton and boron. In the meeting, we will report the p¹¹B non-thermal fusion probability and clarify its contribution to p¹¹B inertial fusion.

 

[1] S. Eliezer, et al., Phys. Plasmas 23, 050704 (2016).

[2] T. Weaver et al., UCRL-74938 (LLNL, Livermore, CA, 1973).

[3] A. Tentori et al., Nucl. Fusion 63, 086001 (2023).

[4] W. M. Nevins et al., Nucl. Fusion 40, 865 (2000).   

Ignition and burn characteristics of p11B+LiDT+DT fuel

Though p¹¹B fuel is expected to be a neutron-free (or low-neutron-emission) fusion fuel, its ignition is extremely difficult to be achieved than that of DT fuel due to the small fusion cross section and large radiation loss. Use of DT ignitor is one way to ignite p¹¹B fuel though the advantage of low neutron emission in p¹¹B fuel is diminished. When the DT ignitor is use, the fuel should be cooled to cryogenic temperature to solidify DT. To avoid this, the solid target at room temperature (such as LiDT fuel) has been proposed by Blue Laser Fusion Inc. In the present study, we numerically investigated possibilities of ignition and burning of p¹¹B+LiDT+DT fuel on the basis of the burn simulations where  a spherically compressed plasma was set as the initial condition, assuming a maximum compression. The simulations were performed by changing the fuel arrangement of DT, LiDT and p¹¹B layers and compositions of p¹¹B and LiDT layers. It is found that DT neutrons plays important roles in ignition and the following propagation burn. In the meeting, we will discuss the ignition requirement, burn characteristics and emitted neutron properties.   

Aneutronic High-gain Target Design with Advanced Thermonuclear Fuels

We at Blue Laser Fusion, Inc. aim to realize the aneutronic laser fusion demonstration power plant in the 2030s. We are designing and developing all solid targets that can achieve a target gain of 100 with less neutron emission and cheaper cost by the direct-drive fast ignition laser fusion scheme. The key points in our target design are to achieve high fusion gain by ignition and burning our advanced fusion fuels of the multi-layered target of DTLi and pB11 that are initially in solid phase at room temperature with DT reaction as the initial ignition source, to achieve high areal density implosion that enables 14.1 MeV neutrons to be moderated in the target, mainly in the pB11 layer, and to achieve fast ignition that realizes highly efficient and reliable ignition. We will present the baseline of our target design and discuss the research topic.   

PLX-BETHE: Target Formation and Integrated Experiments for Plasma-Jet-Driven Magneto-Inertial Fusion (PJMIF)

Plasma-jet-driven magneto-inertial fusion (PJMIF) is an alternative approach to controlled nuclear fusion that aims to utilize a line-replaceable dense plasma liner as a repetitive spherical compression driver. In this experiment, the first measurements of the formation of a spherical argon plasma liner formed from 36 discrete pulsed plasma jets are obtained on the Plasma Liner Experiment (PLX). Properties including liner uniformity and morphology, plasma density, temperature, and ram pressure are assessed as a function of time throughout the implosion process. The results indicate an apparent transition from initial kinetic inter-jet interpenetration to a collisional regime near stagnation times, in accordance with theoretical expectations. A lack of primary shock structures between adjacent jets during flight implies that arbitrarily smooth liners may be formed by corresponding improvements in jet parameters and control. These measurements facilitate the benchmarking of computational models and understanding the scaling of plasma liners toward fusion-relevant energy density.   

Mining magnetized liner inertial fusion data: trends in stagnation morphology

In the magnetized liner inertial fusion concept, a cylindrical metallic tube filled with fusion fuel is imploded with the goal of producing a one-dimensional plasma column at thermonuclear conditions. However, self-emission x-ray images of the stagnating fuel plasma show rich structure indicative of three-dimensional effects. It is not yet fully understood which experimental input conditions have the greatest contribution to the development of three-dimensional structure that can degrade confinement and performance. We demonstrate the use of a linear regression method on a set of preprocessed experimental data to explore potential correlations between inputs and stagnation structure. Our results indicate that several unexplored effects may play a role in modifying development of structure. For example, we provide the first indications that increasing the initial applied magnetic field may substantially reduce kink-like structure in the stagnated fuel plasma. In conjunction with several counter-intuitive null results, we expect the observed correlations will encourage further experimental, theoretical, and simulation-based studies. We note that the method used in this work is general and may be applied to explore other experimentally measured quantities.   

Improvements to FLASH Transport Models for Simulating Magnetized HED Systems

Magneto-inertial fusion is a promising path forward in the pursuit of commercially viable fusion power devices. Simulating such experiments typically involves the use of radiation hydrodynamic codes like FLASH, which is maintained by the Flash Center at the University of Rochester and being co-developed by Pacific Fusion as a target design tool. The charged particle transport in these models is characterized through transport coefficients, such as diffusivity, viscosity, and thermal and electrical conductivity. Accurately modeling transport coefficients throughout the wide range of parameters sampled by HED simulations can be challenging. Recent work by Stanton and Murillo [1,2] employs an effective Boltzmann approach within Chapman-Enskog theory to obtain analytic transport coefficients for weakly- to moderately-coupled plasmas. In this project, we explore the implementation of the Stanton-Murillo transport models into FLASH and show the impact for simulations of magnetically driven imploding liners for pulsed magnetic fusion energy applications.

[1] L. G. Stanton and M. S. Murillo, Phys. Rev. E, 93(4) 043203, (2016)

[2] L. G. Stanton and M. S. Murillo, Phys. Plasmas, 28(8), (2021)   

Studies of preheat-induced mix in MagLIF targets

Magnetized Liner Inertial Fusion (MagLIF) is an inertial confinement fusion concept that preheats a magnetized fuel prior to compression and has the potential to reach high thermonuclear fusion yields. During the laser preheating stage, the higher-density liner material can blow off the liner wall and mix into the lower-density D2 fuel via x-ray ablation or impact from the late-time blast wave. Adding a magnetic field allows the laser energy to be deposited deeper within the plasma causing the expanded coating to become uniformed. We plan to show preliminary data analysis from a scaled MagLIF experiment executed on Omega to characterize and diagnose the mixing of material from the inner surface of the target preheating stage. We will show density profiles of the liner material and an assessment of possible mixing with the D2 fuel as the laser-generated x-rays and blast wave interact with the liner.   

Dynamics of single-mode rippled shock in laser-irradiated targets using streaked optical pyrometry

The behavior of non-uniform shocks and interfaces, generated by modulated laser irradiation or surface perturbations, is crucial for understanding systems ranging from inertial confinement fusion to laboratory astrophysics and material properties at high-energy-density conditions. This study investigates the evolution of a single-mode rippled shock in a polystyrene-fused silica sample using streaked optical pyrometry (SOP). Experiments utilize OMEGA-EP laser beams to drive a multimegabar (>3 Mbar) shock into the sample with a preimposed single-mode interface perturbation. Pyrometry measurements of time-resolved thermal emission from the shock front reveal the transfer of single-mode perturbation to the incident shock and its subsequent damped oscillation as the rippled shock traverses fused silica. We describe an analysis framework for interpreting rippled-shock pyrometry data, correlating spectral radiance measurements with rippled-shock velocities. By integrating these velocities, we reconstruct the shock front amplitude evolution, demonstrating an oscillatory decay. We also compare the experimental data with synthetic pyrometry images generated from FLASH hydrodynamic simulations.   

Utilizing the OMEGA High-Resolution Velocimeter (OHRV) to Quantify Shock-Front Non-Uniformities in Wetted Foams

Foams printed using 2-Photon Polymerization (2PP) and wetted with liquid DT provide a promising new target platform for understanding target degradation mechanisms in inertial confinement fusion (ICF) implosions. These wetted foam targets are less expensive and easier to produce than solid ice layered targets, enabling more rapid fielding of dense-fuel implosions. Radiation hydrodynamic simulations of polar direct drive liquid DT-wetted foams have shown increased laser-target coupling, increased fusion yield, and reliable ignition [1]. Experimental characterization of DT-wetted foams to validate these simulations and better understand the target platform is vital. Using OMEGA’s Capseed Campaign platform, planar shock fronts will be generated within wetted foams, and their nonuniformities will be diagnosed with the OHRV. By varying fill structures and densities, the shock-front nonuniformities and hydrodynamic instabilities seeded by the microstructure of a wetted foam can be characterized. This will provide insight into whether wetted foams may prove to be a tunable platform for investigating hydrodynamic instabilities in ICF implosions.

[1] R. Olson et al., Wetted Foam ICF Target Concept: IFE Workshop white paper (2022).   

New regimes of frontier science on the National Ignition Facility laser

A selection of results from the NIF Discovery Science program will be presented. Examples include nuclear reactions relevant to stellar nucleosynthesis. Equations of state at very high pressures relevant to planetary cores, brown dwarf interiors, and white dwarf envelopes are being measured, and show that ionization can significantly affect the compressibility of matter. Relativistically hot plasmas and target-normal sheath acceleration (TNSA) of protons are being studied on the NIF ARC laser. Experiments to study magnetic reconnection at high energy densities are underway. High velocity, low density interpenetrating plasmas that generate collisionless astrophysical shocks, magnetic fields, bursts of neutrons, and that accelerate particles relevant to cosmic ray generation are being studied. NIF experiments have demonstrated strong suppression of heat conduction in a replica of galaxy-cluster turbulent plasmas. Experiments on Rayleigh-Taylor instabilities at high Reynolds number, relevant to supernova explosions, are being carried out. And hydrodynamic instability experiments have been done in classical regimes (non-stabilized), ablatively stabilized, radiative shock stabilized, and strength stabilized configurations. Examples from these different regimes will be shown.   

Effect of pulse duration and coupling efficiency on laser generated shocks in multi-material targets

The use of lasers as an alternative x-ray source to ablate and drive shocks in a target is an attractive possibility due to their wider availability. A practical application is the ablation of multi-material satellite components such as solar panels. It is therefore crucial to compare x-ray-driven and laser-driven shocks results. The first step for such a goal has been to validate the scaling of ablation pressure in a range of laser-pulse durations at constant laser intensity. However, the comparison between experiments and simulations exhibits discrepancies in the shock characteristics in the short (~100 ps) and long (~10 ns) pulse durations of the scaling.

In this work, we present a comparison between experimental and simulation shock conditions through the adjustment of laser conversion efficiencies. Quantities such as shock breakout time and shock velocity are matched and compared for the pulse durations of our scaling, investigating the modified plasma characteristics. Moreover, we address discrepancies between experimental and simulation density profiles encountered in late plasma expansion. Finally, we provide a physical explanation for the modified laser conversion efficiency, addressing skin effect and collisionless absorption for short pulses, and the development of laser plasma instabilities for long ones.   

Simulating bubble and microjet formation in high velocity plasma jets

Studying the evolution of plasma flows and blast wave dynamics is important for understanding astrophysical phenomena, such as accretion processes, supernova remnant dynamics, and radiative outflows from stellar jets. Laser plasma ablation experiments are a promising method for creating analogous plasma jets and supersonic outflows. Recent experiments at OMEGA using a newly developed radiography system reveal the formation of a single-mode blast-driven instability with bubbles, small-scale spike morphology, and turbulent mixing in the presence of a low density foam. We will present simulations of target configurations in HYDRA and PERSEUS to reproduce these structures and study the blase wave transit in the target cavity. These simulations will be used as a benchmark for further target design for laser-driven ablation experiments. This material is based upon work supported by the US DOE OFES under Award No. DE-SC0017951 and US DOE NNSA University of Rochester "National Inertial Confinement Program" under Award No. DE-NA0004144.   

Implementation of Short-Time-Scale Laser Deposition Physics in Radiation-Hydrodynamics code HELIOS-CR

HELIOS-CR is a 1-D radiation-magnetohydrodynamics code that is used to simulate the dynamic evolution of plasmas created in high energy density physics (HEDP) experiments. The energy sources include lasers, radiation sources, electric currents (in cylindrical geometry), and particle beams. The laser deposition model in the code utilizes ray tracing algorithms for different 1D geometries. The refraction is computed using a geometrical optics model with a plasma refractive index being governed by local values of electron density. Following the conventional approach, the laser energy is deposited in the plasma using an inverse Bremsstrahlung model when the electron density is less that the critical density. Although the deposition models have been extensively tested for simulations that include longer nanosecond-scale laser pulses, they may not be adequate for the ultra-short pulses. We will discuss development of a new model for the laser energy deposition suitable for sub-picosecond laser pulses. In this approach, Maxwell's equations will be solved explicitly to obtain the radiation field and will account for laser-matter interaction in the presence of steep density gradients. We will also discuss the need to re-evaluate the models for the thermal fluxes and electron–ion relaxation in the plasmas with extreme temperature gradients,   

Thermodynamic Consistency and Monotonicity of Equations of State in Simulated Pulsed- Power-Driven Explosions of Cylindrical Conductors.

Megaampere pulsed-power-driven explosions of conductors are an important area of study for char- acterizing hydrodynamic instabilities relevant to magneto-inertial fusion concepts such as magnetized- liner inertial fusion (MagLIF). Experiments such as the Mykonos electrothermal instability II (METI-II) experiment provide insight on the physics of conductive materials brought from solid conditions to high energy density plasma (HEDP) regimes in less than 0.1 μs. Understanding the rapid phase changes in the conductor is key to understanding the electrothermal instability (ETI). In simulated recreations of these experiments, the proper choice of equations of state (EOS), con- stitutive models, and subsequent careful usage are of critical importance for making consistent comparisons to measurements. This work considers resistive magnetohydrodynamic (MHD) simu- lation results from the Lawrence Livermore National Laboratory (LLNL) code Ares in the analysis of different approaches to monotonic reconstruction of SESAME tabular EOS for aluminum that exhibit van der Waals loops.   

In-Flight Measurement of Ten-Fold Compression of Magnetic Field

In the last decade, simulation and theory have increased demand for the ability to magnetize experiments to the kiloTesla level. Physical limitations on energy storage mean that fields of this magnitude over a usable volume can only be achieved by compression. Field compression has been used to generate inferred kiloTesla fields, but such fields are usually inaccessible due to the high density, hot plasma surrounding them. In this talk we will present a field compression platform on the EP laser system at the Laboratory for Laser Energetics that could be used to apply a compressed magnetic field to a clear, usable experimental volume. An initial test of this platform was diagnosed with proton radiography and an in-flight field compression of roughly 10 times the initial field was measured. These experiments indicate that the platform can be expanded to both increase the field compression and usable volume for longer periods than the initial experiment.   

An overview of recent results from the PUFFIN group at MIT

We present recent work by the PUFFIN group at MIT, including:

Oblique shock experiments on COBRA, which using an exploding wire array to drive magnetized plasma flows to interact with an angled, insulating wedge. The shock opening angles are measured using shadowgraphy, enabling the plasma adiabatic index to be measured.

Radiatively cooled reconnection experiments on Z, where we observe the formation and collapse of a reconnection layer using a suite of X-ray diagnostics which reveal the existence of bright hotpots within the layer which correspond to plasmoids in our 3D MHD simulations. We also observe the structure and evolution of the reconnection layer using laser shadowgraphy.

Guide field reconnection on MAIZE, using tilted wire arrays which embed an out-of-plane magnetic field in the reconnection layer. We observe a rotation of the outflows from the reconnection layer on global scales (>10 d_i) indicative of the interplay between the Hall field created by two-fluid effects and the guide field.

The status of the new PUFFIN facility at MIT, a new 1 MA peak current, 1.5 us rise time pulsed-power generator, which is designed to drive plasmas in a quasi-steady-state suitable for studying the growth of instabilities and the development of magnetised turbulence.   

Experimental and synthetic shadowgraphy to diagnose a radiatively-cooled magnetic reconnection layer on Sandia's Z Machine

The MARZ (Magnetic Reconnection on Z) campaign fields dual exploding aluminum wire arrays on Sandia’s Z Machine. On this platform, we diagnose magnetic reconnection in the presence of both strong radiative cooling and the plasmoid instability. Achieving this combination of plasma processes -- a unique capability among laboratory investigations -- makes MARZ especially relevant to extreme astrophysical environments.

Our collaboration simulates the entire MARZ load hardware in GORGON, a 3D resistive MHD code. We construct synthetic shadowgrams by ray-tracing probe beam photons through 3D electron density meshes; ray-transfer matrix analysis includes finite aperture effects. These synthetic images predict: electron density peaking in the layer center, MHD shocks outside the layer, instabilities apparent as axial structures, and the layer growing denser in time.

On the MARZ4 shot in April 2024, we employed LIONZ (Laser Imaging On Z) for shadowgraphic imaging. We analyze the four LIONZ shadowgrams, each frame with 3-ns exposure, spanning a 36-ns window around peak x-ray emission from the layer. Features in these experimental images are qualitatively consistent with simulations. However, some differences are evident, which will help better constrain initial conditions in GORGON.   

Developing a Pulsed-Power-Driven Guide-Field Magnetic Reconnection Platform on MAIZE

We present new analyses and models of our pulsed-power-driven guide-field magnetic reconnection platforms on MAIZE. For both platforms, we use a dual exploding wire array, where plasma ablated from the arrays advects magnetic fields (~2 T) and collides between the arrays where a reconnection layer forms. In our first design, we attempted to prescribe an externally applied magnetic field (0-2 T) using a Helmholtz coil, which we found did not add a guide field to the plasma flows, as the external field was frozen out. For our second design (without a Helmholtz coil), we tilt the two arrays in opposite directions, re-orienting their azimuthal magnetic fields into orthogonal anti-parallel and guide-field components, embedded in the flows. We diagnose the plasma with B-dots, and two simultaneous Mach-Zehnder laser interferometers along orthogonal lines of sight. We observe a layer which rotates by an angle that increases with the guide field strength. An analytical model of the layer shape shows the tilted hardware geometry cannot explain this, and 3D GORGON simulations show no rotation with MHD alone. We propose the observed layer rotation is due to an interaction between the Hall field and the guide field.   

Development of experiments for shock waves in magnetic fields at the 1 MA pulsed power generator

The 1 MA Zebra pulsed power machine at UNR generates magnetic fields of 1-3 MG that can be used for many applications. We are developing experiments to study shock waves in gaseous and plasma media in strong magnetic fields. Shock waves are generated by a laser beam focused into a gas jet in the field region of 2-3 MG. Diagnostics include interferometry, shadowgraphy, and schlieren at wavelengths of 1064 nm, 532 nm, and 266 nm. Magnetic fields in the shocked plasma are measured using Faraday rotation diagnostics. The first results from laser diagnostics are presented, including visualization of shock waves, as well as the dynamics and structure of shock waves in the gas jet.     

Measurments of the Magnetic Field Distribution in Gas-Puff Z-Pinch Implosions Using a Faraday Rotation Diagnostic

Gas-puff Z-pinch implosions are magnetically driven implosions of an annular plasma sheath which is compressed onto the Z-axis. Understanding the current distribution which generates the driving magnetic field within the imploding plasma sheath is key to understanding the implosion dynamics. A diagnostic which is non-perturbative and offers good spatial and temporal resolution is key to determining the exact nature of this distribution. This poster presents new results in measuring the magnetic field distribution in gas puffs, from which the current distribution can be calculated. The gas-puff Z-pinches under study are generated on the 1-MA COBRA generator at Cornell University using a triple gas puff nozzle, where outer and inner annular plasma sheaths collapse onto a central target jet, compressing it. These plasmas are generated with a current rise time of approximately 100ns using argon gas. The Faraday Rotation diagnostic is combined with interferometry measurements to measure the plasma density and supplemented with gated visible-UV light self-emission images, XUV (extreme ultraviolet) quadrant camera images, and PCD (photo-conducting diodes) signals to diagnose the implosion dynamics and timings.   

Implosion Dynamics and Instability Growth of Double and Triple Gas-Puff Z-Pinches with and without Axial Magnetic Fields on COBRA

Gas puff Z-pinches are powerful sources of intense X-rays that generate high energy densities and temperatures with potential applications in thermonuclear fusion. However, Magneto Rayleigh Taylor Instabilities (MRTI) present a significant challenge in achieving a stable pinch. MRTI can be mitigated through tailored density profiles and an external pre-embedded axial magnetic field. Previous work using the multiphysics radiation-MHD code HYDRA has shown that higher-atomic-number inner and outer liners generally improved fuel compression and neutron yield in triple gas puffs, except when changing the inner liner from Argon to Krypton [1]. In this experiment, we utilized a triple nozzle gas injector on the COBRA (~0.9MA, 220ns on long pulse mode) driver to study the implosion dynamics and MRTI growth of Neon, Argon, and Krypton inner and outer liners with a Xenon doped H2 target with and without a 0.3 Tesla magnetic field. The plasma dynamics were characterized using an XUV framing camera, filtered soft X-ray photodiodes, a Mach-Zehnder interferometer, a laser shearing interferometer, and an X-ray spectrometer. Experimental results and comparison with MHD simulations will be presented at the meeting.

Sources: [1]  J. Narkis, F. Conti, and F. N. Beg., Phys. Rev. E 105, 045205 (2022).   

Study of Multi-Shell Gas Puff Z-Pinches with and without Axial Magnetic Field

Gas Puff Z-Pinches are a well-studied source of high energy density plasmas and a promising candidate for nuclear fusion [1]. The growth of the Magneto Rayleigh-Taylor Instability (MRTI), however, limits its prospects as a reliable fusion concept. The application of a pre-embedded axial magnetic field can mitigate this growth, but simulations predict it also causes a decrease in the plasma temperature achieved at stagnation [2]. To combat this effect, multiple annular gas puff shells can be used with lower axial magnetic field strengths to mitigate MRTI growth. Experiments conducted on the 1MA, 200ns rise time COBRA generator at Cornell University investigated the combined effects of these mitigation techniques. Time gated XUV images captured the growth of the MRTI during the implosion, and X-ray spectroscopy provided data on the peak plasma temperatures. Simulations from the collisional-radiative spectral analysis code PrismSpect were compared to experimental spectra to determine the plasma parameters. Results of this analysis will be presented.

[1] J. L. Giuliani and R. J. Commisso, IEEE Transactions on Plasma Science 43, 2385 (2015).

[2] J. Narkis et al., Phys. Rev. E 104, L023201 (2021).   

Kinetic study of dynamic Z-pinches with an energy-conserving particle code

Simulations of dynamic Z-pinches in a deuterium gas are conducted using the energy-conserving implicit particle-in-cell (PIC) code PICNIC. One-dimensional simulations are used to examine how the pinch formation process depends on the initial plasma density for a fixed initial radius and pinch current. The density is varied to explore from weakly to strongly collisional regimes and to study the so-called shock-flash yield produced at stagnation. Two-dimensional RZ simulations are performed to investigate the significance of 1) anomalous resistivity in the low-density periphery of the pinch and 2) neutron yield associated with beam-target fusion that occurs during the nonlinear stage of unstable m=0 sausage modes. In addition to the physics studies, a performance analysis of the implicit PIC code when applied to the dynamic Z-pinch is presented.   

Simulations of self-magnetization in expanding high-energy-density plasmas

Plasma magnetization is one of the fundamental challenges in both laboratory and astrophysical plasmas. Most high energy density (HED) laser experiments on magnetic reconnection, magnetized and unmagnetized collisionless shocks rely on either Biermann or Weibel mechanism to generate the magnetic fields of interest. Multiple HED experiments have observed the formation of ion-scale magnetic filaments of megagauss strength, though their origin remains debated. Theories based on Particle-in-Cell (PIC) simulations have been proposed to explain magnetization, including plasma interpenetration-driven Weibel [1], temperature gradient-driven Weibel [2], and adiabatic expansion-driven Weibel [3]. Here we consider laser intensity of 10¹³-10¹⁵ W/cm² relevant to HEDP and ICF experiments, where collisions must be considered. We develop a first principles model with collisional 2D PIC simulations including interaction with a laser ray tracing and laser heating module [4] to simulate plasma ablation, expansion, and subsequent magnetization in planar geometry, effectively suppressing the Biermann battery. Results show that the plasma rapidly self-magnetizes, generating plasma beta of 100 (β=2μ₀n_eT_e/B²)  with the Hall parameter ω_ceτ_ei>1 within 1 ns, and the plasma dynamics is largely consistent with the expansion-driven Weibel hypothesis. Implications of plasma magnetization for heat flux suppression are also discussed.   

Design, Construction, and Characterization of a 1 kJ Solid-State Switched Pulsed Power System Discharged Through a Coaxial Plasma Gun

With an increased interest in high speed and reliable pulse power switching over the recent years and with the proliferation of semi-conductor manufacturing technologies, the Thyristor or Silicon Controlled Rectifier (SCR) has seen renewed interest for pulse switching. Often however, pulse power system design requires knowing load parameters for correct switch sizing and gate driver design to prevent unwanted thyristor breakdown. This is made difficult when the load is a variable plasma breakdown whose properties itself are dependent on the switch turn-on properties. This poster covers research done on a Thyristor switched pulse power system as operated with a basic coaxial plasma gun (CPG.) The primary components analyzed in this research are: 1.) RLC behavior of the CPG. 2.) Turn-on mechanics of the thyristor switch. 3.) Effect of varying gate driver parameters on switch operation. 4.) Timing of fiber optic signal link to activate the gate driver. 5.) Interrelation between thyristor turn-on profile and CPG operation. This modular pulse power system is rated for 6.5 kV and >1 kJ. The thyristor used has a jitter of under 500 ns and can handle surge currents of over 95 kA. The turn-on time of the CPG discharge is approximately 4 µs with a full 5τ pulse length of ~100 µs. Switch losses are less than 1%. By understanding the relationship between the thyristor and the variable plasma load, better switch designs can be incorporated to increase timing control without sacrificing CPG performance or system safety.   

Opposition Research: A novel platform on the Z Machine to study radiation flow in complex geometries

The dynamics of radiation-driven Marshak waves flowing over opaque obstructions is of interest in astrophysics, for instance to understand how radiation waves from supernova interact with the surrounding, inhomogeneous stellar media. Further, HED experiments studying radiation flow around obstructions can provide valuable data to benchmark numerical models of radiation transport in complex geometries. We present a novel platform on the Z Machine, Opposition Research, to study radiation flow in complex geometries. X-rays from a hohlraum drive radiation waves in two opposing foam packages; one to study complex radiation flow and the other to serve as a fiducial, providing an integrated drive measurement that can be used to calibrate rad-hydro simulations. Radiography is used to measure the position of the blast wave produced in each package as the radiation wave becomes subsonic. We present results showing the left-right drive symmetry of the hohlraum, demonstrating platform viability for experiments with complex radiation flow geometries. We also demonstrate calibration of simulations with a combination of measured blast wave positions and emission-based measurements of the hohlraum x-ray drive.   

Laboratory study of magnetized collisionless shock formation in an oblique geometry and the effect of the ion-electron collisions

Collisionless shocks are ubiquitous in astrophysics and a possible source of the highest-energy cosmic rays (CRs) in our universe. Significant work has been done recently, that highlights the dependence of collisionless shock formation mechanisms on the amplitude of the ambient B-field and its orientation relative to the flow. Recent laboratory measurements of quasi-perpendicular shocks, a configuration with the B-field perpendicular to the flow, revealed the underlying acceleration mechanisms and the interaction between reflected and inflowing ions. However, other magnetic geometries, such as quasi-parallel and oblique, are less well understood.

We will present a new platform for the Janus Laser Facility to investigate the formation of oblique magnetized collisionless shocks. The new platform design relies on the interpenetration of a magnetized high-density (n_i~10¹⁹-10²⁰cm^-3) background plasma with a fast, low-density (n_i~10¹⁷- 10¹⁸ cm^-3) plasma. The dynamics of the interactions were observed using interferometry and streaked optical pyrometry diagnostics. The measurements suggest that a magnetized shock is formed early in the interaction of the two flows, within ~1.5 – 3 ns, driven by a magnetic piston. These measurements are compared with FLASH calculations to reveal the underlying mechanisms of the shock formation.    

Kinetic simulations of magnetized collisionless shock experiments on the Z Machine

Magnetized collisionless shocks are formed when a super-magnetosonic flow encounters a magnetic obstacle, and the resulting shockwave forms on length scales much shorter than the particle mean free path. Recent laboratory experiments have demonstrated the ability to drive magnetized collisionless shocks [1-2], but the exact mechanisms of particle heating and non-stationary shock behavior remain open questions due to the limited domain sizes of previous experiments. A new experimental platform, MagShockZ, has been developed to study high-Mach-number magnetized collisonless shocks over large domains on the Z Machine at Sandia National Laboratories by combining a pulsed-power-driven exploding wire array and a laser-driven magnetic piston. We present results from quasi-1D and 2D simulations that modeled the experiments using the particle-in-cell (PIC) code OSIRIS and radiative MHD code FLASH. The initial conditions for the PIC simulations were extracted from FLASH simulations of the laser-target interaction. The resulting shock formation and propagation were then modeled with OSIRIS. We use synthetic diagnostics to compare the OSIRIS results directly to experimental results.   

Laboratory Investigation of Coupling Between Explosive Piston Plasma and Partially Ionized, Magnetized Plasma

In many astrophysical systems, such as the solar atmosphere, interstellar medium, planetary atmospheres, and accretion disks, partially ionized plasmas (PIP) play a crucial role in their dynamics. The presence of neutral particles significantly impacts the growth of instabilities, energy transport, and coupling processes between plasma species. This study explores the interaction between a high-energy piston plasma created via laser irradiation of a solid target and a partially ionized, magnetized plasma, focusing primarily on planetary atmospheres. A novel gas puffing system was implemented on the Large Plasma Device at UCLA to locally increase the neutral fraction near the target surface, enabling the examination of its effects on the laminar electric fields that predominantly influence the interaction. The study investigates the effects of decreased ionization fractions in helium and hydrogen plasmas to understand the formation and evolution of diamagnetic cavities in a PIP.   

Measurements of Plasma Jet Velocity in Conical Wire Arrays Z-pinches Using Collective Thomson Scattering

To characterize the axial propagation velocity of a plasma jet emitted by a conical wire array and its dependence on the aperture angle of the conical array, Thomson scattering measurements were conducted. Plasma jets emitted by conical wire array Z-pinches have been extensively studied as a controlled laboratory setting to study plasma outflows from newborn stars in the form of Herbig-Haro objects in astrophysics. One of the key parameters to evaluate plasma collisionality is the propagation velocity of it, since mean free path has a strong dependence on this quantity (~v^4).

The plasma jet is produced by a conical wire array Z-pinch consisting of 16 wires (50 μm diameter each) acting as the load for the Llampudken pulsed power generator (~400 kA, ~350 ns). To study the variation in the propagation velocity of the plasma jet as a function of base angle of the cone, specifically 20°, 30°, and 40° inclination angles from vertical.

The primary diagnostic tool used was collective Thomson scattering provided by a 1J, 5ns FWHM Nd:YAG laser, connected to a Czerny-Turner spectrometer coupled with a 4QuikE ICCD camera. Spatial resolution is achieved using a linear optical fibre bundle, resolving the axial direction of the jet. Our experimental results show that for all different inclination angles, the propagation velocity variates with the axial position, increasing as it approaches to the upper part of the jet. This result agrees with several numerical simulations, providing an accurate experimental benchmark. Our results also show that jet velocity increases with the base angle of the conical configuration. Further details, potential applications, and results from complementary diagnostics, such as optical spectrometry and Moiré Schlieren deflectometry, will be presented and discussed.   

Designs of experiments for studies of strongly collimated magnetized laser-driven plasma

The solar corona is the outermost atmosphere of the Sun that is comprised of strongly magnetized plasma. The design for upcoming laboratory astrophysics experiments at the Omega laser facility of the University of Rochester to study the collimation of magnetized plasma jets due to a strong background magnetic field will be shown. In our experiments, a thin plastic foil is heated with laser beams producing a plasma plume from the rear surface of the foil. A 400 kGauss magnetic field is applied co-axially to a plume. Simulations of the experiments using the FLASH code show collimation of the plasma due to the applied magnetic field. In preparation for the experiments, synthetic diagnostic data have been produced based on the simulated profiles. Synthetic images of the plume backlighted by a Gd foil show a distinct difference in the morphology of the plasma with and without a magnetic field. The synthetic proton radiography images show an accumulation of magnetic field at the edge of the plasma jets. The temperature is estimated using synthetic x-ray spectroscopy.   

Investigating radiative cooling and magnetic fields with x-ray driven plasma jets on the MAGPIE pulsed-power generator

We present experiments investigating colliding plasma flows and jets driven by the x-rays emitted from wire array z-pinch implosions. Ablating wedge targets produces collimated, supersonic (M~2-3) plasma jets. By varying target material, we have measured the effects of radiative cooling on these jets using diagnostics such as interferometry and Thomson scattering. The x-ray drive platform also allows us to explore the effects of magnetic field, for example its role in suppressing structures formed within the plasma.   

Simulations of collimated laser-plasma jets in the ambient low beta condition and astrophysical relevance

The collimation mechanism of solar coronal jets is investigated using FLASH simulations in preparation for upcoming laser-driven magnetized plasma jet experiments at the OMEGA facility. Plasma outflows in the direction of the applied 50T magnetic fields are modeled analogous to outflows from the solar corona holes expunged along open magnetic field lines. Plasma parameters such as Euler number, Alfvén velocity and beta are kept as the scale invariant between laboratory experiments and solar corona. Simulations indicate that collimated plasma jets are able to be produced by applying external magnetic fields in our designed experimental conditions, where the collimation effect can be enhanced by increasing external magnetic field strengths. The mechanism is magnetic fields inside outflows are pushed out by the advective transport to form a high magnetic pressure and low beta region, consequently outflows are compressed by the magnetic pressure difference causing jet collimation.   

2-D Kinetic Simulations of Biermann-battery Magnetic Field Generation and Current Sheet Formation in High-Energy-Density Plasmas

Magnetic reconnection is a ubiquitous process in astrophysical plasmas, and drives phenomena like galactic jets and solar flares. The Biermann battery effect generates magnetic fields due to non-collinearities in density and temperature gradients of a plasma. This effect is thought to be responsible for creating primordial magnetic fields and altering heat and energy transport, as well as reconnection rates. In a laser-produced plasma, strong density and temperature gradients generate large (~MG) Biermann fields that can collide with other laser-produced plasmas to undergo magnetic reconnection. Recent experiments at the NIF and OMEGA laser facilities provide a platform for such laboratory astrophysics studies.

Here, using the fully kinetic particle-in-cell (PIC) code PSC, we perform a 2-D simulation of magnetic reconnection with a first-principles laser-ray tracing module. We model an experiment at the NIF of two laser-generated plasma plumes in a magnetic reconnection geometry colliding and study the evolution of Biermann-battery magnetic fields and current sheet formation. Further analysis of the current sheet, where magnetic energy converts to bulk particle motion and thermal energy, are done to determine size, temperature, reconnection rate, and kinetic effects.   

Investigation of the ion Weibel instability: hybrid kinetic simulations in support of laser-driven experiments at OMEGA

In astrophysical plasmas, high magnetic fields are commonplace but their origins and amplification processes responsible for generating them are not entirely understood. One candidate for the amplification of magnetic fields is the ion Weibel instability. Counter streaming plasma flows in the presence of seed fields form current filaments causing the initial magnetic field to grow. The nonlinear dynamics of current filaments are not understood, especially at late times, and theoretical predictions differ depending on what physics is included. Previous experiments have demonstrated the development of Weibel-generated current filaments from interpenetrating ion flows but have not recorded data at late enough times to differentiate between models. New experiments have been performed at OMEGA to observe the nonlinear dynamics of current filaments at later times (t >10ns). Interpretation of this data depends on our understanding of the experimental plasma conditions including the ion-ion collisionality, and resistivity. In this work, initial hybrid kinetic simulations of the latest experiments are presented to complement the single-position Thomson scattering measurements of plasma conditions, and to model the linear and subsequent nonlinear growth of the Weibel instability.   

High-energy-density Targets Fabricated by Michigan Target Research and Fabrication (MiTRF)

Michigan Target Research and Fabrication (MiTRF), located at the University of Michigan, has the distinct capability of fabricating targets for a variety of high-energy-density (HED) physics experiments. We have been assembling targets for the Omega Laser Facility, the Jupiter Laser Facility, the Z machine, and several other facilities for over a decade. We have a thoughtful and comprehensive approach to the target fabrication process that serves the broader HED science community, including researchers from National Laboratories, Universities, and Private Industry. We provide comprehensive modeling support, fabrication, characterization and metrology of individual components and finished targets, hand delivery to the facility, as well as experimental support. We work closely with several partners, including Dana Design machine shop, which provides high-precision machined components, and other machined parts to support experimental needs. MiTRF has recently become a LaserNetUS facility node to support LaserNetUS experiments. This allows us the opportunity to significantly contribute to the broader HED community by providing targets for many experimental configurations fielded at a variety of experimental facilities. We have developed a network within the target building community to provide high-quality targets that meet experimental specifications, relying on each entities' specialties to maximize efficiency and target quality.   

Basic Science User Programs at the Omega Laser Facility

The Omega Laser Facility at the University of Rochester’s Laboratory for Laser Energetics includes the 60-beam 30 kJ OMEGA Laser System and the four-beam high-energy, high-intensity OMEGA EP Laser System. The OMEGA EP short-pulse beam (up to two) or the tunable-wavelength long-pulse beam can also be transported to the OMEGA chamber for joint operations. The two lasers share over 100 facility-supported diagnostics and perform over 2000 highly diagnosed experiments annually. Three programs provide general user access for basic research, with nearly one-third of the total facility time granted through peer-reviewed proposal processes (the National Laser Users’ Facility and Laboratory Basic Science funded by the National Nuclear Security Administration, and LaserNetUS funded by the Department of Energy’s Office of Fusion Energy Sciences). The user programs currently support the education and training of 80 graduate students from over 20 universities. This presentation will provide an overview of the basic science user programs and highlight user research in broad high-energy-density (HED) scientific areas including (but not limited to) laboratory astrophysics, high-pressure material properties and phase-transition dynamics relevant to planets and exoplanets, magnetized HED plasmas, equations of state, warm dense matter, relativistic laser–plasma interaction and intense beam physics, nuclear physics, and inertial fusion energy.   

Verification and Validation Testing of the FLEXO Extended-MHD Code

FLEXO (Flux-Limited Extended-MHD Ohm’s Law) is a production-line extended- magnetohydrodynamics (MHD) code being developed at Sandia National Laboratories for modeling high-energy-density (HED) plasma in a pulsed-power device, primarily with applications to the study of inertial confinement fusion (ICF).  We present recent results from verification and validation testing of various modeling capabilities in FLEXO, including hydrodynamics, MHD, extended-MHD, multi-material modeling, and adaptive mesh refinement.  These include the acceleration of a flyer plate, for which flyer velocities are compared against experimentally-measured values, several tests of Hall physics, and a number of tests for which a multi-material result with two identical materials is compared against a single-material result.

SAND2024-07896A   

Probing the deviation from the Braginskii heat transport model due to kinetic effects in HED-relevant conditions

Non-local heat transport caused by kinetic effects is expected to be prevalent in many high energy density (HED) plasma systems. While the impact of kinetic ions has been studied, less is known about the impact of kinetic electrons. Hydrodynamic and hybrid kinetic codes which are often used to simulate HED systems typically assume fluid electrons. While some non-local fluid heat flux models exist [1-3], a local Braginskii/Spitzer-Härm-type heat flux model is most commonly used in such codes. This model is only strictly valid in regimes where the electron Knudsen number, defined as the electron mean free path over the temperature gradient length scale, is small. In this study the fully kinetic code iFP [4,5] is used to quantify the deviation from the Braginskii model across a range of electron Knudsen numbers, including regimes with highly kinetic electrons. Systems are initialized in locally Maxwellian states with temperature gradients to allow for heat transport. The resulting heat flux is compared with the Braginskii model.

 

This work was performed under the auspices of the National Nuclear Security Administration of the U.S. Department of Energy at Los Alamos National Laboratory, managed by Triad National Security, LLC under contract 89233218CNA000001.

 

[1] Luciani et al., Phys. Rev. Lett., 51 (1983) 1664.

[2] Bendib et al., Phys. Fluids, 31 (1988) 711.

[3] Schurtz et al., Phys. Plasmas, 7 (2000) 4238.

[4] Taitano et al., Comp. Phys. Comm., 263 (2021) 107861.

[5] Anderson et al., J. Comp. Phys., 419 (2020) 109686.   

Computational Modeling of Pulsed Power Experiments

A variety of pulsed power applications and experiments have been modeled with a magnetohydrodynamics code. The smallest scale devices are exploding wires and foils (I_peak ~ 1 kA, t_rise ~ 100 ns). Here, Joule heating dominates and there is strong coupling to the circuit. In density/temperature phase space, the material uniformly follows liquid/vapor coexistence line. The integral action is shown to compare with historic experiments. When, the material expansion is included, a well-defined asymptotic value is observed at burst. Opening switches are typically used in pulse shaping to decrease the risetime. They operate similarly to wires and foils, but typically at larger spatial and current scales (I_peak > 1 MA, t_rise > 2 ms).  Small, fast generators such as the Mykonos driver at Sandia (I_peak ~ 1 MA, t_rise ~ 100 ns) provide a testbed for fundamental physics. One series of experiments investigated a machined sine wave surface on a current carrying rod with and without a dielectric coating. In simulation, trough overheating is quantitively compared to theory and qualitatively compared to experiments. Variations in the initial peak-to-peak amplitude and wavelength are consistent with experiments. The Sandia Z-machine is currently the largest pulsed power driver (I_peak ~ 25 MAmps, t_rise ~ 100 ns). The modeling of fundamental science experiments investigating the magnetic Rayleigh-Taylor instability has shown good agreement with data.   

Active optimization of sheath field accelerated proton beams from intense, high-repetition-rate laser-solid interactions

Active feedback control of high-repetition-rate (~Hz), high-intensity laser systems for optimization of high energy density (HED) science applications is a rapidly-evolving area of research, drawing on advancements in machine learning and high performance computing to accelerate the pace of discovery. A recent experiment at the ELI-Beamlines facility employed the L3-HAPLS laser in conjunction with a multivariate Bayesian optimizer. This optimizer, trained on data from a Proton Beam Imaging Energy Spectrometer (PROBIES), generated control directives for the laser's spectral dispersion, which were adjusted via an Acousto-Optic Programmable Dispersive Filter (AOPDF), demonstrating the potential of these advanced capabilities.

 

During this experiment over 3500 shots were fired onto 10 µm thick copper foils, irradiated at >10²¹ W/cm² with 30 fs, 8 J pulses of 800 nm light at a shot rate of 0.2 Hz, generating relativistic electrons, a broad spectrum of (predominantly bremsstrahlung) x-rays and an intense electric sheath field which accelerated protons to peak energies of ~20 MeV. In addition to the spectrally-resolved proton beam spatial profile provided by PROBIES, proton time-of-flight and Thomson parabola spectrometer diagnostics were run for cross-calibration, a Rep-rated Electron Positron Proton Spectrometer (REPPS) was used for correlation against the fast electron output and the x-ray radiographic properties of the source were tested utilizing a micro-scale test object fabricated through focused ion beam milling. We present some early results of the analysis of this experiment and discuss options for future development of the technique.   

Utilization of the MongoDB Repository for Information and Archiving (MORIA) framework to manage large data sets for machine-learning applications

As the high-energy-density (HED) physics community moves towards high-repetition-rate (HRR) operation in the ~0.01-10 Hz regime, a new paradigm of data management must be adopted [Feister, HPLSE 11 (2023]. Data acquisition from experimental subsystems, including the laser, targetry, and performance diagnostics, must be synchronized and archived in real time (~10-100MB/s, ~1-10 PB/yr). The database architecture should be flexible and expandable, depending on the application or experimental campaign and driven by the FAIR (Findable, Accessible, Interoperable, and Reusable) Guiding Principles [Wilkinson, Scientific Data 3 (2016)]. To this end, General Atomics (GA) has begun development of a NoSQL-database framework, the MongoDB Repository for Information and Archiving (MORIA). An organizational schema has been implemented that shifts scientific HED data organization from a shot-based to a diagnostic-based approach in order to increase archival and retrieval efficiency that lends itself to optimization applications. MORIA has been installed at the GALADRIEL facility and has demonstrated 1Hz archival of multiple laser and target diagnostics for thousands of shots. An overview of the database implementation will be given and results from a recently developed machine learning model for inferring and controlling the compressed pulse shape will be shown.   

Frequency Resolved Optical Gating Analysis with a Deep Neural Network for Ultrafast Pulse Characterization

Frequency Resolved Optical Gating (FROG) is a widely used technique for characterizing ultrafast laser pulses, providing both the amplitude and phase of the electric field. Accurate and rapid analysis of FROG data is crucial for advancing applications in ultrafast science. In this work, we introduce a novel approach for analyzing FROG data using a deep neural network. Our algorithm can operate in two modes: untrained and trained.  In the untrained mode, the neural network does not require pre-training on a large dataset. With just a single measurement, it can reconstruct the pulse as efficiently as traditional phase retrieval algorithms. In the trained mode, the neural network is pre-trained on a large set of synthetic or experimental data. Notably, our network architecture does not require ground truth data for training. It can be directly trained with FROG measurements without prior knowledge of the actual pulse shape. The trained neural network significantly reduces computational time and offers a powerful tool for real-time pulse characterization, which is particularly beneficial for high-repetition rate applications. We will demonstrate with data taken from both the ALEPH laser facility and the Scarlet laser facility.

 

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Tensor Networks Algorithms to Describe Strong Correlations in Heterogeneous, Non-ideal Plasmas

Inertial fusion energy plasmas involve highly heterogeneous and nonequilibrium plasmas. Standard simulation techniques for obtaining transport and equation of state properties, such as the hypernetted-chain approximation, typically assume slowly varying conditions far from boundaries; such bulk homogeneous calculations can be a poor approximation for highly transient high energy density plasmas. Incorporating complex boundary conditions in a highly heterogeneous environment can be achieved using the Yvon-Born-Green hierarchy with Salpeter closure. In the fully heterogeneous limit, even the pair correlation function, g(r), becomes a six dimensional function, g(r,r'), which is further coupled to even higher order correlation functions, g⁽ⁿ⁾(r₁, ..., r_n) generating the hierarchy, and making obtaining these correlations intractable with standard techniques. This high dimensionality make these equations perfect candidates for tensor network techniques, which decompose high-dimensional objects into a series of tensor products of lower-dimensional objects.    

Impact of profile effects on yield ratios from shock driven inertial confinement fusion implosions

The S-factor for reactions between two deuterons (DD) has been successfully measured (relative to DT) in both compressive [1] and shock-driven inertial confinement fusion (ICF) implosions. In contrast, measuring the D³He S-factor (relative to DD) has generated contradictory results for compressive [2] and shock-driven implosions, with ICF-inferred D³He S-factors spanning the range from lower than expected for lower-temperature compressive implosions to much higher than expected for hotter shock-driven implosions. This impacts the feasibility of using the D³He-implosion platform for bound and/or plasma screening studies as well as for implosion-based nuclear astrophysics studies in general, and needs to be understood. Comparison to radiation hydrodynamic simulations suggests that the high temperature results might be explained by considering the effects of spatial gradients in temperature and density, i.e., profile effects. This poster will discuss these results along with ratios of multiple different reactions from implosions with DT³He gas fill, considering paths forward for S-factor measurements from ICF implosions with ³He in the fill gas.   

Asymptotic scaling laws for the stagnation conditions of Z-pinch implosions

In most high-energy-density science schemes, energy is rapidly coupled to a target in a convergent geometry that drives an implosion and elevates its density, temperature, and pressure upon stagnation. The conditions realized at stagnation are connected to those of the in-flight shell, which are primarily determined by the drive pressure, shell entropy, and geometry of the implosion. In contrast to laser-driven spherical implosions, which typically reach a maximum drive pressure early in the implosion that remains constant thereafter, magnetically-driven cylindrical implosions result in a monotonically increasing drive pressure. We theoretically investigate the implications of this difference by analyzing the implosion trajectory in the aspect ratio/ Mach number parametric plane, and find that, while laser-driven shells expand in-flight while maintaining a fixed density at their exterior, magnetic drive results in decreasing thickness and increasing density during the implosion. We discuss the consequences of these trends on implosion performance, including their potential impact on stability and relationships between drive pressure and fusion yield in ICF experiments.

 

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.   

Onset of Detachment in the DIII-D Closed Divertor for an n = 3 Resonant Magnetic Perturbation

A stable solution for detachment during resonant magnetic perturbation (RMP) application has been achieved in EMC3-EIRENE for the closed divertor at DIII-D. For this work, the particle flux rollover which indicates the onset of detachment in the closed divertor is determined for a discharge with an n = 3 perturbation at high plasma densities and H-Mode. EMC3-EIRENE has the capability to model detachment scenarios by linearizing the electron energy sink term from the neutral gas interaction. Plasma response from the MARS-F code is used as an input for the magnetic equilibrium and the RMP current amplitude is varied to determine its effect on the scrape-off layer (SOL) plasma. The plasma is modeled with an input power into the SOL of 5.5 MW, and chemically sputtered carbon impurities. Spatially uniform anomalous diffusion of the main ion species and impurities are defined as 0.3 m²/s, and energy diffusivity is 1 m²/s.  The present modeling suggests that the detachment rollover for the upper closed divertor in the presence of an n = 3 perturbation occurs for a control parameter of the deuterium puffing rates ~ 290 Torr-L/s (220 Torr-L/s in this experiment). These background scrape-off layer plasma solutions will then be used in ERO2.0 to determine the characteristics of carbon transport during H-Mode in a resonant magnetic perturbation scenario for a changing magnetic footprint.