Session UO8: MF: Tokamaks, Stellarators, and Compact Fusion Reactors

Main ion charge exchange spectroscopy on JET in preparation for the DT campaign

Due to the reduction in intrinsic carbon charge exchange (CX) emission following the changeover to the beryllium/tungsten JET ITER-like wall, alternative methods have been deployed for measuring the core ion temperatures: i) injection of extrinsic impurities, such as neon, at a cost of increased fuel dilution; and ii) analysis of D-alpha CX emission from the main fuel ions. This work presents extensive validation efforts towards routine high-quality profiles of T$_{\mathrm{i}}$ and v$_{\mathrm{tor}}$ from a recently commissioned main ion CX diagnostic. The system shares sightlines with the Ne CX spectrometers on two opposing views of the neutral beams, allowing for comparisons between the obtained datasets. Modelling shows a small (\textless 3{\%}) impact on T$_{\mathrm{i}}$ due to spatial averaging effects, whereas the CX cross-section effects could lead to an underestimation of T$_{\mathrm{i}}$ by about 10{\%} for T$_{\mathrm{i}}$ approaching 10 keV. The data quality implications on T$_{\mathrm{i}}$, $\nabla $T$_{\mathrm{i}}$, T$_{\mathrm{i}}$/T$_{\mathrm{e}}$ and v$_{\mathrm{tor}}$ on the physics outputs from core transport power balance analysis are presented in the context of preparations for high P$_{\mathrm{fus}}$ DT operations in 2020.   

The resilience of highly dissipative exhaust scenarios at JET to seed impurity mixes and divertor geometry

A fusion power plant with an ITER like lower single null divertor requires more power dissipation, f$_{\mathrm{diss}}$, between the core plasma and the divertor target plates than existing devices and ITER. A f$_{\mathrm{diss}}$ \textgreater 90{\%} of the total heating power will be needed, accounting for losses from radiation, perpendicular transport and CX processes. Thereby more than 70{\%} being radiation on closed field lines (cf. ITER 30{\%}). Completely detached divertor targets with T$_{\mathrm{e}}$ \textless 5eV will reduce erosion in an environment with a large fraction of radiating seed impurities. On JET with metal PFCs highly dissipative regimes with completely detached divertor targets and small ELM regimes have been achieved using a variety of seeding species at the highest available heating powers of up to \textasciitilde 29MW. Varying the admixture of Ar and N$_{\mathrm{2}}$ alters the ratio of core to divertor radiation but not the achievable f$_{\mathrm{diss}}$ with completely detached divertor targets. For the same fueling gas throughput confinement in unseeded JET ILW discharges with an open horizontal divertor (CC) is improved compared to vertical target geometry (VV). However, with N$_{\mathrm{2}}$ as well as with Kr seeding the maximum achievable f$_{\mathrm{diss}}$ are equal for both configurations, with confinement being equal and degraded compared to CC but similar to unseeded VV conditions.   

Improved density feedback control range and response time on DIII-D using 3D fields

A new algorithm for plasma density feedback control using adaptive, non-axisymmetric (3D) magnetic fields was implemented on DIII-D, enabling a radiative divertor in high performance, Super H-mode discharges. For conventional density feedback by gas valves the natural density at zero gas flow presents a hard limit. The application of 3D magnetic fields in low collisionality plasmas leads to the so-called ``density pumpout'', i.e.\ a reduction below the natural density. The newly designed control algorithm combines both actuators allowing for a wider range of density targets. Furthermore, 3D fields have a faster plasma density response than gas puff, improving control response time. With this, accurate control of the density trajectory is achieved, as is necessary for access to the Super H-mode regime with increased pedestal performance. Moreover, it was demonstrated that the pedestal density could be sustained at favorable conditions while the divertor density was increased by injection of impurities to integrate a radiative divertor scenario. The ability to loosen the tight coupling of pedestal and divertor density by 3D field density feedback allows to explore solutions for core-edge integrated scenarios.   

I-Mode Exhaust Experiments on ASDEX Upgrade

Experiments conducted on ASDEX Upgrade (AUG) investigating the compatibility of dissipative divertors with high performance I-modes show plasmas exhibit a prompt, I/L back-transition prior to significant reductions in divertor heat flux when seeding nitrogen into the private flux region. The I-mode regime is currently being explored as a candidate reactor scenario, leveraging its demonstrated access to enhanced energy confinement, H$_{\mathrm{98}}$ \textgreater 1.0, while maintaining L-mode-like impurity confinement and avoiding large ELMs. Recently published attempts to integrate I-mode with detached divertor operation on Alcator C-Mod are introduced to give context to new AUG experiments at 1.0 MA, 2.5 T, with densities of 4-5e19 m$^{\mathrm{-3}}$. In AUG, ECH heating of \textasciitilde 2.2 MW is used to enter into a stationary I-mode, reaching pedestal-top T$_{\mathrm{e}}$ \textasciitilde 900 eV. Private flux N$_{\mathrm{2}}$ seeding at rate necessary to cause reductions in divertor heat flux, 3.0-6.0e21 el/s, results in an I/L back-transition within 1-2   

Impact of negative triangularity on turbulent transport in TCV: From validated simulations to basic understanding

Negative triangularity tokamak discharges lie beyond the standard operating regimes but have been experimentally found to potentially lead to significant improvements in the plasma confinement. The physical mechanism leading to this improvement and in particular how edge triangularity, which is rapidly diminishing towards the magnetic axis, can influence the behaviour of the plasma at all radial positions still remains unclear. \\ We will discuss global gyrokinetic GENE simulations used to reproduce the transport level measured in negative triangularity L-mode TCV plasmas. Dedicated synthetic diagnostic have been successfully used to compare GENE results to experimental fluctuation measurements. Furthermore, the impact and beneficial effects of $\delta<0$ in turbulent regimes other than TEM dominated plasmas will be addressed.   

Development of a laser collective scattering system for measuring short-scale turbulence on HL-2A Tokamak

A CO$_{\mathrm{2}}$ laser collective scattering system has been successfully developed to measure short-scale density fluctuations at the HL-2A tokamak. At present, there are two detective channels with homodyne detection. It can measure the radial wavenumber k$_{\mathrm{r}}$ in a range of 10 \textasciitilde 40 cm$^{\mathrm{-1}}$. Both the main beam and local oscillator beams pass through plasmas tangentially. Taking the advantages of the toroidal curvature of the magnetic field lines and the small turbulence wavevector being perpendicular to the field lines, local measurements of the scattering light are available. Besides the small-scale turbulent fluctuations, a lot of MHD activities can also be detected by this system. And the local oscillator beams passing through the plasma (far-forward scattering) is the reason why this high-wavenumber diagnostic system can detect the macro-instabilities. The methods of separating small-scale and large-scale fluctuation signals are introduced also. During the last experimental campaign, lots of phenomena associated with small-scale turbulence on HL-2A tokamak have been observed by this diagnostic system.   

Propagation of Soliton-like density pulses following pellet injection in DIII-D H-mode Plasmas

Pellet fueling from the high field side of tokamaks has been shown to effectively fuel H-mode plasmas and is planned for fueling in ITER. New observations have been made on DIII-D of pellet ablation striations from HFS injected pellets leading to density pulses with soliton like structure propagating along field lines in both directions from the ablation location. The pulses are observed in the interferometer chordal signals with high time resolution and are not expected from hydrodynamic simulations of pellet cloud expansion and ExB polarization drifts. The density pulses are observed to persist for as many as 5 toroidal revolutions and accelerate, consistent with the ion sound speed of the cloud being reheated during the expansion. When large ELMs are triggered by the pellets the density pulses are only observable by chords passing through the HFS. Observation details are presented and implications for refining deposition models and the fueling of large burning plasmas are discussed.   

Drift effects on W7-X edge heat and particle fluxes

Classical particle drifts are known to have substantial impacts on fluxes of particles and heat through the edge plasmas in both tokamaks and stellarators, resulting in asymmetric loading of divertor targets. Here we present results from the first dedicated investigation of drift effects in the W7-X stellarator. In low-density plasmas, the main driver of the asymmetries appears to be poloidal ${E}\times{B}$ drift flows due to radial electric fields in the edge plasma, analogous to a similar effect seen in tokamaks. Such flow patterns led to up-down asymmetries in the target fluxes including radial offsets of the strike lines. The explanation of these effects as a consequence of poloidal ${E}\times{B}$ drift flows is supported by a comparison of the locations of the asymmetric features with the footprints of key topological regions of the edge magnetic field on the divertor. In higher-density plasmas, the target fluxes were quite different and the underlying drift mechanisms are not yet as well understood. Also, unlike in the low-density case, the upper and lower targets collected non-ambipolar currents with opposite signs that inverted upon field reversal.   

Towards steady state high performance ECRH plasmas at W7-X

The operation phase OP1.2 in 2018/19 demonstrated that high performance plasma could be achieved with ECRH only. The excellent confinement and heat exhaust properties could not be sustained for steady state since W7-X was neither equipped with a steady state pellets system nor with cooled plasma facing components. Both are in preparation for the next campaign in 2021, where steady state operation is envisaged. The ECRH system, which consists of 10 gyrotrons, successfully operated with the O2-mode polaristion at densities of 1.5 10$^{\mathrm{20}}$ m$^{\mathrm{-3}}$ and a power up to 7.5 MW. Here the electron ion coupling is strong enough that T$_{\mathrm{i}}$ was approaching T$_{\mathrm{e}}$ reaching values of up to 3.5 keV. High density ECRH operation also enabled stationary detachment at the divertor for up to 30 s. The long pulse high power operation exhibits the limits of the actual ECRH installation. In particular the atmospheric power transmission suffered from high humidity levels in the experimental hall. In addition high density operation requires more ECRH power. Therefore extensive up-grades will be performed in the next 3 years, including the development of an 1.5 MW gyrotron, preparing 2 additional gyrotron positions, improving the O2 launch geometry and a strong reduction of the humidity in the transmission line.   

Characterization of pre-molten SnLi/Sn by surface techniques and ICP-OES. Effects induced during its interaction with stainless steel, molybdenum and tungsten substrate

The employment of liquid metals as plasma facing components in nuclear fusion reactors supposes an alternative to the traditional use of refractory high Z materials such as tungsten. Within the liquid metals approach to the problem, the use of tin-lithium alloys could combine the advantages of using the most usual pure liquid metals: tin (Sn) and lithium (Li), For this purpose, the University of Illinois has started an extensive experimental campaign in order to synthetize the material, characterize it and study its interaction with stainless steel, molybdenum and tungsten. The first depth profile characterization by Time of Flight-Secondary Ion Mass Spectrometry (SIMS-ToF) of SnLi samples, previously molten is presented in this work. Additionally, the composition of the alloy specimens was absolutely quantified by using ICP-OES in order to check the reproducibility of the alloy synthesis process. The surface characterization was extended to additional techniques as SEM and profilometry in order to shed light on the observed effects. The preliminary results indicated high reproducibility in the absolute composition of the alloy by ICP-OES, thus demonstrating a proper synthesis process, also showing no influence of the interaction with the substrate and melting on the alloy post-mortem composition. On the other hand, SIMS-ToF corroborated the presence of Sn and Li on the pre-molten samples, pointing out a stronger interaction of the alloy with stainless steel compared to tungsten and molybdenum.   

Recent Advances in High-Fidelity Irradiations of REBCO High-Temperature Superconductors and Implications for High-Field Fusion Devices

REBCO, or rare-earth barium copper oxide, is a family of high-temperature superconductors (HTS) that is being examined as a magnet material for future fusion devices. Compared to low-temperature superconductors, REBCO can carry greater current densities at higher temperatures and external magnetic fields, enabling it to be used to generate extremely strong magnetic fields. As fusion power density scales with the fourth order of magnetic field strength, the stronger fields produced by REBCO magnets enable the design of compact fusion devices with large power outputs. However, the response of REBCO to irradiation by fusion neutrons is far from fully understood, and the extent to which REBCO magnets can sustain large neutron fluences will determine the lifetimes of compact fusion devices. Further complicating radiation damage studies of REBCO is that neutron irradiation is often difficult, costly, and causes significant activation. To emulate the neutron damage that will be seen in future fusion reactors, recent upgrades to experimental facilities at MIT have enabled newly high-fidelity proton irradiations of REBCO, and the results from these experiments may hold large implications for high-field fusion devices.   

Baseline Plasma Diagnostics Development on the Helicon Plasma Experiment (HPX)

HPX at the Coast Guard Academy Plasma Lab (CGAPL) continues to progress toward utilizing the reputed high densities (10$^{\mathrm{13}}$ cm$^{\mathrm{-3}}$ and higher) at low pressure (.01 T) of helicons, for eventual high temperature and density diagnostic development in future laboratory investigations. HPX has installed an Impedans Langmuir probe and constructed an RF-shielded triple probe experimental diagnostic to compare the plasma's density, temperature, and behavior during experiments. Our 2.5 J YAG laser Thomson Scattering (TS) system operates at its first and second harmonic, 532 and 1064 nm respectively. It utilizes a high-performance VPH grating spectrometer and a charge coupled device (CCD) camera with a range of 380-1090 nm with a resolution of 1024x1024 for second harmonic (532 nm) photon emissions. At 1064 nm, a new polychromator has been procured from General Atomics optimized for TS measurements of 5 eV \textless T$_{\mathrm{e}}$ \textless 2000 eV over a 109-degree scattering angle. Progress on the construction of the RF coupling system, Helicon Mode development, and observations from Thomson Scattering, particle, and electromagnetic scattering diagnostics will be reported.