Session NI02: Fundamental Processes in Plasmas: Low temperature to Fusion

Nonlinear interaction between ultra-high-power ultra-short microwave pulses with gas/plasma

During the last decade, ultra-powerful (≥500 MW) sub-nanosecond (~0.5 ns) high-power microwave (HPM) sources (X- and K-band) were developed, which allow microwave-plasma/gas interaction studies in a nonlinear regime never encountered before. In this talk, we present the results of this research carried out at the Plasma Physics and Pulsed Power Laboratory, Technion – Israel Institute of Technology over the last 5 years. Several new phenomena, such as ionization-induced self-channeling of the HPM pulse [1,2], HPM-driven plasma wakefield excitation [3-5], nonlinear complete absorption of the HPM pulse [6], and its super-luminal propagation [7] were observed by experiments and confirmed by theoretical and numerical studies.   

High-speed plasma measurements with a plasma impedance probe

Plasma impedance probes (PIPs) are a type of RF probe that primarily measure electron density.  This work introduces two advancements to existing PIP techniques: a streamlined analytical model for interpreting PIP-monopole measurements and techniques for achieving $geq 1$ MHz time-resolved PIP measurements.  The model's improvements include introducing sheath thickness as a measurement and providing a more accurate method for measuring electron density and damping over a larger range of conditions.  The model is validated by a quasi-static numerical simulation which compares the simulation with measurements, identifies sources of error, and provides probe design criteria for minimizing uncertainty.  The improved time resolution is achieved by introducing higher-frequency hardware, updated analysis algorithms, and a more rigorous approach to RF calibration.  Finally, the new model and high-speed techniques are applied to two datasets: a 4 kHz plasma density oscillation resolved at 100 kHz with densities ranging between 2 x 10¹⁴ to 3 x 10¹⁵ m^-3 and a 150 kHz oscillation resolved at 4 MHz with densities ranging between 4 x 10¹⁴ to 6 x 10¹⁴ m^-3.    

Influence of Strong Coupling in Atmospheric Pressure Plasmas: Disorder-Induced Heating, Transport, and Implications for PIC Simulation

Molecular dynamics (MD) simulations are used to study how strong ion-ion correlations influence cold atmospheric pressure plasmas (CAPP). Results show that strong ion coupling significantly increases the ion temperature via disorder-induced heating (DIH). This is followed by ion-neutral temperature relaxation, effectively heating the neutral gas within a nanosecond timescale - demonstrating fast neutral gas heating. A thermodynamics-based model is shown to predict the equilibrium temperature. This model demonstrates good agreement with experimental measurements of the neutral gas temperature in CAPP discharges. Building upon this understanding, the discussion will transition into the development of a new model for ion and neutral gas diffusion. This model places emphasis on the influence of strong Coulomb coupling in ion-ion interactions, and its consequential impact on neutral gas dynamics via the increase in temperature. Three regimes are identified that underscore the importance of both ion-neutral and ion-ion interactions across diverse ionization fractions. The model was validated via MD over a wide range of ionization fractions. Finally, the discussion will critically evaluate the suitability of the commonly employed particle-in-cell (PIC) method for CAPP. It is found that the PIC method struggles to capture the physical effects of DIH due to stringent prerequisites on macroparticle weight and grid resolution. Additionally, our research unveils a novel numerical artifact, artificial correlation heating (ACH), which is connected to the effective coupling strength associated with the macroparticles weight. This newly identified numerical heating mechanism delineates an upper boundary in density or macroparticle weight for the applicability of PIC simulations.   

Diocotron Drift Mode Suppression for Increased Particle Transfer in a Multi Cell Trap

For the creation of a positron-electron (pair) plasma the A Positron Electron eXperiment (APEX) collaboration needs large quantities of positrons. To accumulate many positron is an experimentally challenging task [1]. To achieve this task, we have designed and constructed a new prototype multi cell Penning-Malmberg trap (MCT) [2]. This MCT includes a master cell, and three storage cells (one on-axis, and two off-axis). With this device we will test and improve the plasma transfer to the off-axis cells while mitigating losses. The goal is to transfer and stack multiple plasma pulses in one (or more) off-axis trap.

 

In this contribution, we discuss the dynamics during the transport to the off-axis cells. These dynamics are dominated by competing diocotron drift modes [3] that can lead to significant particle losses. We present techniques to suppress these modes that mitigate losses and center the plasma in the off-axis cells. With these techniques, we significantly improved the transfer. In addition, we were able to demonstrate the first consecutive transfer and confinement in two different off-axis cells. The confinement in multiple off-axis traps is a milestone for the future use of a MCT at the NEPOMUC positron source in Munich.   

Turbulence suppression in plasmas around the Greenwald density limit on EAST and DIII-D

Recent high poloidal beta (βP) scenario experiments on EAST and DIII-D have made coordinated breakthroughs for high confinement quality at high density near the Greenwald limit. Experiments on EAST have nearly doubled the ion temperature at normalized density (fGr) ~0.9, overcoming a long standing limitation in long pulse operations. Experiments on DIII-D have achieved fGr above 1 simultaneously with normalized confinement (H98y2) well above 1, as required in fusion reactor designs but never before verified in experiments. Density gradient amplification of turbulence suppression at high βP can explain both of these achievements. EAST long pulse H-mode experiments have reached a world record duration of 400 seconds, but with Ti<   

Nonlinear edge-localized modes as plasmoid-mediated reconnection bursts

Understanding the physics of edge instabilities, in particular as described by a self-consistent model of the nonlinear evolution, remains a challenging problem in magnetic-fusion confinement. Here, using full extended MHD simulations, we investigate the nonlinear evolution of Peeling-Ballooning Edge-Localized Modes (P-B ELMs) and the associated physics of fast magnetic reconnection triggered by the formation of thin current sheets and their secondary instabilities in the late dynamical phase of the instabilities. In an earlier study, low-n edge non-axisymmetric reconnecting current-sheet instabilities, and the onset of plasmoid instabilities for given SOL current sheets, were examined [Ebrahimi PoP 24, 056119 2017]. Here, by including the critical effect of plasma pressure gradients in DIII-D discharges, we will present new results, and a more complete picture of ELM dynamics by examining the sequential stages of the linear instability, as well as the early and late nonlinear phases. Large-scale axisymmetric current sheets, as well as small-scale current sheets, are formed as the coherent P-B ELM filaments nonlinearly evolve. It is observed that, at high Lundquist numbers, these current sheets break during a reconnection burst, i.e. a secondary exponential growth of intermediate modes followed by relaxation due to the suppression of P-B drive. [F. Ebrahimi and A. Bhattacharjee, Nucl. Fusion, under review] We find that as the linearly unstable intermediate-n ballooning modes and the nonlinearly driven peeling low-n modes grow and saturate, it is during a fast-reconnection phase mediated by plasmoid instability, where nonlinear expulsion of finger-like currents (plasmoids) of finite amplitude ballooning modes occur. Preliminary results on the stability of strong negative triangularity shaping plasmas using extended MHD model will also be presented.