Session ZI01: MFE VI: Waves, Energetic Particles, and Runaway Electrons

Current Drive and Alpha-Channeling-Driven Rotation

The electric field associated with a planar electrostatic wave has no average momentum, and thus as the wave damps, the momentum of the plasma has to be conserved. How, then, can waves be used to extract charge and drive currents, both of which rely on transferring momentum to the plasma? We show how wave-mediated momentum exchange between nonresonant and resonant particles can drive net currents in spite of the constraint [1]. This momentum-exchange current drive can provide a possible magnetogenesis mechanism in astrophysical settings [2]. Applying our results to the wave-mediated diffusion of hot alpha particles in a magnetized plasma, we show that, for some waves, the hot ion charge is not extracted, but is instead cancelled by a return current in the nonresonant particles [3]. This cancellation current is analogous to the zeroth-order cancellation of cross-field currents in classical transport [4], and analogies with collisional rotation relaxation [5] will be discussed. Finally, we show how spatial wave structure can enable charge extraction. This charge extraction in turn permits ExB rotation to be driven by alpha channeling, providing a possible path to hot-ion-mode fusion. 

[1] I. E. Ochs and N. J. Fisch, Momentum-Exchange Current Drive by Electrostatic Waves in an Unmagnetized Collisionless Plasma, Phys. Plasmas, 27, 062109 (2020). [2] I. E. Ochs and N. J. Fisch, Magnetogenesis by Wave-driven Momentum Exchange, ApJ 905, 13 (2020). [3] I. E. Ochs and N. J. Fisch, Nonresonant Diffusion in Alpha Channeling, PRL 127, 025003 (2021). [4] I. E. Ochs and N. J. Fisch, Favorable Collisional Demixing of Ash and Fuel in Magnetized Inertial Fusion, PRL 121, 235002 (2018). [5] E. J. Kolmes et al., Radial Current and Rotation Profile Tailoring in Highly Ionized Linear Plasma Devices, Phys. Plasma 26, 082309 (2019).    

Visualization of Fast Ion, Phase-space Flow Driven by Alfvén Instabilities

Fast ion migration across a broad range of phase space, induced by multiple Alfvén Eigenmodes (AE) is measured for the first time by a novel imaging neutral particle analyzer [1] combined with a newly developed neutral beam modulation technique in the DIII-D tokamak. The unprecedented phase space resolution enabled by the INPA reveals details of fast ion transport physics and has identified three key features: (1) The transport promptly occurs immediately following neutral beam ionization, forming a phase space ‘hole’ at the injected energy and the radial location of minimum safety factor (q_min). (2) Fast ions move radially outward towards the plasma peripheral region at a reduced energy, where the thermalization process is much faster than that at the birth location. As a result, a pile-up of fast ions, i.e., larger population than neoclassical predictions, is observed, when the edge AE amplitude is reduced. (3) Phase-space tomography of the INPA data [2] reveals the formation of a high-energy tail of fast ions, with energies exceeding the injection energy of beams in the plasma core, interior to the q_min location, representing inward transport of the fast ions by AEs. Moreover, the migration is also found to affect the temporal evolution of AE amplitudes and radial structures as predicted by nonperturbative modeling.The measured flow pattern can be qualitatively interpreted by nonlinear hybrid kinetic-Magnetohydradynamic code (MEGA). A direct comparison to the calculated phase-space islands along the phase-space flow streamline highlights the key role of the pitch angle scattering on the formation of phase space flow.

 

[1] X.D. Du et al., Nucl. Fusion 58, 082006 (2018).

[2] X.D. Du et al., Nucl. Fusion 60, 112001 (2020).   

Runaway electron suppression in MST tokamak plasmas with RMP: simulation and experiment

Nonlinear MHD simulations using the NIMROD code have provided an explanation for the differing effects of $m=1$ and $m=3$ resonant magnetic perturbations (RMPs) on runaway electron (RE) confinement in Madison Symmetric Torus (MST) tokamak plasmas. Correlating experiments with simulated magnetic topology shows that REs are suppressed only when the applied RMP imposes magnetic chaos in the outer region of the plasma [Munaretto, et al., NF 60, 046024 (2020)]. Without an RMP, discharges with $q(0)<1$ and $2 

Prevention of disruption driven runaway electrons with a passive non-axisymmetric coil in the SPARC tokamak

A significant challenge to the development of the tokamak as a fusion energy source is the unwanted formation of relativistic electron beams during a plasma disruption. These runaway electrons (RE) are driven by an inductive electric field during the current quench (CQ), and high current machines capable of confining burning plasmas may be particularly susceptible. The SPARC tokamak is designed as a compact (R₀ = 1.85 m and a = 0.57 m), high-field (B₀ = 12.2 T) tokamak capable of reaching Q > 2 in D-T fueled H-mode plasmas. With a flattop current of I_p = 8.7 MA, and a minimum expected disruption duration of τ_CQ > 3.2 ms, a loop voltage of ~5 kV may lead to RE beam currents of up to 4.3 MA if not mitigated. This work examines a non-axisymmetric runaway electron mitigation coil (REMC) that is passively driven by the disruption CQ and excites MHD modes which produce stochastic fields and thus scatter energetic electrons. A 3D finite element code (COMSOL) determines the current induced in the REMC from the plasma current decay (up to 590 kA) and resulting external magnetic field perturbations. The NIMROD 3D MHD code then models the excitation of plasma MHD in the presence of this perturbation. Orbit-following code ASCOT5 computes advection and diffusion coefficients of the radial transport of REs in a stochastic field. Finally, the 1D radial transport solver in the DREAM framework evolves the electric field and the RE generation throughout the disruption. Whereas candidate REMC toroidal symmetries of n = 2 and 3 show little to no mitigation of RE formation, the n = 1 REMC excites a rich spectrum of saturated plasma modes early (0.7 ms) in the CQ leading to a near fully stochastic magnetic field and complete prevention of RE beam formation. Design of the REMC is proceeding, including structural support against electromagnetic loads, and options for a disabling switch.   

Self-consistent simulation of resistive kink instabilities with runaway electrons

It has been observed in experiments that MHD instabilities can be excited in a post-disruption plasma with large runaway electron current. The instabilities can cause significant loss of runaway electrons to the wall due to stochastic magnetic fields. In this work, we use the MHD code M3D-C1 combining with a fluid model for runaway electrons to simulate the nonlinear evolution of MHD instabilities in the runaway electron final loss event in DIII-D shot 177040. The simulation of relativistic runaway electrons is optimized with a method of characteristics to reduce numerical instabilities and save simulation time. It is found that the dominant MHD instability is a (2,1) resistive kink mode, which can grow within tens of microseconds due to the large resistivity in the post-disruption plasma. Runaway electrons are lost as the stochastic region grows and only a small population near the core can remain. The plasma current converts from runaway electron current to Ohmic current due to the induction electric field, and new current profile is more peaked near the core which can lead to (1,1) kink instability. The deposition area of lost REs on the wall is also calculated. Given the good agreement with experiment, the simulation model provides a reliable tool to study macroscopic plasma instabilities in existence of runaway electron current, and can be used to support future studies of runaway electron mitigation strategies in ITER.   

DREAM: a fluid-kinetic framework for tokamak disruption runaway electron simulations

Runaway electrons generated during a tokamak disruption pose a severe threat to future reactor-scale devices. Due to the exponential sensitivity of the runaway generation rate to the plasma current, robust avoidance and mitigation schemes cannot be fully validated in today's medium-size tokamaks. Comprehensive and validated runaway electron generation models are thus essential for the development of such schemes. In this contribution we present the Disruption Runaway Electron Analysis Model (DREAM), a new simulation tool specifically designed to study the generation of runaway electrons during tokamak disruptions. The tool combines 1D fluid models for the background plasma (electric field, temperature, poloidal flux, ion charge states) with either fluid or kinetic models for the electrons in tokamak geometry. To enable accurate and efficient simulations of the whole disruption, electrons are separated into three sub-populations based on their energies, allowing different models to be used for thermal, superthermal, and relativistic electrons simultaneously. Notably, the thermal and runaway electrons can be treated using conventional fluid models, while the superthermal electrons are evolved using a reduced kinetic equation, providing precise accounting of the transient---and thus inherently kinetic---hot-tail runaway generation mechanism. In addition to the novel treatment of electrons, DREAM incorporates a number of physical mechanisms which have never before been brought together in a complete, self-consistent disruption simulation, including radial transport of heat and electrons, dynamic evolution of ion charge states, collisions with partially ionized atoms, the effect of passive conducting structures on the electric field, and hyperresistivity. The first studies  conducted with DREAM indicate that fast electron radial transport may provide a path to effective runaway electron avoidance in ITER.