Magnetic turbulence is observed at the beginning of the current quench in intended TEXTOR disruptions. Runaway electron (RE) suppression has been experimentally found at magnetic turbulence larger than a certain threshold. Below this threshold, the generated RE current is inversely proportional to the level of magnetic turbulence. The magnetic turbulence originates from the background plasma and the amplitude depends strongly on the toroidal magnetic field and plasma electron density. These results explain the previously found toroidal field threshold for RE generation and have to be considered in predictions for RE generation in ITER.

DIII-D experiments at low density (n_{e}∼10^{19}  m^{-3}) have directly measured whistler waves in the 100-200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limit-cycle-like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission that follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.

Realization of a high-current (approaching 1 MA) post-disruption runaway electron (RE) beam in DIII-D yields controlled access to very low edge safety factor (q a ) conditions. This enables unique observation and study of low-order kink instabilities in post-disruption plasmas where the current is carried entirely by relativistic REs. The conventional external kink stability boundary (in terms of q a and internal inductance, ℓ i ) is found to accurately predict the operational space of the RE beam, with q a limited to ≈2. Kink instabilities appear with a characteristic growth rate of a few tens of microseconds (which is comparable to the Alfven time) and ultimately cause complete loss of the RE population on a similar time-scale. This characteristic RE loss time is significantly faster than observations away from the q a ≈2 stability limit and implies both higher peak heat loading but also less chance of destructive magnetic to kinetic energy conversion via RE beam regeneration. With large enough kink amplitude no RE beam regeneration is observed, indicating the magnetic to kinetic energy conversion was inhibited. Instability structure analysis reveals that early instabilities at high q a ( 4 ⪆ ) are likely internal or resistive kinks (at higher poloidal mode number), while at q a ≈ 2 the most destructive instabilities are either internal or external kinks with low-order poloidal mode number (m=2). The HXR loss magnitude is found to be proportional to the perturbed magnetic field and exhibits a helical spatial pattern. These observations are novel for present-day tokamaks yet will potentially be very common in high current tokamaks such as ITER, where predicted RE beam equilibrium evolutions cross the q a ≈ 2 stability boundary.

Novel spatial, temporal, and energetically resolved measurements of bremsstrahlung hard-x-ray (HXR) emission from runaway electron (RE) populations in tokamaks reveal nonmonotonic RE distribution functions whose properties depend on the interplay of electric field acceleration with collisional and synchrotron damping. Measurements are consistent with theoretical predictions of momentum-space attractors that accumulate runaway electrons. RE distribution functions are measured to shift to a higher energy when the synchrotron force is reduced by decreasing the toroidal magnetic field strength. Increasing the collisional damping by increasing the electron density (at a fixed magnetic and electric field) reduces the energy of the nonmonotonic feature and reduces the HXR growth rate at all energies. Higher-energy HXR growth rates extrapolate to zero at the expected threshold electric field for RE sustainment, while low-energy REs are anomalously lost. The compilation of HXR emission from different sight lines into the plasma yields energy and pitch-angle-resolved RE distributions and demonstrates increasing pitch-angle and radial gradients with energy. DOI: 10.1103/PhysRevLett.118.255002 Introduction.-Reaching mega-ampere currents and mega-electron volt (MeV) energies during fast shutdown events, runaway electrons (REs) pose perhaps the greatest operational risk to tokamak fusion reactors such as ITER [1][2][3][4]. Because of the severe potential for damage to the reactor walls, opportunities for empirical tuning of RE control actuators will be limited. Instead, a first-principles predictive understanding is needed, and present-day experiments fill a crucial need in validating theoretical predictions of RE dissipation.Classical theories for relativistic RE generation in tokamaks based on the effects of Coulomb collisions (small angle [5] and secondary avalanche [6]) determine the critical electric field (E C ) for the growth of RE populations. Further work highlighted the important role of synchrotron damping in elevating the threshold electric field above E C [7,8], and several experiments have since yielded evidence of the elevated threshold [9][10][11][12]. These observations motivated the development of a rigorous analytical theory [13] and computational tools [14][15][16][17][18][19] that clarified the importance of the effects of pitch-angle scattering and synchrotron damping. Alongside quantifying the enhancement of the threshold field, these works predict phase-space circulation around an attractor resulting in a pileup of REs at specific energies potentially resulting in nonmonotonic features in the RE distribution function (f e ). While important to the RE dissipation rate and thus the prospects for control, neither

Areas of agreement and disagreement with present-day models of RE evolution are revealed by measuring MeV-level bremsstrahlung radiation from runaway electrons (REs) with a pinhole camera. Spatially-resolved measurements localize the RE beam, reveal energy-dependent RE transport, and can be used to perform full two-dimensional (energy and pitch-angle) inversions of the RE phasespace distribution. Energy-resolved measurements find qualitative agreement with modeling on the role of collisional and synchrotron damping in modifying the RE distribution shape. Measurements are consistent with predictions of phase-space attractors that accumulate REs, with non-monotonic features observed in the distribution. Temporally-resolved measurements find qualitative agreement with modeling on the impact of collisional and synchrotron damping in varying the RE growth and decay rate. Anomalous RE loss is observed and found to be largest at low energy. Possible roles for kinetic instability or spatial transport to resolve these anomalies are discussed.