Edge Localized Modes Cycle Research Articles

HL‐2A's ELM cycle simulations by integrating BOUT++'s drift MHD and transport code

AbstractA new integrating model has been developed to couple tokamak edge multiscale magnetohydrodynamic (MHD) events and transport simulations, such as edge‐localized mode (ELM) cycles. As a proof of principle, we first start from a set of three‐field two‐fluid model equations, which includes the pressure, current, and vorticity. The equations are separated into the slowly evolving part of the axisymmetric component by taking a time average of the axisymmetric component. The time‐averaged fluxes, which are quadratic in fluctuating quantities, act as driven terms for the time‐averaged axisymmetric quantities that determine the plasma transport, and therefore the large‐scale evolution of the plasma profiles. Then the HL‐2A's ELM cycles are simulated using the model. Good agreements of ELM size and pedestal recovery time have been achieved for the solutions obtained from the coupled simulation compared with experiment. For one ELM cycle simulation, the coupled code can achieve a speedup of a factor of up to 30 over standalone code.

Modeling transient edge plasma transport with dynamic recycling

The work presents numerical simulation studies of the role that dynamic plasma recycling on the main wall and divertor target surfaces plays in transient edge plasma transport phenomena, such as edge localized modes (ELMs). The studies are performed by coupling the edge plasma transport code UEDGE [Rognlien et al., J. Nucl. Mater. 196–198, 347 (1992)] and the wall reaction–diffusion transport code FACE [Smirnov et al., Fusion Sci. Technol. 71, 75 (2017)]. The two-dimensional, time-dependent, two-way coupling of the codes, in a realistic tokamak geometry, is accomplished using the Integrated Plasma Simulator framework [Elwasif et al., in 18th Euromicro Conference on Parallel, Distributed and Network-Based Processing (PDP 2010), Pisa, Italy (IEEE, 2010), pp. 419–427] for all modeled material plasma boundaries. The simulations show that dynamic plasma recycling has substantially different characteristics on the main wall and on the divertor plates. It is demonstrated that during an ELM cycle the outer wall can dynamically absorb and release a number of particles comparable to that expelled by the ELM from the core plasma, by far exceeding the dynamic retention capacity of the divertor surfaces. The resulting evolution of the edge and divertor plasma conditions during an ELM cycle is analyzed.

Local measurements of the pedestal magnetic field profile throughout the ELM cycle on DIII-D

New high speed localized measurements of the pedestal magnetic field during the edge localized mode (ELM) cycle of a DIII-D High confinement mode (H-mode) discharge indicate a temporally and spatial complex redistribution of the edge current density profile, jedge. The measurement technique extracts the magnetic field magnitude, B, via the spectral separation of Stark-split neutral beam radiation in the pedestal. Single spatial channel measurements from a novel spatial heterodyne spectrometer are validated in discharges with core current profile changes. The technique measures Stark-splitting changes that imply B changes as small as 1 mT with high time resolution (50 μs). At normalized poloidal flux ψn=1.0, B appears saturated in the inter-ELM period and then rapidly decreases in <200 μs by ∼1%, before edge recycling emission begins to increase. Radially inboard of jedge, B increases at the ELM crash. The behavior is consistent with a rapid collapse of jedge at the ELM crash and subsequent pedestal recovery. In some discharges, at ψn<0.96, changes in B are observed throughout the ELM cycle. In others, B recovers and is relatively stable until a few ms leading up to the next crash. Measurements of B during the H-mode transition show a large increase at ψn=1 with little change at ψn=0.9, consistent with the formation of the edge bootstrap current density peak. The ψn=0.9 spectrum is complicated by predicted changes to the Stark component intensities with density at the L–H transition.

Fast-ion Dα spectroscopy diagnostic at KSTAR.

A fast-ion Dα (FIDA) diagnostics system was installed for core and edge measurements on KSTAR. This system has two tangential FIDA arrays that cover both blue- and redshifted Dα lines (cold: 656.09nm) in active views along the neutral beam 1A centerline. The spectral band is 647-662.5nm, and it covers the Doppler shift of the emission from the maximum energy of the neutral beam (100 keV). A curved filter strip with a motorized stage adequately prevents saturation of the electron multiplying charge-coupled device signal by the cold Dα line from the plasma edge. From comparisons of the measured spectra and FIDASIM modeling code, the FIDA spectra are well matched quantitatively. Moreover, the first measurements show that the FIDA radiance agrees with the neutron rate in the time trace during external heating and perturbation. In addition, responses are observed in the core FIDA radiance during the edge-localized mode cycle.

The impact of ELM mitigation on tungsten source in the EAST divertor

Divertor tungsten (W) erosion source during edge localized mode (ELM) bursts in EAST is investigated based on optical emission spectroscopy on atomic neutral tungsten line emission at 400.9 nm. Both temporal evolution and total source strength are analyzed and compared in three different ELM mitigation schemes including natural ELMs, resonant magnetic perturbations (RMP) and 2.45 GHz lower hybrid wave (LHW). The mitigation of W source caused by single ELM impact at the divertor target is mainly attributed to the reduction of the W source production in the ELM decay phase. However, with the increase of ELM frequency, the time-averaged intra-ELM W source during an ELM cycle rises and dominates the evolution of the total W source with increasing fraction, which exhibits strong correlation with the core W level in the RMP scheme. In the natural ELM scheme, the pedestal electron temperature is found not only to control the effective W sputtering yield at target during ELMs, thus influencing the intra-ELM W source, but also to be related to the delay time between the divertor ELM WI emission and the core extreme ultraviolet bolometer (XUV) signal which can scale with the parallel ion transit time. Furthermore, the delay time is found to have more consistent dependence on the pedestal plasma collisionality. The rise time of intra-ELM WI emission increases when RMP or LHW is applied, revealing a different ELM mitigation mechanism in comparison with the natural ELM scheme. The temporal profiles of the intra-ELM WI emission in different ELM mitigation schemes are compared and the potential mechanism is discussed. Besides, the ELM mitigation effects with RMP are found to be asymmetric at the outer and inner divertor targets, which is characterized by the different variations of the intra-ELM W source strength and the rise time of WI emission with the ramp-up of RMP coil current. Divertor partial detachment is achieved simultaneously with strong ELM mitigation under a suitable RMP phase difference without additional gas puff.

Interpretative SOL modeling throughout multiple ELM cycles in DIII-D

Abstract Coupling between the UEDGE (edge fluid model), GINGRED (grid generation) and CAKE (equilibrium reconstruction) codes opens the door for automated interpretative scrape-off-layer (SOL) analysis over entire discharges, providing information that is essential in efforts to couple the SOL to core transport codes. In this work, we utilize new developments in the autoUEDGE code (Izacard et al. 2018) to investigate the behavior of the DIII-D SOL during the temporal evolution of an edge-localized mode (ELM) cycle. Modeled temperature and density profiles in UEDGE are automatically matched to experimental measurements by iteratively and self-consistently adjusting transport coefficient profiles in the plasma edge. This analysis is completed over multiple ELM cycles of a well-diagnosed discharge with long ( ∼ 100 ms ) inter-ELM periods. Directly after the ELM crash, a short period of high-density, low-temperature conditions is observed in Langmuir probe measurements at the outer divertor. This regime is associated with enhanced D α emission and incident particle flux, suggesting that the divertor enters a period of high recycling after an ELM crash. After about ∼ 25 ms , divertor conditions return to their pre-ELM conditions and remain there for several tens of milliseconds. Using the autoUEDGE code, the SOL is modeled as a function of ELM cycle using upstream profiles as input. The 2D modeling successfully reproduces both divertor Thomson scattering measurements and the experimentally observed divertor dynamics. Though the recycling is kept fixed throughout the modeling, changes in particle fluxes are consistent with local experimental recycling changes induced by ELMs. Agreement between modeling and observation suggests a strong link between upstream profiles and the high-recycling divertor conditions directly following large type-I ELMs.

Ion temperature measurement techniques using fast sweeping retarding field analyzer (RFA) in strongly intermittent ASDEX Upgrade tokamak plasmas.

This manuscript presents a new method of interpreting the ion temperature (Ti) measurement with a retarding field analyzer (RFA) that accounts for the intermittent/turbulent nature of the scrape off layer (SOL) plasmas in tokamaks. Fast measurements and statistical methods are desirable for an adequate description of random fluctuations caused by such intermittent events as edge localized modes (ELMs) and blobs. We use a RFA that can sweep its current-voltage (I-V) characteristics with up to 10 kHz. The RFA uses an electronics compensation stage to subtract the capacitive pickup due to the finite connecting cable capacitance, which greatly improves the signal-to-noise ratio. In the 10 kHz case, a single I-V characteristic is obtained in time, which is an order of magnitude faster than the ELM cycle. The fast sweeping frequency allows us to reconstruct the Ti probability density function (PDF), which we use as the Ti representation. The boundary conditions that we place on the I-V characteristics when calculating the Ti values impact the resulting Ti PDF. If the boundaries are insensitive to the plasma fluctuations, then the most probable Ti value of the PDF (20 eV-25 eV) is similar to the Ti value obtained via the classical conditional averaging method (20 eV-27 eV). However, if the boundary conditions follow the fluctuations, then the PDF-based method gives a substantially higher most probable Ti value (35 eV-60 eV). Overall, we show that a fast sweeping RFA diagnostic should be used in intermittent SOL plasmas to reconstruct the PDF for accurate Ti measurements.

Dynamics of the pedestal transport during edge localized mode cycles at ASDEX Upgrade

The dynamic behaviour of the ion and electron energy, particle and momentum transport measured during type-I edge localized mode (ELM) cycles at ASDEX Upgrade is presented. Fast measurements of the ion and electron temperature profiles revelead that the ion and electron energy transport recover on different timescales, with the electrons recovering on a slower timescale (Cavedon et al 2017 Plasma Phys. Control. Fusion 59 105007). The dominant mechanism for the additional energy transport in the electron channel that could cause the delay in the electron temperature gradient () recovery is attributed to the depletion of energy caused by the ELM. The local sources and sinks for the electron channel in the steep gradient region are much smaller compared to the energy flux arriving from the pedestal top, indicating that the core plasma may dictate the local dynamics of the recovery during the ELM cycle. A model for the edge momentum transport based on toroidal torque balance that takes into account the existence of poloidal impurity asymmetries has been developed. The analysis of the profile evolution during the ELM cycle shows that the model captures the dynamics of the rotation both before the ELM crash and during the recovery phase.

Overview of physics studies on ASDEX Upgrade

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG.Stable high-density H-modes with MW m−1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated.Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and Er allow for inter ELM transport analysis confirming that Er is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of ‘natural’ no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback.Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle—measured for the first time—or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO.

Inter-ELM pedestal localized fluctuations in tokamaks: Summary of multi-machine observations

Abstract A variety of experimental studies on pedestal localized fluctuations appearing in between crashes of edge localized modes (ELMs) across several tokamaks have been reviewed and summarized. The onset of the inter-ELM fluctuations is correlated with the evolution of the pedestal gradients. Three profile recovery phases are extracted, which are interlinked with the onsets of different kinds of pedestal fluctuations. Across machines it is found that the pedestal fluctuations can be assorted into at least three categories. These are determined by the fluctuation onset in the ELM cycle, observed frequency range and radial location in the pedestal. Further, the categories might be also related to different instabilities. Similar observations at various machines may point to a underlying generation mechanism that acts similarly for presently accessible pedestal parameter ranges.

Scaling of the toroidal structure and nonlinear dynamics of ELMs on ASDEX Upgrade

Edge localized modes (ELMs) are magnetohydrodynamic (MHD) instabilities that cause fast periodic relaxations of the strong edge pressure gradient in tokamak fusion plasmas. Magnetic pick-up coils allow the extraction of toroidal mode numbers n during the ELM cycle including the nonlinear crash on ASDEX Upgrade, providing a good comparability to nonlinear 3D MHD codes. This paper investigates how the mode numbers before and during the ELM crash change with a variation of plasma parameters. It is found that the toroidal structure size during the crash is similar to the one existing slightly before and always has a low n = 1–7 range. Furthermore, in the nonlinear phase of the ELM n does not show a clear trend with most peeling-ballooning relevant parameters such as normalized pressure gradient, bootstrap current density or triangularity, whereas a strong decrease of n with edge safety factor q95 is observed in agreement with nonlinear modeling in the here investigated high collisionality region. A simple geometric model is presented, which is capable of explaining the q scaling by existence of ballooned structures that minimize n.

Estimating the performance of lithium beam measurements of current density and electron density in an H-mode pedestal.

The lithium beam is an effective diagnostic tool for investigation of stability and particle transport in the pedestal. It was used successfully to measure edge current density on DIII-D, achieving qualitative agreement with neoclassical models. Electron density profiles were also measured. Proposed upgrades will continue these measurements with higher reliability as well as explore edge current measurements using spectroscopy. The optics will be redesigned to optimize throughput and aperture broadening and to replace the photomultiplier tubes with avalanche photodiodes. The new system will yield detailed measurements of the pedestal, complementing existing diagnostics for investigating pedestal stability, edge localized mode cycle, and particle transport through the pedestal.

Ion heat transport dynamics during edge localized mode cycles at ASDEX Upgrade

The edge ion heat transport is analyzed in ASDEX Upgrade (AUG) by combining a comprehensive set of pedestal measurements with both interpretive and predictive modelling. The experimentally determined ion heat diffusivities, , are compared with neoclassical theory and the impact of edge localized modes (ELMs) on the edge ion heat transport level is studied in detail. Pedestal matching experiments in deuterium and hydrogen plasmas show that the inter-ELM pedestal remains close to the neoclassical value. The additional power needed in hydrogen to get similar pedestal temperatures as in deuterium plasmas mostly affects the electron heat channel, i.e. the electron heat diffusivity increases while the ion heat diffusivity stays at the same level within the uncertainties. Sub-ms measurements of the edge ion temperature allows us to extend the analysis to the entire ELM cycle. During the ELM crash, the ion heat transport is increased by an order of magnitude. The perturbed heat flux increases first at the separatrix, i.e. first the separatrix ion temperature increases, leading to a flatter ion temperature gradient, followed by a decrease of the whole pedestal profile. The ion heat transport returns to its pre-ELM neoclassical level 3–4 ms after the ELM crash.

Nonlinear coupling induced toroidal structure of edge localized modes

Edge localized modes (ELMs) are magnetohydrodynamic (MHD) instabilities that cause fast periodic relaxations of the strong edge pressure gradient in tokamak fusion plasmas. A novel diagnostic method allows the extraction of toroidal mode numbers, rotation velocities and spatial information during the ELM cycle including the crash. While mode number branches –6 and –10 are dominant just before the ELM crash, during the ELM crash –5 are observed in typical discharges with type-I ELMs in the tokamak experiment. These findings are compared to results from nonlinear MHD simulations. Although is linearly dominant, nonlinear coupling in which is particularly important leads to the dominance of –5 during the ELM crash, in excellent agreement with experimental observations. The simultaneous occurrence of these modes over a wide radial region leads to high stochasticity and thus increased transport.

Pedestal and Er profile evolution during an edge localized mode cycle at ASDEX Upgrade

The upgrade of the edge charge exchange recombination spectroscopy diagnostic at ASDEX Upgrade has enabled highly spatially resolved measurements of the impurity ion dynamics during an edge-localized mode cycle (ELM) with unprecedented temporal resolution, i.e. 65 μs. The increase of transport during an ELM induces a relaxation of the ion, electron edge gradients in impurity density and flows. Detailed characterization of the recovery of the edge temperature gradients reveals a difference in the ion and electron channel: the maximum ion temperature gradient is re-established on similar timescales as , which is faster than the recovery of . After the clamping of the maximum gradient, Ti and Te at the pedestal top continue to rise up to the next ELM while ne stays constant which means that the temperature pedestal and the resulting pedestal pressure widen until the next ELM. The edge radial electric field Er at the ELM crash is found to reduce to typical L-mode values and its maximum recovers to its pre-ELM conditions on a similar time scale as for ne and Ti. Within the uncertainties, the measurements of Er align with their neoclassical predictions for most of the ELM cycle, thus indicating that Er is dominated by collisional processes. However, between 2 and 4 ms after the ELM crash, other contributions to flow, e.g. zonal flows or ion orbit effects, could not be excluded within the uncertainties.

Toroidal mode number determination of ELM associated phenomena on ASDEX Upgrade

In highly confined tokamak plasmas periodically appearing edge localized modes (ELMs) are accompanied by mode-like magnetohydrodynamic (MHD) activities with defined toroidal mode numbers.Here the method of determining toroidal mode numbers n on the ASDEX Upgrade tokamak with a toroidally spread magnetic pick-up coil array is reviewed and improved by taking into account intrinsic coil phases. ELM synchronization is used to characterize inter-ELM MHD activity and their development during the ELM cycle in terms of their mode numbers. The mode number development is correlated with the development of the pedestal parameters which shows that the inter-ELM modes cause transport across the pedestal. An estimation of the position of the modes is done via a comparison between the mode velocities and the plasma rotation profile at the edge.Results show that during the ELM cycle MHD modes appear at several positions in the strong gradient region with clearly defined toroidal structures in the range of n = 1–10. These structures of inter-ELM modes are preserved during the ELM crash where also a strong n = 0 phenomenon occurs.

Toward integrated multi-scale pedestal simulations including edge-localized-mode dynamics, evolution of edge-localized-mode cycles, and continuous fluctuations

The high-fidelity BOUT++ two-fluid code suite has demonstrated significant recent progress toward integrated multi-scale simulations of tokamak pedestal, including Edge-Localized-Mode (ELM) dynamics, evolution of ELM cycles, and continuous fluctuations, as observed in experiments. Nonlinear ELM simulations show three stages of an ELM event: (1) a linear growing phase; (2) a fast crash phase; and (3) a slow inward turbulence spreading phase lasting until the core heating flux balances the ELM energy loss and the ELM is terminated. A new coupling/splitting model has been developed to perform simulations of multi-scale ELM dynamics. Simulation tracks five ELM cycles for 10 000 Alfvén times for small ELMs. The temporal evolution of the pedestal pressure is similar to that of experimental measurements for the pedestal pressure profile collapses and recovers to a steep gradient during ELM cycles. To validate BOUT++ simulations against experimental data and develop physics understanding of the fluctuation characteristics for different tokamak operation regimes, both quasi-coherent fluctuations (QCFs) in ELMy H-modes and Weakly Coherent Modes in I-modes have been simulated using three dimensional 6-field 2-fluid electromagnetic model. The H-mode simulation results show that (1) QCFs are localized in the pedestal region having a predominant frequency at f≃300−400 kHz and poloidal wavenumber at kθ≃0.7 cm−1, and propagate in the electron diamagnetic direction in the laboratory frame. The overall signatures of simulation results for QCFs show good agreement with C-Mod and DIII-D measurements. (2) The pedestal profiles giving rise to QCFs are near the marginal instability threshold for ideal peeling-ballooning modes for both C-Mod and DIII-D, while the collisional electromagnetic drift-Alfvén wave appears to be dominant for DIII-D. (3) Particle diffusivity is either smaller than the heat diffusivity for DIII-D or similar to the heat diffusivity for C-Mod. Key I-mode simulation results are that (1) a strong instability exists at n≥20 for resistive ballooning mode and drift-Alfvén wave; (2) the frequency spectrum of nonlinear BOUT++ simulation features a peak around 300 kHz for the mode number n = 20, consistent with a reflectometer measurement at nearby position; (3) the calculated particle diffusivity is larger than the calculated heat diffusivity, which is consistent with a key feature of the I-mode pedestal with no particle barrier.

Excitation of edge plasma instabilities and their role in pedestal saturation in the HL-2A tokamak

In HL-2A, the characteristics of the edge plasma instabilities and their effects on the dynamical evolution of the pedestal in H-mode plasmas have been investigated. In the edge pedestal region with steep pressure gradient, a quasi-coherent mode (QCM) has been observed in density fluctuations with a frequency range of 50–100 kHz. It appears during the edge localized mode (ELM)-free period after the L–H transition and prior to the first ELM. A threshold in the pedestal density gradient has been identified for the excitation of this mode. The QCM can also be observed during inter-ELM periods. It is excited early in the inter-ELM period, and disappears when the ELM onset starts. The radial wave-number of the mode is estimated with two radially separated reflectometers. It shows that the mode is radially propagating inward. The poloidal wave number estimated with the Langmuir probes is kθ ~ 0.43 cm−1. The mode propagates poloidally in the electron diamagnetic direction in the plasma frame. The toroidal mode number, deduced from Mirnov signals, is n ~ 7. The corresponding poloidal mode number is m ~ 21 according to the local safety factor value. The analysis for the dynamical evolution of the pedestal during the ELM cycle clearly shows that the mode is excited before the ELM onset. During and after the ELM crash, the mode disappears. It suggests that the QCM is driven by the pedestal density gradient, and the mode in return regulates the pedestal density evolution.

Intermediate-k density and magnetic field fluctuations during inter-ELM pedestal evolution in MAST

Measurements of local density and magnetic field fluctuations near the pedestal top, conditionally averaged over the edge localized mode (ELM) cycle, have been made in Mega Amp Spherical Tokamak (MAST). A Doppler backscattering (DBS) system installed at MAST was used to measure intermediate-k density fluctuations at the top of the pedestal. A novel diagnostic technique combining DBS with cross-polarization scattering (CP-DBS) enabled magnetic field fluctuations to also be locally measured at similar wave numbers. Polarization isolation and other effects for CP-DBS are discussed. Both measurements were used in a series of high-β –4.5) MAST plasmas with large type-I ELMs with an period where microtearing modes (MTMs) had been predicted to be unstable in similar conditions (Dickinson 2012 Phys. Rev. Lett. 108 135002). The measured density fluctuation level increased by a factor of about 4 between 2 and 4 ms after the ELM, which was correlated with the recovery of the density profile while the temperature pedestal height continued to increase slowly. Magnetic field fluctuations showed different temporal behaviors, slowly increasing throughout the ELM cycle as the local β increased. Linear GS2 calculations show both MTM and electron temperature gradient (ETG) modes unstable at similar wave numbers as the measurements (although with more overlap between ETG wave numbers and diagnostic spectral resolution) at the top of the pedestal, along with kinetic ballooning modes are unstable lower in the pedestal (at larger wavelengths). The inferred ratio of fluctuation levels from experiment was . The comparable ratios from GS2 were for the MTM and for the ETG. Both the experimental wave number range and the fluctuation ratio are more similar to the linear characteristics of the ETG than the MTM. These results imply that intermediate-k fluctuations due to the ETG play a role in inter-ELM pedestal evolution.

Electron kinetics inferred from observations of microwave bursts during edge localized modes in the mega-amp spherical tokamak.

Recent measurements of microwave and x-ray emission during edge localized mode (ELM) activity in tokamak plasmas provide a fresh perspective on ELM physics. It is evident that electron kinetics, which are not incorporated in standard (fluid) models for the instability that drives ELMs, play a key role in the new observations. These effects should be included in future models for ELMs and the ELM cycle. The observed radiative effects paradoxically imply acceleration of electrons parallel to the magnetic field combined with rapid acquisition of perpendicular momentum. It is shown that this paradox can be resolved by the action of the anomalous Doppler instability which enables fast collective radiative relaxation, in the perpendicular direction, of electrons accelerated in the parallel direction by inductive electric fields generated by the initial ELM instability.