M. Podesta

Snowflake divertor configuration studies in National Spherical Torus Experimenta)

Experimental results from NSTX indicate that the snowflake divertor (D. Ryutov, Phys. Plasmas 14, 064502 (2007)) may be a viable solution for outstanding tokamak plasma-material interface issues. Steady-state handling of divertor heat flux and divertor plate erosion remains to be critical issues for ITER and future concept devices based on conventional and spherical tokamak geometry with high power density divertors. Experiments conducted in 4–6 MW NBI-heated H-mode plasmas in NSTX demonstrated that the snowflake divertor is compatible with high-confinement core plasma operation, while being very effective in steady-state divertor heat flux mitigation and impurity reduction. A steady-state snowflake divertor was obtained in recent NSTX experiments for up to 600 ms using three divertor magnetic coils. The high magnetic flux expansion region of the scrape-off layer (SOL) spanning up to 50% of the SOL width λq was partially detached in the snowflake divertor. In the detached zone, the heat flux profile flatt...

Electrostatic instabilities, turbulence and fast ion interactions in the TORPEX device

Electrostatic turbulence, related structures and their effect on particle, heat and toroidal momentum transport are investigated in TORPEX simple magnetized plasmas using high-resolution diagnostics, control parameters, linear fluid models and nonlinear numerical simulations. The nature of the dominant instabilities is controlled by the value of the vertical magnetic field, Bv, relative to that of the toroidal field, BT. For Bv/BT > 3%, only ideal interchange instabilities are observed. A critical pressure gradient to drive the interchange instability is experimentally identified. Interchange modes give rise to blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from electrostatic probe measurements using pattern recognition methods. The observed values span a wide range and are described by a single analytical expression, from the small blob size regime in which the blob velocity is limited by cross-field ion polarization currents, to the large blob size regime in which the limitation to the blob velocity comes from parallel currents to the sheath. As a first attempt at controlling the blob dynamical properties, limiter configurations with varying angles between field lines and the conducting surface of the limiter are explored. Mach probe measurements clearly demonstrate a link between toroidal flows and blobs. To complement probe data, a fast framing camera and a movable gas puffing system are installed. Density and light fluctuations show similar signatures of interchange activity. Further developments of optical diagnostics, including an image intensifier and laser-induced fluorescence, are under way. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector in scenarios for which the development from linear waves into blobs is fully characterized. A theory validation project is conducted in parallel with TORPEX experiments, based on quantitative comparisons of observables that are defined in the same way in the data and in the output of numerical codes, including 2D and 3D local and global simulations.

Energetic Particle Instabilities in Fusion Plasmas

Remarkable progress has been made in diagnosing energetic particle instabilities on present-day machines and in establishing a theoretical framework for describing them. This overview describes the much improved diagnostics of Alfven instabilities and modelling tools developed world-wide, and discusses progress in interpreting the observed phenomena. A multi-machine comparison is presented giving information on the performance of both diagnostics and modelling tools for different plasma conditions outlining expectations for ITER based on our present knowledge.

High spatial sampling global mode structure measurements via multichannel reflectometry in NSTX

Global modes?including kinks and tearing modes (f ~ 400?kHz)?play critical roles in many aspects of plasma performance. Their investigation on NSTX is aided by an array of fixed-frequency quadrature reflectometers used to determine their radial density perturbation structure. The array has been recently upgraded to 16 channels spanning 30?75?GHz (ncutoff = (1.1?6.9) ? 1019?m?3 in O-mode), improving spatial sampling and access to the core of H-mode plasmas. The upgrade has yielded significant new results that advance the understanding of global modes in NSTX. The GAE and CAE structures have been measured for the first time in the core of an NSTX high-power (6?MW) beam-heated H-mode plasma. The CAE structure is strongly core-localized, which has important implications for electron thermal transport. The TAE structure has been measured with greatly improved spatial sampling, and measurements of the TAE phase, the first in NSTX, show strong radial variation near the midplane, indicating radial propagation caused by non-ideal MHD effects. Finally, the tearing mode structure measurements provide unambiguous evidence of coupling to an external kink.

The dependence of H-mode energy confinement and transport on collisionality in NSTX

Understanding the dependence of confinement on collisionality in tokamaks is important for the design of next-step devices, which will operate at collisionalities at least one order of magnitude lower than in the present generation. A wide range of collisionality has been obtained in the National Spherical Torus Experiment (NSTX) by employing two different wall conditioning techniques, one with boronization and between-shot helium glow discharge conditioning (HeGDC+B), and one using lithium evaporation (Li EVAP). Previous studies of HeGDC+B plasmas indicated a strong increase of normalized confinement with decreasing collisionality. Discharges with lithium conditioning discussed in the present study generally achieved lower collisionality, extending the accessible range of collisionality by a factor of two. While the confinement dependences on dimensional, engineering variables of the HeGDC+B and Li EVAP datasets differed, collisionality was found to unify the trends, with the lower collisionality lithium conditioned discharges extending the trend of increasing normalized confinement time, BTτE, with decreasing collisionality when other dimensionless variables were held as fixed as possible. This increase of confinement with decreasing collisionality was driven by a large reduction in electron transport in the outer region of the plasma. This result is consistent with gyrokinetic calculations that show microtearing and electron temperature gradient (ETG) modes to be more stable for the lower collisionality discharges. Ion transport, near neoclassical at high collisionality, became more anomalous at lower collisionality, possibly due to the growth of hybrid TEM/KBM modes in the outer regions of the plasma.

Non-linear dynamics of toroidicity-induced Alfvén eigenmodes on the National Spherical Torus Experiment

The National Spherical Torus Experiment (NSTX, [M. Ono et al., Nucl. Fusion 40, 557 (2000)]) routinely operates with neutral beam injection as the primary system for heating and current drive. The resulting fast ion population is super-Alfv#19;enic, with velocities 1 < vfast=vAlfven < 5. This provides a strong drive for toroidicity-induced Alfv#19;en eigenmodes (TAEs). As the discharge evolves, the fast ion population builds up and TAEs exhibit increasing bursts in amplitude and down-chirps in frequency, which eventually lead to a so-called TAE avalanche. Avalanches cause large (≤ 30%) fast ion losses over ~ 1 ms, as inferred from the neutron rate. The increased fast ion losses correlate with a stronger activity in the TAE band. In addition, it is shown that a n = 1 mode with frequency well below the TAE gap appears in the Fourier spectrum of magnetic fluctuations as a result of non-linear mode coupling between TAEs during avalanche events. The non-linear coupling between modes, which leads to enhanced fast ion transport during avalanches, is investigated.

Core transport of lithium and carbon in ELM-free discharges with lithium wall conditioning in NSTX

Core transport of intrinsic carbon and lithium impurities is analysed in H-mode discharges in NSTX. The application of lithium coatings on graphite plasma-facing components led to high-performance H-mode discharges with edge localized mode (ELM) suppression and resulted in core carbon accumulation. Lithium ions did not accumulate and had densities less than 1% of carbon densities. Core transport codes NCLASS, NEO and MIST are used to assess the impact of lithium evaporative coatings on impurity transport. The disappearance of ELMs, due to changes in the electron pressure profiles, together with modifications in neoclassical transport, due to changes in main ion temperature and density profiles, explains the core carbon accumulation in discharges with lithium coatings. Residual anomalous transport in the pedestal region is needed to explain the experimental carbon density profile shape and evolution. The enhancement in neoclassical lithium particle diffusivities due to the high carbon concentration is partially responsible for the low lithium core concentration.

Measurements of core lithium concentration in a Li-conditioned tokamak with carbon walls

The National Spherical Torus Experiment (NSTX Ono et al 2000 Nucl. Fusion 40 557) is exploring the use of lithium as candidate plasma-facing material to handle the large power flux to the wall of fusion devices. This paper reports on the measurements of lithium concentration in the plasma core during the 2010 NSTX experimental campaign, during which 1.3?kg of lithium was evaporated into the NSTX vessel. It is shown that lithium does not accumulate in significant amounts inside the plasma, resulting in an upper bound for the measured lithium concentration that is well below 0.1% of the electron density for a broad range of experimental conditions. Carbon, which constitutes the primary material of the NSTX inner wall, remains the dominant plasma impurity even after large amounts of lithium, of the order of hundreds of milligrams, are evaporated into the vacuum vessel.

Advances in high-harmonic fast wave physics in the National Spherical Torus Experiment

Improved core high-harmonic fast wave (HHFW) heating at longer wavelengths and during start-up and plasma current ramp-up has now been obtained by lowering the edge density with lithium wall conditioning, thereby moving the critical density for perpendicular fast-wave propagation away from the vessel wall. Lithium conditioning allowed significant HHFW core electron heating of deuterium neutral beam injection (NBI) fuelled H-mode plasmas to be observed for the first time. Large edge localized modes were observed immediately after the termination of rf power. Visible and infrared camera images show that fast wave interactions can deposit considerable rf energy on the outboard divertor. HHFW-generated parametric decay instabilities were observed to heat ions in the plasma edge and may be the cause for a measured drag on edge toroidal rotation during HHFW heating. A significant enhancement in neutron rate and fast-ion profile was measured in NBI-fuelled plasmas when HHFW heating was applied.

Fast-ion energy loss during TAE avalanches in the National Spherical Torus Experiment

Strong toroidal Alfven eigenmode (TAE) avalanches on NSTX, the National Spherical Torus Experiment (Ono et al 2000 Nucl. Fusion 40 557) are typically correlated with drops in the neutron rate in the range 5–15%. In previous studies of avalanches in L-mode plasmas, these neutron drops were found to be consistent with modelled losses of fast ions. Here we expand the study to TAE avalanches in NSTX H-mode plasmas with improved analysis techniques. At the measured TAE mode amplitudes, simulations with the ORBIT code predict that fast ion losses are negligible. However, the simulations predict that the TAE scatter the fast ions in energy, resulting in a small (≈5–6%) drop in fast ion β. The net decrease in energy of the fast ions is sufficient to account for about 50% of the drop in neutron rate, redistribution for ≈40%, and fast ion losses account for only ≈10%. This loss of energy from the fast ion population is comparable to the estimated energy lost by damping from the Alfven wave during the burst. The previously studied TAE avalanches in L-mode are re-evaluated using an improved calculation of the potential fluctuations in the ORBIT code near the separatrix.