N. Tsujii

I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod

An improved energy confinement regime, I-mode, is studied in Alcator C-Mod, a compact high-field divertor tokamak using ion cyclotron range of frequencies (ICRFs) auxiliary heating. I-mode features an edge energy transport barrier without an accompanying particle barrier, leading to several performance benefits. H-mode energy confinement is obtained without core impurity accumulation, resulting in reduced impurity radiation with a high-Z metal wall and ICRF heating. I-mode has a stationary temperature pedestal with edge localized modes typically absent, while plasma density is controlled using divertor cryopumping. I-mode is a confinement regime that appears distinct from both L-mode and H-mode, combining the most favourable elements of both. The I-mode regime is investigated predominately with ion ∇B drift away from the active X-point. The transition from L-mode to I-mode is primarily identified by the formation of a high temperature edge pedestal, while the edge density profile remains nearly identical to L-mode. Laser blowoff injection shows that I-mode core impurity confinement times are nearly identical with those in L-mode, despite the enhanced energy confinement. In addition, a weakly coherent edge MHD mode is apparent at high frequency ~100–300 kHz which appears to increase particle transport in the edge. The I-mode regime has been obtained over a wide parameter space (BT = 3–6 T, Ip = 0.7–1.3 MA, q95 = 2.5–5). In general, the I-mode exhibits the strongest edge temperature pedestal (Tped) and normalized energy confinement (H98 > 1) at low q95 ( 4 MW). I-mode significantly expands the operational space of edge localized mode (ELM)-free, stationary pedestals in C-Mod to Tped ~ 1 keV and low collisionality , as compared with EDA H-mode with Tped . The I-mode global energy confinement has a relatively weak degradation with heating power; leading to increasing H98 with heating power.

Observations of core toroidal rotation reversals in Alcator C-Mod ohmic L-mode plasmas

Direction reversals of intrinsic toroidal rotation have been observed in Alcator C-Mod ohmic L-mode plasmas following modest electron density or toroidal magnetic field ramps. The reversal process occurs in the plasma interior, inside of the q = 3/2 surface. For low density plasmas, the rotation is in the co-current direction, and can reverse to the counter-current direction following an increase in the electron density above a certain threshold. Reversals from the co- to counter-current direction are correlated with a sharp decrease in density fluctuations with k(R) >= 2 cm(-1) and with frequencies above 70 kHz. The density at which the rotation reverses increases linearly with plasma current, and decreases with increasing magnetic field. There is a strong correlation between the reversal density and the density at which the global ohmic L-mode energy confinement changes from the linear to the saturated regime.

Scaling of the power exhaust channel in Alcator C-Mod

Parametric dependences of the heat flux footprint on the outer divertor target plate are explored in EDA H-mode and ohmic L-mode plasmas over a wide range of parameters with attached plasma conditions. Heat flux profile shapes are found to be independent of toroidal field strength, independent of power flow along magnetic field lines and insensitive to x-point topology (single-null versus double-null). The magnitudes and widths closely follow that of the “upstream” pressure profile, which are correlated to plasma thermal energy content and plasma current. Heat flux decay lengths near the strike-point in H- and L-mode plasmas scale approximately with the inverse of plasma current, with a diminished dependence at high collisionality in L-mode. Consistent with previous studies, pressure gradients in the boundary scale with plasma current squared, holding the magnetohydrodynamic ballooning parameter approximately invariant at fixed collisionality—strong evidence that critical-gradient transport physics plays ...

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified, since these processes occur in a particular range of the collisionality.

Studies of turbulence and transport in Alcator C-Mod H-mode plasmas with phase contrast imaging and comparisons with GYRO

Recent advances in gyrokinetic simulation of core turbulence and associated transport requires an intensified experimental effort to validate these codes using state of the art synthetic diagnostics to compare simulations with experimental data. A phase contrast imaging (PCI) diagnostic [M. Porkolab, J. C. Rost, N. Basse et al., IEEE Trans. Plasma Sci. 34, 229 (2006)] is used to study H-mode plasmas in Alcator C-Mod [M. Greenwald, D. Andelin, N. Basse et al., Nucl. Fusion 45, S109 (2005)]. The PCI system is capable of measuring density fluctuations with high temporal (2kHz–5MHz) and wavenumber (0.5–55cm−1) resolution. Recent upgrades have enabled PCI to localize the short wavelength turbulence in the electron temperature gradient range and resolve the direction of propagation (i.e., electron versus ion diamagnetic direction) of the longer wavelength turbulence in the ion temperature gradient (ITG) and trapped electron mode range. The studies focus on plasmas before and during internal transport barrier fo...

Observation of ion cyclotron range of frequencies mode conversion plasma flow drive on Alcator C-Mod

At modest H3e levels (n3He/ne∼8%–12%), in relatively low density D(H3e) plasmas, n¯e≤1.3×1020 m−3, heated with 50 MHz rf power at Bt0∼5.1 T, strong (up to 90 km/s) toroidal rotation (Vϕ) in the cocurrent direction has been observed by high-resolution x-ray spectroscopy on Alcator C-Mod. The change in central Vϕ scales with the applied rf power (≤30 km s−1 MW−1), and is generally at least a factor of 2 higher than the empirically determined intrinsic plasma rotation scaling. The rotation in the inner plasma (r/a≤0.3) responds to the rf power more quickly than that of the outer region (r/a≥0.7), and the rotation profile is broadly peaked for r/a≤0.5. Localized poloidal rotation (0.3≤r/a≤0.6) in the ion diamagnetic drift direction (∼2 km/s at 3 MW) is also observed, and similarly increases with rf power. Changing the toroidal phase of the antenna does not affect the rotation direction, and it only weakly affects the rotation magnitude. The mode converted ion cyclotron wave (MC ICW) has been detected by a pha...

Studies of turbulence and transport in Alcator C-Mod ohmic plasmas with phase contrast imaging and comparisons with gyrokinetic simulations

Recent advances in gyrokinetic simulation have allowed for quantitative predictions of core turbulence and associated transport. However, numerical codes must be tested against experimental results in both turbulence and transport. In this paper, we present recent results from ohmic plasmas in the Alcator C-Mod tokamak using phase contrast imaging (PCI) diagnostic, which is capable of measuring density fluctuations with wave numbers up to 55 cm −1 . The experiments were carried out over the range of densities covering the ‘neo-Alcator’ (linear confinement time scaling with density, electron transport dominates) to the ‘saturated ohmic’ regime. We have also simulated these plasmas with the gyrokinetic code GYRO and compared numerical predictions with experimentally measured turbulence through a synthetic PCI diagnostic method. The key role played by the ion temperature gradient (ITG) turbulence has been verified, including measurements of turbulent wave propagation in the ion diamagnetic direction. It is found that the intensity of density fluctuations increases with density, in agreement between simulation and experiments. The absolute fluctuation intensity agrees with the simulation within experimental error (±60%). In the saturated ohmic regime, the simulated ion and electron thermal diffusivities also agree with experiments after varying the ion temperature gradient within experimental uncertainty. However, in the linear ohmic regime, GYRO does not agree well with experiments, showing significantly larger ion thermal transport and smaller electron thermal transport. Our study shows that although the short wavelength turbulence in the electron temperature gradient (ETG) range is unstable in the linear ohmic regime, the nonlinear simulation with kθ ρs up to 4 does not raise the electron thermal diffusivity to the experimental level, where kθ is the poloidal wavenumber and

20 years of research on the Alcator C-Mod tokamak

The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only high-power radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing components—approaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated the critical role of cross-field transport in divertor operation, edge flows and the tokamak density limit. C-Mod developed the I-mode and the Enhanced Dα H-mode regimes, which have high performance without large edge localized modes and with pedestal transport self-regulated by short-wavelength electromagnetic waves. C-Mod has carried out pioneering studies of intrinsic rotation and demonstrated that self-generated flow shear can be strong enough in some cases to significantly modify transport. C-Mod made the first quantitative link between the pedestal temperature and the H-modes performance, showing that the observed self-similar temperature profiles were consistent with critical-gradient-length theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) mode-conversion, ICRF flow drive, demonstration of lower-hybrid current drive at ITER-like densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on C-Mod provided the first observation of non-axisymmetric halo currents and non-axisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included.

ICRF Mode Conversion Flow Drive on Alcator C-Mod

We have carried out a detailed study of ion cyclotron range of frequency (ICRF) mode conversion (MC) flow drive on the Alcator C-Mod tokamak including its dependence on plasma and RF parameters. The flow drive efficiency is found to strongly depend on the 3He concentration in D(3He) plasmas, a key parameter separating the ICRF minority heating regime and MC regime. At +90° antenna phasing (waves in co-Ip direction) and dipole phasing (waves symmetrical in both directions), we find that ΔV0, the change in the core toroidal rotation velocity, is in the co-Ip direction, increases with RF power and with Ip (opposite to the 1/Ip intrinsic rotation scaling). The flow drive efficiency decreases at higher plasma density and also at higher antenna frequency. The observed flow drive efficiency in H-mode has been small due to the unfavourable density scaling. The flow drive effect at −90° phasing appears to be saturated or decrease at high RF power. The up–down asymmetry in the MC to the ion cyclotron wave may be the key to understand the flow drive mechanism.

Measurements of ion cyclotron range of frequencies mode converted wave intensity with phase contrast imaging in Alcator C-Mod and comparison with full-wave simulations

Radio frequency waves in the ion cyclotron range of frequencies (ICRF) are widely used to heat tokamak plasmas. In ICRF heating schemes involving multiple ion species, the launched fast waves convert to ion cyclotron waves or ion Bernstein waves at the two-ion hybrid resonances. Mode converted waves are of interest as actuators to optimise plasma performance through current drive and flow drive. In order to describe these processes accurately in a realistic tokamak geometry, numerical simulations are essential, and it is important that these codes be validated against experiment. In this study, the mode converted waves were measured using a phase contrast imaging technique in D-H and D-3He plasmas. The measured mode converted wave intensity in the D-3He mode conversion regime was found to be a factor of ∼50 weaker than the full-wave predictions. The discrepancy was reduced in the hydrogen minority heating regime, where mode conversion is weaker.