J. Hosea

Initial results from coaxial helicity injection experiments in NSTX

Coaxial helicity injection has been investigated on the National Spherical Torus Experiment (NSTX). Initial experiments produced 130 kA of toroidal current without the use of the central solenoid. The corresponding injector current was 20 kA. Discharges with pulse lengths up to 130 ms have been produced.

Angular distribution of the bremsstrahlung emission during lower hybrid current drive on PLT

The bremsstrahlung emission from the PLT tokamak during lower-hybrid current drive has been measured as a function of angle between the magnetic field and the emission direction. The emission is peaked strongly in the forward direction, indicating a strong anisotropy of the electron velocity distribution. The data demonstrate the existence of a nearly flat tail of the velocity distribution, which extends out to approximately 500 keV and which is interpreted as the plateau created by Landau damping of the lower-hybrid waves.

Wave-particle studies in the ion cyclotron and lower hybrid ranges of frequencies in alcator C-mod

Abstract This paper reviews the physics and technology of wave-particle-interaction experiments in the ion cyclotron range of frequencies (ICRF) and the lower hybrid (LH) range of frequencies (LHRF) on the Alcator C-Mod tokamak. Operation of fixed frequency (80 MHz) and tunable (40- to 80-MHz) ICRF transmitters and the associated transmission system is described. Key fabrication issues that were solved in order to operate a four-strap ICRF antenna in the compact environment of C-Mod are discussed in some detail. ICRF heating experiments utilizing the hydrogen (H) and helium-3 (3He) minority heating schemes are described, and data are presented demonstrating an overall heating efficiency of 70 to 90% for the (H) minority scheme and somewhat lower efficiency for (3He) minority heating. Mode conversion electron heating experiments in D(3He), D(H), and H(3He) discharges are also reported as well as simulations of these experiments using an advanced ICRF full-wave solver. Measurements of mode-converted ion cyclotron waves and ion Bernstein waves using a phase contrast imaging diagnostic are presented and compared with the predictions of a synthetic diagnostic code that utilizes wave electric fields from a full-wave solver. The physics basis of the LH current profile control program on Alcator C-Mod is also presented. Computer simulations using a two-dimensional (velocity space) Fokker Planck solver indicate that ~200 kA of LH current can be driven in low-density H-mode discharges on C-Mod with ~3 MW of LHRF power. It is shown that this off-axis LH current drive can be used to create discharges with nonmonotonic profiles of the current density and reversed shear. An advanced tokamak operating regime near the ideal no-wall β limit is described for C-Mod, where ~70% of the current is driven through the bootstrap effect. The LH power is coupled to C-Mod through a waveguide launcher consisting of four rows (vertically) with 24 guides per row (toroidally). A detailed description of the LH launcher fabrication is given in this paper along with initial operation results.

Fusion neutron production during deuterium neutral-beam injection into the PLT tokamak

Fusion neutron emission of 1.5 × 1014 neutrons s−1 and 2 × 1013 neutrons/pulse has been observed for PLT deuterium discharges with up to 2.5 MW of deuterium neutral-beam injection. The neutron time evolution and magnitude are consistent with theoretical calculations of the fusion reactions caused by energetic injected ions which are confined and slow down classically. The factor-of-two accuracy in the absolute neutron calibration is the major uncertainty in the comparison with theory. Neutron sawtooth oscillations ( 3%) are observed which can also be explained classically.

Initial physics results from the National Spherical Torus Experiment

The mission of the National Spherical Torus Experiment (NSTX) is to extend the understanding of toroidal physics to low aspect ratio (R/a approximately equal to 1.25) in low collisionality regimes. NSTX is designed to operate with up to 6 MW of High Harmonic Fast Wave (HHFW) heating and current drive, 5 MW of Neutral Beam Injection (NBI) and Co-Axial Helicity Injection (CHI) for non-inductive startup. Initial experiments focused on establishing conditions that will allow NSTX to achieve its aims of simultaneous high-bt and high-bootstrap current fraction, and to develop methods for non-inductive operation, which will be necessary for Spherical Torus power plants. Ohmic discharges with plasma currents up to 1 MA and with a range of shapes and configurations were produced. Density limits in deuterium and helium reached 80% and 120% of the Greenwald limit respectively. Significant electron heating was observed with up to 2.3 MW of HHFW. Up to 270 kA of toroidal current for up to 200 msec was produced noninductively using CHI. Initial NBI experiments were carried out with up to two beam sources (3.2 MW). Plasmas with stored energies of up to 140 kJ and bt =21% were produced.

Modelling of the electron distribution based on bremsstrahlung emission during lower-hybrid current drive on PLT

Lower-hybrid current drive requires the generation of a high-energy electron tail anisotropic in velocity. Measurements of bremsstrahlung emission produced by this tail are compared with the calculated emission from reasonable model distributions. The physical basis and the sensitivity of this modelling process are described, and the plasma properties of current-driven discharges which can be derived from the model are discussed.

Overview of the Alcator C-Mod program

Research on the Alcator C-Mod tokamak has emphasized RF heating, self-generated flows, momentum transport, scrape-off layer (SOL) turbulence and transport and the physics of transport barrier transitions, stability and control. The machine operates with P-RF up to 6 MW corresponding to power densities on the antenna of 10 MW m(-2). Analysis of rotation profile evolution, produced in the absence of external drive, allows transport of angular momentum in the plasma core to be computed and compared between various operating regimes. Momentum is clearly seen diffusing and convecting from the plasma edge on time scales similar to the energy confinement time and much faster than neo-classical transport. SOL turbulence and transport have been studied with fast scanning electrostatic probes situated at several poloidal locations and with gas puff imaging. Strong poloidal asymmetries are found in profiles and fluctuations, confirming the essential ballooning character of the turbulence and transport. Plasma topology has a dominant effect on the magnitude and direction of both core rotation and SOL flows. The correlation of self-generated plasma flows and topology has led to a novel explanation for the dependence of the H-mode power threshold on the del B drift direction. Research into internal transport barriers has focused on control of the barrier strength and location. The foot of the barrier could be moved to larger minor radius by lowering q or B-T. The barriers, which are produced in C-Mod by off-axis RF heating, can be weakened by the application of on-axis power. Gyro-kinetic simulations suggest that the control mechanism is due to the temperature dependence of trapped electron modes which are destabilized by the large density gradients. A set of non-axisymmetric coils was installed allowing intrinsic error fields to be measured and compensated. These also enabled the determination of the mode locking threshold and, by comparison with data from other machines, provided the first direct measurement of size scaling for the threshold. The installation of a new inboard limiter resulted in the reduction of halo currents following disruptions. This effect can be understood in terms of the change in plasma contact with the altered geometry during vertical displacement of the plasma column. Unstable Alfven eigenmodes (AE) were observed in low-density, high-power ICRF heated plasmas. The damping rate of stable AEs was investigated with a pair of active MHD antennae.

Ion cyclotron range of frequencies stabilization of sawteeth on Tokamak Fusion Test Reactor

Results obtained from experiments utilizing high‐power ion cyclotron range of frequencies (ICRF) heating to stabilize sawtooth oscillations on Tokamak Fusion Test Reactor (TFTR) [Hawryluk et al., Plasma Phys. Controlled Fusion 33, 1509 (1991)] are reviewed. The key observations include existence of a minimum ICRF power required to achieve stabilization, a dependence of the stabilization threshold on the relative size of the ICRF power deposition profile to the q=1 volume, and a peaking of the equilibrium pressure and current profiles during sawtooth‐free phases of the discharges. In addition, preliminary measurements of the poloidal magnetic field profile indicate that q on axis decreases to a value of 0.55±0.15 after a sawtooth‐stabilized period of ∼0.5 sec has transpired. The results are discussed in the context of theory, which suggests that the fast ions produced by the ICRF heating suppress sawteeth by stabilizing the m=1 magnetohydrodynamic (MHD) instabilities believed to be the trigger for the sawt...

Spectral effects on fast wave core heating and current drive

Recent results obtained with high harmonic fast wave (HHFW) heating and current drive (CD) on NSTX strongly support the hypothesis that the onset of perpendicular fast wave propagation right at or very near the launcher is a primary cause for a reduction in core heating efficiency at long wavelengths that is also observed in ICRF heating experiments in numerous tokamaks. A dramatic increase in core heating efficiency was first achieved in NSTX L-mode helium majority plasmas when the onset for perpendicular wave propagation was moved away from the antenna and nearby vessel structures. Efficient core heating in deuterium majority L-mode and H-mode discharges, in which the edge density is typically higher than in comparable helium majority plasmas, was then accomplished by reducing the edge density in front of the launcher with lithium conditioning and avoiding operational points prone to instabilities. These results indicate that careful tailoring of the edge density profiles in ITER should be considered to limit radio frequency (rf) power losses to the antenna and plasma facing materials. Finally, in plasmas with reduced rf power losses in the edge regions, the first direct measurements of HHFW CD were obtained with the motional Stark effect (MSE) diagnostic. The location and radial dependence of HHFW CD measured by MSE are in reasonable agreement with predictions from both full wave and ray tracing simulations.

Fast-ion orbit effects during ion cyclotron range of frequency experiments on the Princeton Large Torus

The angular dependence of the flux of fast neutrals was measured in conjunction with Ion Cyclotron Range of Frequency experiments on the Princeton Large Torus. A regime was found where the energetic tail of the charge-exchange neutral flux was strongly anisotropic. Unexpectedly, however, the flux from co-going ions was peaked at an angle intermediate between perpendicular and parallel. This could be the consequence of a preferential filling of that class of banana orbits whose tips are located within the ion cyclotron resonance layer.