Steady state tokamak equilibria without current drive are found. This is made possible by including the potato bootstrap current close to the magnetic axis. Tokamaks with this class of equilibria do not need seed current or current drive, and are intrinsically steady state. {copyright} {ital 1997} {ital The American Physical Society}
Summary form only given. Numerical thermalization induced by discrete-particle effects was observed in particle-in-cell (PIC) simulations 1 , which show that the thermal relaxation time scales with N D2 and N D for one-dimensional (1D) and two-dimensional (2D) models 2 , respectively. The parameter N D denotes the number of particles in a Debye length (1D) or a Debye square (2D). However, it was found recently that the thermal relaxation time is anomalously shortened to scale with N D while adding the Monte-Carlo collisions in a 1D PIC simulation. 3,4 In this work, we examine the numerical thermalization in two-dimensional ES PIC simulations with the consideration of electron-ion collision and electron-neutral collision. 4 Our results show that the thermal relaxation time is less sensitive to the collision frequency as compared to the 1D cases, and the thermal relaxation time remains to scale with N D as predicted by the theoretical analysis using the Balescu-Lenard-Landau kinetic theory 5 .
X rays emitted by electrons channeling through a periodically distorted lattice are studied. The lattice distortion works as a wiggler. The intensity of the radiation due to the resonance scattering of the wiggler field can be made comparable with that of the channeling radiation.
Equilibrium and stability analyses have identified a class of tokamak configurations with conventional safety factor profiles (q0∼qmin≳1) at moderately high li(li∼1.0), and high normalized β(βN∼3.5–4.0), that are stable to the ideal n=1 kink without the requirement of wall stabilization. In contrast to previously identified high li, high βN equilibria, these configurations have high bootstrap current fractions (fBS∼50%–70%); they require only modest central current drive for maintaining steady state and are therefore compatible with advanced tokamak (AT) operation. Strong plasma shaping is crucial for achieving the high β and high bootstrap fraction simultaneously.
During 1998, the General Atomics (GA) ARIES-Spherical Torus (ST) team examined several critical issues related to the physics performance of the ARIES-ST design, and a number of suggestions were made concerning possible improvements in performance. These included specification of a reference plasma equilibrium, optimization about the reference equilibrium to achieve higher beta limits, examination of three possible schemes for plasma initiation, development of a detailed scenario for ramp-up of the plasma current and pressure to its full, final operating values, an assessment of the requirement for electron confinement, and several suggestions for divertor heat flux reduction. The reference equilibrium was generated using the TOQ code, with the specification of a 100%, self-consistent bootstrap current. The equilibrium has {beta} = 51%, 10% below the stability limit (a margin specified by the ARIES-ST study). In addition, a series of intermediate equilibria were defined, corresponding to the ramp-up scenario discussed. A study of the influence of shaping on ARIES-ST performance indicates that significant improvement in both kink and ballooning stability can be obtained by modest changes in the squareness of the plasma. In test equilibria the ballooning beta limit is increased from 58% to 67%. Also the maximum allowable plasma-wall separation for kink stability can be increased by 30%. Three schemes were examined for noninductive plasma initiation. These are helicity injection (HICD), electron cyclotron heating (ECH)-assisted startup, and inductive startup using only the external equilibrium coils. HICD startup experiments have been done on the HIT and CDX devices. ECH-assisted startup has been demonstrated on CDX-U and DIII-D. External coil initiation is based on calculations for a proposed DIII-D experiment. In all cases, plasma initiation and preparation of an approximately 0.3 MA plasma for ARIES-ST appears entirely feasible.
Electron cyclotron heating (ECH) is a useful tool in global transport and local confinement studies. Operational experience with the inside launch ECH system on DIII-D shows that reliable operations are possible with power densities up to 0.7 GW/m[sup 2] in vacuum waveguide. Global confinement is roughly predicted by the Rebut-Lallia or ITER-89P scaling law, but direct analysis indicates a nearly linear scaling with toroidal field not found in these scaling laws. Local transport studies with off-axis heating clearly show inward transport in the electron fluid. This implies that diffusive and critical gradient models cannot completely describe plasma transport.
