Optimum confinement of ECH plasmas in Heliotron E

Abstract

The optimum confinement of plasmas produced by electron cyclotron heating (ECH) in Heliotron E and some other significant transport characteristics of ECH plasmas have been studied experimentally by changing the magnetic field configuration. It is found that the plasma confinement can be improved by minor changes of the magnetic field configuration. The main results are as follows: (1) Plasmas with a minor radius reduced (by means of a limiter) to two thirds of the value in the original (non-limited) configuration have optimum confinement properties when the magnetic axis is shifted inward by 20 mm. (2) The electron pressure in an optimized plasma configuration (confirmed by neutral beam injection) is higher than that in a standard ECH plasma. (3) The insertion of a carbon limiter causes the plasma line density to increase, with no significant degradation of the core temperature or density. (4) Magnetic field perturbations hardly affect the temperature and density profiles, unless the magnetic axis is shifted outward by a large (50 mm) distance. (5) Transport analysis indicates that significant improvement in the confinement properties can be achieved by slightly manipulating the magnetic field configuration.

Electron beam acceleration and potential formation induced by the Compton scattering of extraordinary waves

“…, we can similarly find that (71) approximately coincides with (28). The ratio of acceleration rates for the Compton scattering of m = ±1 and m = 0 is also found to be given by (31), and it is very small since |k 2 ⊥ ν tb /k ω ce | Ӷ 1. Consequently, the acceleration and deceleration due to the Compton scattering of m = 0 are much stronger than those of m = ±1.…”

“…Here, ω ce is the electron cyclotron frequency. Consequently, the nonlinear scattering of the extraordinary wave may provide an effective means for an acceleration of high-energy or relativistic electrons [29][30][31][32][33][34][35].…”

Experiments on Li pellet injection into Heliotron E

“…The enhanced ablation rate in the edge plasma region may result from ablating by fast ions during a strong NBI pulse (near the inner and outer regions in figure 7(a)) or from the fast-electron population during a ECR pulse (near the outer region in figure 8(a)) which were not taken into account by the ablation model used. Note that experiments on hydrogen pellet injection into Heliotron E plasmas [15] showed a similar enhanced evaporation of the pellet due to both fast ions and electrons.…”