Jung-soo Kim

100% noninductive operation at high beta using off-axis ECCD in DIII-D

The Advanced Tokamak (AT) program on DIII-D is developing the scientific basis for steady-state, high-performance operation in future devices. The key element of the program is to demonstrate sustainment of 100% noninductive current for several seconds at high beta. Guided by integrated modeling, recent experiments using up to 2.5 MW of off-axis electron cyclotron current drive (ECCD) and up to 15 MW neutral beam injection (NBI) with q95 ≈ 5 have sustained ≈100% of the plasma current noninductively for 1 s at high beta (β ≈ 3.6%, βN ≈ 3.4, above the no-wall limit) with qmin ≥ 1.5 and good confinement (H89 ≈ 2.3). Integrated modeling using both empirical and theory-based models is used to design experiments and to interpret their results. These experiments have achieved the parameters required for the ITER Q=5 steady-state scenario, and the same modeling tools are applied to ITER AT scenario development.

Control of plasma profiles in DIII-D discharges

Active control of plasma profiles is an essential requirement for operating within plasma stability limits, for steady-state operation and for optimization of the plasma performance. In DIII-D, plasma profiles have been actively controlled using various actuators in the following manner: (a) real time closed loop control of the q profile evolution using electron cyclotron heating and neutral beam injection as actuators; (b) active control of the density and pressure profiles in quiescent H-mode and quiescent double barrier plasmas using electron cyclotron current drive (ECCD) and pellet injection; (c) active control of the edge profiles to suppress edge localized modes using resonant magnetic perturbation with toroidal mode number n = 3, (d) real time control of the current density profile to suppress neoclassical tearing modes using localized deposition of co-ECCD.

Evidence for modified transport due to sheared E×B flows in high‐temperature plasmas

Sheared mass flows are generated in many fluids and are often important for the dynamics of instabilities in these fluids. Similarly, large values of the E×B velocity have been observed in magnetic confinement machines and there is theoretical and experimental evidence that sufficiently large shear in this velocity may stabilize important instabilities. Two examples of this phenomenon have been observed in the DIII–D tokamak. In the first example, sufficient heating power can lead to the L‐H transition (transition from low‐mode to high‐mode confinement), a rapid improvement in confinement in the boundary layer (narrow region just inside the last closed flux surface) of the plasma. For discharges with heating power close to the threshold required to get the transition, changes in the edge radial electric field are observed to occur prior to the transition itself. In the second example, certain classes of discharges with toroidal momentum input from neutral beam injection exhibit a further improvement of co...

The effect of a single blade limiter on energetic neutral beam particles in doublet III

Energetic beam ion collisions with the main limiter can be a significant power loss process under certain operating conditions in Doublet III. Furthermore, these collisions may cause measurable damage to the limiter itself. Under low current and low toroidal field conditions (e.g., Ip = 290 kA and BT = 6.3 kG), 20–38% of the inferred absorbed beam power may be deposited directly on the ion drift side of the limiter by the beam ions. However, for higher plasma current and toroidal fields (e.g., Ip = 290 kA and BT = 15 kG), the fraction of inferred absorbed beam power deposited on the limiter is reduced to < 10%. Monte Carlo code simulations show that this loss of beam power is primarily a result of the large poloidal and toroidal gyro-orbits of the energetic beam ions. Other factors which may enhance beam ion losses to the limiter are (1) large separation distances between the primary limiter and the (outboard) vacuum vessel wall, and (2) plasma density buildup near the plasma edge during high gas puff operation. In addition, our data suggests enhanced plasma density and recycling near the limiter. This localized density can cause appreciable premature ionizations of the incoming beam neutrals and thus reduce the effective plasma heating of the beamline which is immediately upcurrent of the limiter. The prematurely-ionized beam particles from this adjacent beamline are responsible for much of the damage to the ion drift side of the limiter. We have found that under certain operating conditions (1) the direct beam heating of the limiter is 50% greater and (2) the stored plasma energy is 10% less when the beamline immediately upcurrent of the limiter heats the plasma. Thus, the relative positions of the limiters to the beamlines are important in designing future tokamaks.