Speaker
Dr
John Ferron
(General Atomics)
Description
Increased confinement and ideal stability limits at relatively high values of the internal inductance (l_i) have enabled an attractive scenario for steady-state tokamak operation to be demonstrated in DIII-D. The potential of the scenario was shown in high elongation and triangularity double-null divertor discharges in which β_N>4.5 was achieved at l_i≈1.3. This high value of β_N just reached the ideal n=1 kink stability limit calculated without the effect of a stabilizing vacuum vessel wall, with the ideal-wall limit still higher at β_N>5.5. Confinement is above the H-mode level with H_98≈1.8. This type of discharge is a candidate for a reactor that could either operate stably at β_N≈4 without the requirement for a nearby conducting wall or n≥1 active stabilization coils, or at β_N≈5 with wall stabilization. With the high β_N and relatively high q_95=7, the discharge in the experiment is overdriven with bootstrap current fraction f_BS≈0.8, noninductive current fraction f_NI>1 and negative surface voltage. For ITER, operation at l_i≈1 is a promising option. Improved core confinement at high li could compensate for reduced H-mode pedestal confinement if a low pedestal height results from pedestal physics and/or ELM-stabilization using 3D fields. At l_i≈1, f_BS would be ≈0.5 with the remainder from external current driven efficiently near the axis. This scenario has been tested in the ITER shape in DIII-D at q_95=4.8, so far reaching f_NI=0.7 and f_BS=0.4 at β_N≈3.4 with performance appropriate for the ITER Q=5 mission, H_89 β_N/q_95^2 >0.3. High l_i discharges thus far take advantage of inductively driven current density near the axis as a partial substitute for externally-driven current. Studies with the FASTRAN transport code using the TGLF energy transport model explored how increased current drive power for DIII-D, 9 MW electron cyclotron current drive (ECCD) and 13 MW off-axis beam power, could be applied to maintain a stationary, fully noninductive high l_i discharge. Solutions are found at β_N=4, l_i=1.07, and f_BS=0.5 calculated stable without a conducting wall with ECCD and neutral beam current drive near the axis and at β_N=5 calculated to be stable with the vacuum vessel wall.
This work was supported by the US Department of Energy under DE-FC02-04ER54698, DE-AC52-07NA27344, DE-AC05-00OR22725, DE-AC02-09CH11466, and DE-FG02-04E54761.
| Country or International Organisation | USA |
|---|---|
| Paper Number | PPC/P2-35 |
Primary author
Dr
John Ferron
(General Atomics)
Co-authors
Dr
Christopher T. Holcomb
(Lawrence Livermore National Laboratory)
Dr
Egemen Kolemen
(PPPL)
Dr
Francesca Turco
(Columbia University)
Mr
J. M. Park
(Oak Ridge National Laboratory)
Dr
Rob La Haye
(General Atomics)
Dr
Timothy C. Luce
(General Atomics)
Dr
Wayne M. Solomon
(Princeton Plasma Physics Laboratory)
