Speaker
Mr
Christopher T. Holcomb
(USA)
Description
New off-axis neutral beam injection (NBI) capability on DIII-D has expanded the range of achievable and sustainable current and pressure profiles of interest for developing the physics basis of steady-state scenarios in future tokamaks. Fully noninductive (NI) scenarios are envisioned to have broad current and pressure profiles with elevated minimum safety factors (q_min), high normalized beta (beta_N), and a large fraction of the plasma current I_P sustained by the bootstrap current. Using off-axis NBI, plasmas have been produced with q_min between ~1.3 and ~2.5 to evaluate the suitability for steady-state operation (f_NI≡I_NI/I_P=1). Nearly stationary plasmas were sustained for two current profile relaxation timescales (3 s), with q_min=1.5, beta_N=3.5, f_NI=70%, and performance that projects to Q=5 in an ITER-size machine. The duration of the high beta_N phase is limited only by the available NBI energy. Low-order tearing modes are absent and the predicted ideal-wall n=1 kink beta_N limit is >4. To achieve higher f_NI, higher beta_N is needed to increase the bootstrap current, and higher q_min will decrease the required external current drive near the axis. Experiments to produce plasmas with q_min>2 showed that the use of off-axis NBI results in higher sustained q_min, with q_min at a larger radius (i.e. a broader current profile), and a broader pressure profile. These changes increased the predicted ideal-wall n=1 kink mode beta_N limit from below to above beta_N =4. These plasmas achieved a maximum beta_N=3.2 limited by the available NBI power and reduced confinement (H_98~1) relative to similar plasmas with lower q_min. beta_N=4 with q_min>1.5 was transiently obtained albeit with only 2 out of 5 MW of off-axis NBI available. Off-axis fishbones and low-order tearing modes were observed in the course of the q-profile scan. These studies indicate that obtaining a sustained, high performance, f_NI=1 scenario involves a number of trade-offs related to the choice of q-profile.
This work was supported by the US Department of Energy under DE-AC52-07NA27348, DE-FC02-04ER54698, DE-FG02-04ER54761, DE-AC02-09CH11466, DE-FG02-08ER85195, DE-FG02-08ER549874, DE-AC05-00OR22725, and SC0G804302.
Country or International Organization of Primary Author
USA
Primary author
Mr
Christopher T. Holcomb
(USA)
Co-authors
Mr
Alan W. Hyatt
(General Atomics)
Dr
Edward J. Doyle
(University of California Los Angeles)
Ms
Francesca Turco
(Columbia University)
Dr
Jeremy M. Hanson
(Columbia University)
Dr
Jin Myung Park
(Oak Ridge National Laboratory)
Mr
John R. Ferron
(General Atomics)
Dr
Lei Zeng
(University of California Los Angeles)
Dr
Matthew J. Lanctot
(Lawrence Livermore National Laboratory)
Dr
Michio Okabayashi
(Princeton Plasma Physics Laboratory)
Dr
Peter A. Politzer
(General Atomics)
Dr
Rob J. La Haye
(General Atomics)
Dr
Terry L. Rhodes
(University of California Los Angeles)
Dr
Timothy C. Luce
(General Atomics)
Dr
Tom H. Osborne
(General Atomics)
Dr
Tom W. Petrie
(General Atomics)
Dr
Yongkyoon In
(FAR-TECH, Inc.)
Mr
Yubao Zhu
(University of California Irvine)
