DIII-D RESEARCH OPPORTUNITIES FORUM FOR THE 2013 EXPERIMENTAL CAMPAIGN Review | Direct submission with log-in | Request submission without log-in
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Title	33: High Beta, Steady State Hybrids
Name:	Clinton (Craig) Petty petty@fusion.gat.com	Affiliation:	General Atomics
Research Area:	Steady State Heating and Current Drive	Presentation time:	Not requested
Co-Author(s):		ITPA Joint Experiment :	No
Description:	This experiment will integrate a high beta hybrid plasma with the reactor relevance of Te~Ti and full noninductive current drive. The optimization of the six gyrotrons and six co-beams will allow us to eliminate the residual 10 mV loop voltage of our best previous case, and hopefully lower q_95 from 5.85 to 5.0 at the same B_T. Additionally, the higher heating power, and possibly some broadening of the pressure profile using off-axis NBI, should allow us to increase beta closer to the ideal wall limit, which is above beta_N=4. This experiment will demonstrate that H-mode (hybrid) discharges with q_min~1 are capable of high beta (beta_N~4) operation with >50% bootstrap current fraction. The remaining noninductive current will will be supplied by on-axis sources (except possibly the 150 beam) at high efficiency. The poloidal magnetic flux pumping that is self-generated in hybrid will suppress the sawteeth despite the strong on-axis current drive, which is important for avoiding the 2/1 mode. The higher efficiency for on-axis current drive will offset the modest bootstrap current fraction such that this scenario will satisfy the requirements for FNSF as well as (or better than) the high q_min scenario with strong off-axis current drive.	ITER IO Urgent Research Task :	No
Experimental Approach/Plan:	We expect to have to repeat shots several times to obtain full-length gyrotrons and beams simultaneously. (1) Start by repeating shot 133881. (2) Inject all gyrotrons with central current drive. For the six co-NBI sources, increase the injection voltages as much as possible while maintaining a plasma pulse length of at least 5 seconds. Hopefully we will have already done an experiment to determine if tilting the 150 beam increases the stability limit by broadening the pressure profile. (3) Optimize the dynamic error correction (may use broadband feedback), adjust the plasma shape for optimal pumping. (4) Attempt to increase beta_N using the full heating power. (5) If plasma current is overdriven (i.e. negative loop voltage), then increase plasma current to compensate. The density also can be adjusted.
Background:	The current proposal for FNSF envisions a high q_min advanced tokamak scenario with 70% bootstrap current fraction. While this is compatible with the US view of DEMO, the physics of the high q_min AT scenario is still being developed. There is also an issue regarding the high off-axis current drive efficiency needed for FNSF in this proposal. Here I propose that the low q_min hybrid scenario is compatible with the requirements of FNSF, and it has several advantages. First, the physics basis is well advanced. Long duration hybrid discharge with high beta and high confinement are routinely achieved. Second, because q_min=1 in the hybrid scenario, all of the external current drive can be deposited near the plasma center where the current drive efficiency is the highest (because of the lack of trapped particles and the high electron temperature). While the bootstrap current fraction will be lower in this low q_min hybrid scenario (50% rather than 70%), the increase in the current drive efficiency for central deposition more than makes up for this. Experiments on DIII-D have come very close to demonstrating this scenario using five co-beams and five gyrotrons. Hybrid plasmas with beta_N=3.4 were stably produced with a loop voltage of 10 mV. The loop voltage was a strongly decreasing function of heating power. While the ion and electron temperature were nearly the same outside of rho=0.2, the H-mode confinement factor remained high, H_98=1.4. This result is better than for the typical hybrid regime on DIII-D and is correlated with better than usual electron thermal transport in this LSN plasma shape. Therefore, this proposal will likely lead to the development of a high beta, high confinement, steady state scenario based on the hybrid regime. A half-day experiment in 2010 did not result in improved parameters despite the additional of a sixth co beam source because of 2/1 NTM issues. The evidence is that the lower 2/1 mode limit is related to having a too peaked pressure profile. This could explain several facets of the 2/1 mode onset, such as the dependence on the current evolution and the dependence on the confinement factor. We will need to pay close attention to the peakness of the pressure profile and find ways to decrease it if necessary, such as using the off-axis beam, changing the wall conditions or gas pre-fill levels.
Resource Requirements:	NBI: 6 co sources are needed. 210RT may be used to collect MSE data. ECH: 6 gyrotrons are essential. I-coils: Dynamic error field correction will be used (possibly broadband feedback).
Diagnostic Requirements:	MSE is critical.
Analysis Requirements:	TRANSP for current drive and transport, DCON for stability.
Other Requirements: