Low Density Start-up D. Mueller, R. Raman, M.Bell, S. Gerhardt
Why modify start-up • Low collisionality plasmas on NSTX require low density • Full benefits of Li may require reducing fueling • Historically low density (low fueling) discharges suffer from MHD or locked modes and low performance • CHI start-up (142140) demonstrates forming a large Rp, 300 kA plasma quickly can give stable low li (~0.4) plasmas with ne about 1/3 that in an ohmic comparison at 700 kA (134865) Note: the example was L-Mode • Start-up with low-density plasmas is possible and the plasmas are stable up to the start of flattop
Three approaches to low fueling start-up are proposed • Use zero OH pre-charge to investigate if error fields due to OH X TF are responsible • Mimic the CHI start-up by using high loop voltage to get high Ip early at large Rp, low li • Use very low voltage start-up (~1.5 V/turn) and low Ip ramp rate to investigate if low dIp/dt can reduce MHD and permit lower fueling
Zero OH pre-charge • Start with usual fiducial start-up except OH at zerowith no early gas puff and minimal CS gas • Ramp Ip to about 700 kA and compare results with a 700 kA fiducial • If successful, try ½ OH pre-charge and extend flattop • Note many low ne discharges do not fail until during flattop so challenge will be to get enough duration
High voltage start-up (rapid initiation like CHI) • Try High loop voltage (4+ V, about double standard) • Use 10 to 20% lower PF5 than usual. • PF3/PF5 ratio about 2 times lower (vary from 3 to 1) • Vary prefill as required. Control to the fiducial shape starting at 20 ms., but use lower PF3 to PF5 ratio from CHI case until ~35 ms to get lower li. • PF1AU&L must be adjusted so as to avoid plasma moving up during the ramp-up. • Try to get Rp ~ 10 cm larger until about 70 ms (start of isoflux control) • The CHI shot did not use any LFS gas, but did use 1100T on CS; vary CS injector. • 1-2 NBI sources starting at 20 ms. • Will take at least a few half-days, probably several to explore
Low voltage start-up (breakdown phase, t<40 ms) • We normally use ~2V/turn at breakdown. • Use <1.5 V for breakdown (will require lower prefill) and lower PF5 • During the slow ramp-up (~0.5 - 0.7MA/s), we normally use ~2V/turn for L-Mode or ~ 1V/turn in H-Mode. • The PF3/PF5 ratio can be used to vary the size of the plasma at start-up and to lower li.
Low voltage start-up (slow ramp-up) • Ramp Ip about ½ the rate normally used after 40 ms; try < 0.25 MA/s instead of .06 MA/s; note if the plasma stays in L-Mode it will run out of OH flux at about 1 MA and 0.4 s, if it transitions to H-Mode, at about 500 kA, there might be a few 100 ms of Ip flattop. (May need to induce H-Mode during ramp-up to get a meaningful flattop.) • Control to the fiducial shape starting at 20 ms., but use lower PF3 to PF5 ratio to adjust plasma volume and keep li low like was the case for CHI. This will require several shots to get PF1AU&L right. • Try to get Rp ~ 10 cm larger until about 70 ms (start of isoflux control). • May need to delay start of isoflux control if Ip at 70 ms is not yet high enough (>275 kA) for rtEFIT to make reasonable flux surfaces. • The CHI shot did not use any LFS gas, but did use 1100T on CS; vary CS injector. • 1-2 NBI sources starting at 20 ms • This will take at least a couple half-days to explore
Summary • Will use a 700 kA fiducial shape plasma as comparison so some flattop can be achieved in all scenarios • Each scenario will require dedicated run time, best if it is in ½ day chunks, in increasing complexity: • Low, zero OH 1 Day • Low voltage start-up, low ramp-rate 1-2 Days • High voltage start-up 2-4 Days • Operational space is complex, for example timing of transition to H-Mode has a major influence on ne, duration, stability, etc.; that timing depends on many variables • Need good conditions to ensure that any MHD is not due to the usual suspects