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
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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
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