1. CHI Run Summary for March 10-12, 31 & April 9, 2008 Flux savings from inductive drive of a Transient CHI started plasma (XP817) R. Raman, B.A. Nelson, D. Mueller, T.R. Jarboe et al. 1

2. Tested staged firing capability for the capacitor bank Crowbar system reliability needed to be improved Capacitor bank timing flexibility needed (to fire bank 2 before bank 1) Fast voltage monitoring system worked well Produced a few plasma discharges in the stuffed injector mode to begin Divertor plate conditioning on March 11 March 10: Commissioning the newly installed CHI hardware 2

3. First ran a CHI-only and a CHI+OH discharge from 2007 Then ran about 15 high injector current plasma discharges Injector current was ~30kA Plasma CHI plasma remained attached to the lower divertor plates Filter-scope signals observing the Lower divertor plates saturated Could not observe any signal decreases during conditioning Repeated the CHI-only and CHI+OH discharge Saw small improvements Then conducted 1-hour D 2 GDC + 30min HeGDC + 5g TMB + 2 hour HeGDC Considerably more lines seen in RGA spectrum during D 2 GDC than during HeGDC (mass 18, 20, 28, 32) Indicating that D 2 GDC was more effective in removing surface contaminants. March 11: Conditioned the Lower Divertor Plates 3

4. March 12: Main portion of CHI XP started Very productive day – Good progress made in multiple areas 1.Inductive discharge development that had reduced CS pre-charge First, inductive only discharges were run in which the pre-charge in the central solenoid was successfully reduced to 9kA (from the full value of 24kV). This was an important step as it reduces the OH fringing field making it easier to setup the divertor flux footprints needed for initiating a CHI discharge under conditions of a pre-charged central solenoid. 2.CHI discharge successfully initiated in the injector region with CS pre-charge Over a sequence of several shots, conditions needed for establishing a successful CHI breakdown during the presence of OH fringing fields were established. These CHI flux conditions will be further optimized during a future run. 4

5. 3.Reproduced good temperature CHI discharges with zero CS pre-charge CHI-only discharges were repeated and optimized. As in 2007, the CHI produced discharges grew very rapidly and contacted the absorber region. To reduce the CHI discharge growth rate, the staged firing capability of the new capacitor bank was used for the first time to first discharge a single capacitor, then after a delay - two capacitors. This technique compensated for the new divertor surface conditions and allowed us to re-establish the breakdown conditions used in 2006. The reduced growth rate of the CHI discharge in conjunction with the much lower level of energy now used (about 20kJ), resulted in the CHI discharge consistently reaching temperatures of 10-20eV (as in 2006). The measured temperatures and densities were not much different from those of inductively produced discharges during the first 10ms of a normal inductive startup 5

6. 4.Saw first evidence for coupling to OH with good density and temperature signals. During the four shots that attempted to couple these to induction, the discharge coupled to induction, but electron density and temperature profiles from Thomson scattering showed most of the plasma to be resting on the outer vessel region during the inductive phase (this was later confirmed by EFIT reconstruction). On the last discharge increased vertical field moved the plasma further inboard verifying the Thomson and EFIT results. 6

7. March 31: Demonstrated first good coupling of a CHI produced discharge to induction in NSTX Started with final discharge from March 12 CHI discharges initiated using 1 capacitor (total 7.5kJ initial stored energy) Absorber arcs minimized or reduced Pre-programmed PF coil currents used Discharges beyond 40ms were ramping up rapidly in Ip but were vertically unstable 7

8. March 31: Fast camera fish-eye view of a CHI started discharge. 8 R. Maqueda, Phantom Camera

9. 9 April 9: Demonstrated generation of a high quality discharge using CHI startup target Discharges produced on March 31 were further improved to increase the initial peak in the CHI produced upto 150kA and then successfully ramped up using zero pre-charge in the central solenoid to plasma currents exceeding 600kA. Used feedback control of outer gap and vertical position. Up to two capacitors used (15kJ initial stored energy) The CHI started discharges transitioned into a H-mode, reaching electron temperatures of 500eV and had had low plasma inductance similar to the type of discharges preferred for high-performance operation in NSTX. In some discharges both neutral beam injection and High Harmonic Fast Wave Heating were used.

10. 10 CHI initiated and coupled to OH discharges with one capacitor (black) and with Two capacitors (blue). PF coil programming & gas injection for both discharges are different. More information to come later …..