Solenoid Free Plasma Start-up Mid-Run Summary (FY 2008)

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Solenoid Free Plasma Start-up
 Mid-Run Summary (FY 2008)
     R. Raman and D. Mueller
       Univ. of Wash. / PPPL
        16 April 2008, PPPL

Flux savings from inductive drive of a Transient CHI started plasma (XP817)
                   5 Days - March 10-12, 31 & April 9, 2008

  March 10: Commissioned newly installed CHI hardware
 •Tested staged firing capability for the capacitor bank
      -Crowbar system reliability needed to be improved
      -Capacitor bank timing flexibility needed
 •Fast voltage monitoring system worked well
 •Produced a few plasma discharges in the stuffed injector mode
  March 11: Conditioned the Lower Divertor Plates
•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
     -Could not observe spectroscopy signal decreases during conditioning
•Repeated the CHI-only and CHI+OH discharge
     -Saw small improvements
•Then conducted 1-hour D2GDC + 30min HeGDC + 5g TMB + 2 hour HeGDC
     •Considerably more lines during D2GDC (mass 18, 20, 28, 32)
                  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 24kA).
     - 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.
     - Also helps assess if CHI startup could be used in NSTX during Outer PF startup.
     - Also useful for future CHI + Iron Core startup.

2. CHI discharge successfully initiated in the injector region with CS pre-charge

•   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.

    Staged bank capability allowed establishing good breakdown conditions from 2006

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

•    The reduced growth rate of the CHI discharge in conjunction, 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

            First observation of electron density during the OH coupling phase

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.

      Plasma Current – Thomson
      Profiles are for the red trace

March 31: Demonstrated first good coupling of a CHI produced discharge to induction in NSTX

• Using March 12 shots, discharges initiated using 1 capacitor (total 7.5kJ initial stored energy)
•Pre-programmed PF coil currents used
•Discharges beyond 40ms were ramping up rapidly in Ip but were vertically unstable

March 31: Fast camera fish-eye view of a CHI started discharge

                                    R. Maqueda, Phantom Camera   7
April 9: Demonstrated generation of a high quality discharge using a CHI startup target

  •Discharges produced on March 31 were further improved, 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.

  •These discharges are essentially ready for a test with and without Lithium
      - Increased input power during the first 5ms would reduce flux
      consumption from OH (which is now used to overcome radiative losses in
      these plasmas)

CHI initiated and coupled to OH discharges with one capacitor (color) and an OH only
                                  Discharge (black)

CHI initiated and coupled to OH discharge with two capacitors (color) and OH only
  discharge (black). This discharge had CS gas injection and more NBI power.

 With three capacitors the spectroscopic line intensities increase
(Need to reduce low-Z impurities or provide additional input power)

     Solenoid-Free Start-up and Ramp-up high-priority research

Improve NSTX high performance discharges with CHI

1. Couple inductive ramp-up to CHI plasmas (R08-2)

2. Demonstrate significant ohmic flux savings using CHI
- Test with Lithium
- Further condition the divertor plates to allow operation with >2 capacitors
- Increase CS voltage to 6kV (additional 200kW of input power)

3. Develop operating conditions aimed at improving the control - done
and increasing the current and closed poloidal flux of CHI (for 2009 & 2010)

4. Increase high-performance plasma pulse lengths using CHI startup
- Will naturally happen after step 2

             Conclusions and Plans for Balance of FY 2008

Good coupling of a CHI started discharge to induction has now been
demonstrated in NSTX
 - An important milestone for NSTX, validating compatibility between CHI and
standard inductive operation
- H mode also obtained, validating compatibility with H-mode, an important step
for achieving long-pulse discharges
- Flux savings requires a cleaner divertor plate, more input power, or further
reducing CHI start-up density, a well understood result from HIT-II experiments

Flux savings using CHI (~3 days)
 - Further condition the divertor plates to allow operation with >2 capacitors
 - Test with Helium (suggested by M. Bell) and with D 2 doped with Methane
(suggested by M. Ono)
 - After Li system begins routine operation - Test with Lithium
 - Increase CS voltage to 6kV (additional 200kW of input power)

Test CHI Startup with pre-charged CS (1-2 days)
- Helps prepare for 2009, with Outer PF startup
- Reload & test an outer PF startup discharge with CHI that was tried in 2005