When Does Core Radiation Affect Confinement?

1. When Does Core Radiation Affect Confinement? • It seems that we can get to pretty high Prad / PNBI before it affects stored energy. • If we believe Snyder’s model for the pedestal and a stiff model for transport in the core, then we should be able to radiate Prad = PNBI - PLH with no effect on confinement. • Operate NSTX at a range of currents in ELM-free lithium H-mode, and check the L -> H transition power. • Ramp up the beam power in steps and see at what level core radiation degrades bulk plasma confinement, as it builds up later in the shot. • It may be helpful to ramp up the power during the shot, to avoid too much stored energy in the fast ions while the density is low. • Shot plan, set up and diagnostics in database. • Plan to analyze existing data before XP review. Rob Goldston

2. Use of HHFW to Trigger ELMs • We have seen that HHFW turning off, even for a pulse < 100 msec, causes ELMs in ELM-free lithium H-mode. • Set up low beam-power ELM-free lithium H-mode, which is what worked before. • Use sequences of 300msec, 100msec, 30msec, 10msec, 3msec HHFW pulses, and see if ELMs are triggered. • If even long pulses don’t get ELMs, quit early. • Increase beam power to see if this works when HHFW is a small fraction of input power. • Set up, diagnostics, analysis in database. Rob Goldston

3. Drive Edge Harmonic Oscillations with Modulated Radio Frequency Heating • We have observed signatures of Edge Harmonic Oscillations (EHOs) at 6 kHz in NSTX, as on DIII-D, AUG and JET. • However we are not getting enough density and impurity control from them. • Set up discharges with EHO signatures as seen before. • Modulate HHFW power at frequency of n=1 fundamental EHO, and see if we can amplify EHOs. Scan k|| and BT to find strongest coupling through ponderomotive force or edge currents, etc. • Set up, diagnostics, analysis in database. Rob Goldston

4. Use SPAs to Drive EHOs • We have observed signatures of Edge Harmonic Oscillations (EHOs) at 6 kHz in NSTX, as on DIII-D, AUG and JET. • However we are not getting enough density and impurity control from them. • Set up discharges with EHO signatures as seen before. • Hook up SPAs to drive all HHFW antenna straps in parallel at frequency of n=1 fundamental EHO, and see if we can amplify EHOs. Coupling to saturated kink should be strong. • Set up, diagnostics, analysis in database. • This is prima-facie nuts, except that the resonant frequency of the antenna straps is 200 Hz. Rob Goldston