1. Gas Injection D. Mueller January 26-28, 2010 Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin NSTX Supported by 1

2. NSTX NSTX-PAC-25 – Solenoid Free Start-up (Mueller)February 18-20, 2009 NSTX Gas Injector Locations 2 HFS gas Center Stack Shoulder LFS Bay J Upper LFS Bay J Lower LFS Bay K Top Lower Dome Branch 5 Supersonic Gas LFS (Low Field Side) Injectors are Piezo-electric valves with ~150 Tl/s Center Stack, Shoulder, Branch 5 and Lower Dome are all puff valves Supersonic Gas Injector (SGI) is a moveable high velocity (Mach 5) injector that uses a shaped nozzle to achieve the directional, high velocity flow NSTX fueling rates: Gas valves 0-300 Tl/s NBI ~ 4 Tl/s SGI 30-130 Tl/s Pumping speed Turbo pump ~3400 l/s, NB cryo-panels ~50,000 l/s Vessel volume 28,700 l, NB box volume ~ 50,000 l

3. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Piezo-electric Valves 3 3 piezo valves located on the Low Field side LFS of the vacuum vessel are used for –Prefill gas for breakdown 2 x 10 -5 Torr GP ~ 0.3 x 10 20 D 2 ~.5 x10 20 e - in plasma at 10 ms ~ 100% –Fueling plasma, especially early in the discharge –Used to supply gas for GDC The valves are operated in a pulse-width modulation mode Avoids non reproducible flow vs voltage (max flow rate is more stable over a long time) Each valve is supplied gas from a plenum that is filled to about 1500 T between shots

4. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Example of LFS injection 4 The ion gauge used to measure the prefill pressure has a time response of ~ 50 ms this is too slow to use a standard PID feedback algorithm Instead the requested prefill pressure is used to estimate the amount of gas required and the flow rate of the piezo valve is used to determine how long to open the valve. 50 ms after the valve is closed the vessel pressure is checked against the request up to about t = -200 ms 0-1.2 s 3e-5 Torr Ion Gauge Valve Voltage Micro-Ion Gauge Plenum Pressure 1680 Torr 1720 Torr 120 V 0 0 Reference Plenum pressure has PF and TF pickup

5. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Early LFS gas required to avoid MHD 5 Using the Prefill to supply enough gas to avoid early H-alpha spikes and MHD can result in discharge failing –Solution add early gas puff from LFS injectors –Use of Li increases the required gas puff value

6. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Puff valves are located on the High Field Side HFS of the vacuum vessel and the lower divertor region 6 Used for –Fueling after T0, typically ~ 90 ms – Improves access to H-Mode –Prefill for CHI (Branch5 provides a local gas source in lower gap) The valves are air-pressure- driven, solenoid-actuated fast valves that simply dump the contents of a pre-loaded plenum, there is a delay (~300 ms) from issuing the open command until the valve actually opens.

7. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Flow rate from Puff valves 7 The Lower Dome and Branch 5 valves are connected to their injection points inside the vacuum by large diameter tubes (~1.3 m, 3.5 cm I.D.) and have a short delay of ~16 ms and rapid pump out The Center Stack and Shoulder injectors are routed from the fast solenoid valves to their injection points by long thin tubing (~ 2 m long, 0.457 cm I.D.) –The gas flow rate has a sharp increase to maximum and a long decay afterwards CS Plenum volume.040 l

8. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Supersonic Gas Injector has been used to replace the HFS gas for access to H-Mode 8 The SGI is comprised of -A de Laval converging-diverging graphite nozzle -A commercial piezoelectric gas valve -A diagnostic package (Langmuir probe, thermocouples and magnetic pick-up coils) -mounted on a movable probe at the midplane -flow rate up to 4.55e21 particles/s, Mach number of about 4. Vlad Soukhanovskii SOFE07

9. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Supersonic gas jet penetration mechanism is different than that of conventional gas injection 9  High density plasmoid blocks jet from deep penetration into magnetized plasma  Depth of penetration is ultimately determined by jet pressure and plasma kinetic and magnetic pressure  Desirable for fueling are molecular clustering and/or droplet formation in jet achieved at very high pressure and cryogenic temperatures Gas jet neutral density Plasmoid  Unlike conventional gas injection, penetration depth of supersonic gas jet cannot be described by single neutral particle ionization / charge exchange penetration model  Supersonic gas jet is a low divergence high pressure, high density gas stream with low ionization degree - bulk edge/SOL electrons do not fully penetrate gas jet BtBt References: Rozhansky et al. NF 46 (2006) 367 Lang et. al. PPCF 47 (2005) 1495

10. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Supersonic gas jet fueling efficiency is x 2-5 higher than that of conventional gas injection 10  Instantaneous fueling efficiency (FE) is calculated as dN e /dt *  -1  In ohmic plasmas, FE is a function of SGI-LCFS distance (SGI at  ~ 40 Torr l /s) in LSN configuration  FE in inner wall -limited plasmas higher than in diverted config.’s 0.75 MA Ohmic  FE in LSN H-mode plasmas 0.1-0.4 (SGI at  ~ 65 Torr l /s ~ 4.2 x 10 21 s -1 ). IW-limited L-mode target plasma for lithium experiments

11. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 SGI-U fueling favorably compares to conventional gas injection fueling 11  Three discharges with different fueling are compared: reduced HFS rate + LFS similar to SGI-U reduced HFS + SGI-U at R=1.57 m reduced HFS+SGI-U at R=1.98 m  In the SGI-U-fueled discharges divertor pressure lower divertor recycling lower midplane pressure lower  When SGI-U is closer to separatrix (R=1.57 m vs R=1.98 m) - higher plasma density is obtained  However, all fueling methods result in high divertor ionization source, and monotonic density rise : need active pumping for mitigation Vlad Soukhanovskii EPS08

12. NSTX Physics Operations Course Gas Injection (Mueller)Jan 26-28, 2010 Summary 12 Piezoelectric valves used in pulse-width modulation mode  Prefill  Early fueling  GDC Puff valves various locations and conductances  Fueling / H-Mode access  Chi start-up Supersonic Gas Injector  Fueling / H-mode access  More efficient fueling than normal valves