NSTX Supported by Gas Injection College WM Colorado

NSTX Supported by Gas Injection 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 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 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 1

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

Piezo-electric Valves • 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 1020 D 2 ~. 5 x 1020 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 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 3

Example of LFS injection • 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 3 e-5 Torr 0 120 V 0 Ion Gauge Valve Voltage Micro-Ion Gauge 1720 Torr 1680 Torr Reference Plenum Pressure -1. 2 s NSTX Physics Operations Course Gas Injection (Mueller) Plenum pressure has PF and TF 0 pickup Jan 26 -28, 2010 4

Early LFS gas required to avoid MHD • 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 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 5

Puff valves are located on the High Field Side HFS of the vacuum vessel and the lower divertor region • Used for – Fueling after T 0, typically ~ 90 ms – Improves access to H-Mode – Prefill for CHI (Branch 5 provides a local gas source in lower gap) • The valves are air-pressuredriven, 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. NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 6

Flow rate from Puff valves CS • 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 Plenum volume. 040 l • 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 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 7

Supersonic Gas Injector has been used to replace the HFS gas for access to H-Mode • 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. 55 e 21 particles/s, Mach number of about 4. Vlad Soukhanovskii SOFE 07 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 8

Supersonic gas jet penetration mechanism is different than that of conventional gas injection § 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 Plasmoid § High density plasmoid blocks jet from Bt 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 References: Rozhansky et al. NF 46 (2006) 367 Lang et. al. PPCF 47 (2005) 1495 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 9

Supersonic gas jet fueling efficiency is x 2 -5 higher than that of conventional gas injection 0. 75 MA Ohmic § Instantaneous fueling efficiency (FE) is calculated as d. Ne/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 § FE in LSN H-mode plasmas 0. 1 -0. 4 (SGI at ~ 65 Torr l /s ~ 4. 2 x 1021 s-1). IW-limited L-mode target plasma for lithium experiments NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 10

SGI-U fueling favorably compares to conventional gas injection fueling § 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 EPS 08 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 11

Summary • 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 NSTX Physics Operations Course Gas Injection (Mueller) Jan 26 -28, 2010 12