gas injection d. mueller january 26-28, 2010 culham sci ctr u st. andrews york u chubu u fukui u...

Gas Injection Gas Injection

D. Mueller

January 26-28, 2010

Culham Sci CtrU St. Andrews

York UChubu UFukui U

Hiroshima UHyogo UKyoto U

Kyushu UKyushu Tokai U

NIFSNiigata UU Tokyo

JAEAHebrew UIoffe Inst

RRC Kurchatov InstTRINITI

KBSIKAIST

POSTECHASIPP

ENEA, FrascatiCEA, Cadarache

IPP, JülichIPP, Garching

ASCR, Czech RepU Quebec

College W&MColorado Sch MinesColumbia UComp-XGeneral AtomicsINLJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNew York UOld Dominion UORNLPPPLPSIPrinceton UPurdue USNLThink Tank, Inc.UC DavisUC IrvineUCLAUCSDU ColoradoU MarylandU RochesterU WashingtonU Wisconsin

NSTXNSTX Supported by

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NSTXNSTX NSTX-PAC-25 – Solenoid Free Start-up (Mueller) February 18-20, 2009

NSTX Gas Injector Locations

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HFS gas

CenterStack

Shoulder

LFS Bay J UpperLFS Bay J Lower

LFS Bay K Top

Lower DomeBranch 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/sNBI ~ 4 Tl/sSGI 30-130 Tl/s

Pumping speed Turbo pump ~3400 l/s, NB cryo-panels ~50,000 l/sVessel volume 28,700 l, NB box volume ~ 50,000 l

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

Piezo-electric Valves

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• 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 D2 ~.5 x1020

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


Example of LFS injection

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• 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

0Reference

Plenum pressure has PF and TF pickup


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


Puff valves are located on the High Field Side HFS of the vacuum vessel and the lower divertor region

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• 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.


Flow rate from Puff valves

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• 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


Supersonic Gas Injector has been used to replace the HFS gas for access to H-Mode

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• 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


Supersonic gas jet penetration mechanism is different than that of conventional gas injection

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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 jetneutral 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

Bt

References: Rozhansky et al. NF 46 (2006) 367 Lang et. al. PPCF 47 (2005) 1495


Supersonic gas jet fueling efficiency is x 2-5 higher than that of conventional gas injection

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Instantaneous fueling efficiency (FE) is calculated as dNe/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 MAOhmic

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


SGI-U fueling favorably compares to conventional gas injection fueling

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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 mitigationVlad Soukhanovskii EPS08


Summary

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•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

gas injection d. mueller january 26-28, 2010 culham sci ctr u st. andrews york u chubu u fukui u...

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