XP817: Transient CHI – Solenoid free Plasma Startup XP817: Transient CHI – Solenoid free Plasma Startup and Coupling to Inductionand Coupling to Induction

Office ofScience

R. Raman, B.A. Nelson, D. Mueller, T.R. Jarboe, M.G. Bell et al.,

University of WashingtonPrinceton Plasma Physics Laboratory

NSTX FY08 Results ReviewAugust 6-7, 2008 (PPPL)

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

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

Supported by

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 2

Transient CHI: Axisymmetric reconnection leads to formation of closed flux surfaces

• Demonstration of closed flux current generation– Aided by gas and EC-Pi injection from below divertor plate region

• Demonstration of coupling to induction (2008)– Aided by staged capacitor bank capability

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 3

FY08 Result: Proof-of-Principle Demonstration of Coupling a CHI Started Discharge to Induction

8 Total days: 2 days – commissioning & conditioning

2 days – Testing without Cryo pumping capability

4 days – Full capability

March 10: Commissioned new CHI hardware

- Staged capacitor bank operation

- Fast voltage monitoring system

March 11: Conditioned the lower divertor plates

- Operated in stuffed injector mode

- Conducted first D2GDC (in recent history)

March 12: First day of main XP

- Inductive discharge development that had reduced CS pre-charge

- Started CHI discharge with some pre-charge in CS

- Reproduced good Te CHI discharges with zero CS pre-charge

- Saw first evidence for coupling to OH (good Te and ne signals)

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008

CHI Started Discharges after Inductive Coupling Transition into H-mode and are more reproducible with Li Conditioning

March 31: Inductively coupled discharges reach 180kA and reach Te ~100eV

April 9: Inductively started discharges reach Ip = 600kA and Te = 500eV

- Used position feedback control to increase current

June 2,3: Operated without Cryo Pumping and without Boronization

- Vessel base pressure ~10x higher than usual (4 x 10-7 Torr)

- June 2- Tested CHI discharge formation in He & then switched to D2 plasmas, Ip ~400kA in poor reproducibility discharges

- June 3 - First use of Li evaporation for CHI

- Discharges became reproducible and reached Ip ~600kA

- Tested use of HHFW for heating during the coupling phase to induction

July 8: Used Cryo pumping and 10mg/min Li evaporation

- Central electron temperature reached 1keV

- Ip reached 725kA, discharges transitioned into H-modes

- Discharges with 2min HeGDC + 6min Li were more reproducible than Li-only cases

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FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 5

March 12: First Evidence of CHI Coupling to Induction

• Final four shots coupled to induction

– Te and ne (from Thomson) showed plasma to be resting on outer vessel during inductive phase

– Later confirmed by EFIT

• On last shot increased vertical field moved plasma further inboard verifying Thomson and EFIT results

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 6

March 31: Demonstrated First Good Coupling of CHI Produced Discharge to Induction (in NSTX)

• Started with Final discharge from March 12

– Used 7.5kJ of capacitor bank energy to initiate CHI discharge

– Used pre-programmed PF coil currents to maintain equilibrium

– Discharges >40ms were ramping up in Ip but vertically unstable

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 7

CHI started discharge couples to induction and transitions to an H-mode demonstrating compatibility with high-performance plasma operation

Te & Ne from Thomson

Ti from CHERS- Central Te reaches 800eV

- Central Ti > 700eV

Note the broad density

profile during H-mode phase

• Discharge is under full plasma equilibrium position control− Loop voltage is preprogrammed

CHERS: R. Bell

Thomson: B. LeBlanc

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008

Discharges with Li Conditioning are More Reproducible and reach Higher Currents after Inductive Coupling

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Li makes breakdown more reproducible and the improvements are due to reduced recycling (similar to the effect of Ti-gettering on HIT-II)

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 9

Need auxiliary heating or metal divertor plates to compensate for increased radiated power with more capacitors

128400: 5mF (7.6kJ)

128401: 10mF (15.3kJ)

129402: 15mF (22.8kJ)

Plasma Current• Low-z impurity radiation increases

with more capacitors– High Te in spheromaks (500eV)

obtained with metal electrodes– Test with partial metal outer

divertor plates during FY09– Reverse TF polarity to make

outer vessel cathode• Upper divertor radiation also

increases with more capacitors– Need to reduce absorber arcs– Absorber field nulling coils to be

used during FY09• Assess benefits of partial metal

plates + Absorber coils– Discharge clean divertor with

high current DC power supply– Use 350kW ECH during FY11

FY08 NSTX Results Review (Solenoid-Free Plasma Startup) August 6, 2008 10

First Demonstration of Compatibility of CHI with High-Performance Inductive Operation in a Large ST

• Transient CHI is a proven method to generate closed flux (160kA to date)

– Startup & inductive coupling at 100kA demonstrated on NSTX & HIT-II

– CHI initiated and inductively ramped current reached 700kA in H-mode

– Peak Te ~0.9keV obtained in discharges coupled to induction

• Need to reduce Low-Z impurity radiation to increase current

– Electrode conditioning to be improved for FY09

– Reduce absorber arcs using Absorber PF coils

– Use outer metal divertor plates needed for Liquid Li system

• Beyond 2009

– 350kW ECH + higher power HHFW will help considerably

– Results from metal outer plate tests during 2009 will provide additional data on effect of Low-Z impurities

– Higher voltage needed to increase startup current

– Higher TF will increase current multiplication factor

Closed flux startup currents of about 500kA is possible in NSTX through hardware improvements (not a physics limitation)