Point-source DC Helicity Injection on the Pegasus Toroidal - - PowerPoint PPT Presentation

D.J. Battaglia, APS-DPP Dallas 2008

Point-source DC Helicity Injection on the Pegasus Toroidal Experiment

Devon J. Battaglia M.W. Bongard, B.A. Kujak-Ford, E.T. Hinson, B.T. Lewicki, A.J. Redd, A.C. Sontag and the Pegasus Team University of Wisconsin - Madison

D.J. Battaglia, APS-DPP Dallas 2008

Point-source DC helicity injection is an attractive non-solenoidal startup technique

• Non-solenoid startup is a critical issue for future long-pulse STs

– Would extend efficiency of OH drive and provide j(R) modification on present experiments that already have a central solenoid

• Plasma gun point-source DC helicity injection tested on Pegasus

– Low impurity, high Jinj source – Scalable design ⇒ flexible, compact & no toroidal vacuum break Anode Gun

D.J. Battaglia, APS-DPP Dallas 2008

Outboard point-source injection on Pegasus features a scalable “port-plug” design

Anode Outer limiter 40 cm Plasma guns Current filaments 1 m

D.J. Battaglia, APS-DPP Dallas 2008

Ip ~ 0.1 MA non-solenoidal startup achieved using < 4 kA injected current

Equilibrium reconstruction of similar discharge with Ip = 75 kA at 28 ms M = 2

Filament relaxation Outboard limited Inboard limited 1 m 0.01 βφ 0.29 βp 0.30 li 1.65 κ 1.14 A 0.35 m a 0.40 m R0 0.15 T Bφ,0

D.J. Battaglia, APS-DPP Dallas 2008

Achieved Ip depends on helicity and relaxation constraints

dK dt = 2 J B d3x

V

• 2

t 2 B ds

A

• Helicity balance in a tokamak geometry:

Ip Ap 2R0 Vind + Veff

( )

Veff NinjAinjB,inj

• Vbias

Taylor relaxation of a force-free equilibrium:

µ0Ip µ0Iinj 2RinjwB

,inj

Ip Cp 2Rinjµ0 Iinj w

• 1/ 2
• Assumes system is in steady-state (dK/dt = 0)
• Ip limit depends on the scaling of plasma

confinement via the η term Assumptions:

• Driven edge current mixes uniformly in SOL
• Edge fields average to tokamak-like structure

B = µ0J = B

Ap Plasma area Cp Plasma circumference Ψ Plasma toroidal flux w Edge width

p edge

D.J. Battaglia, APS-DPP Dallas 2008

Max Ip achieved when helicity and relaxation criteria are simultaneously satisfied

Estimated plasma evolution Plasma guns Anode

• Requires Bv ramp for radial force balance & Vind

Relaxation limit Helicity limit Ip max Time

ITF = 288 kA Vbias = 1kV Vind = 1.5 V Iinj = 4 kA w = dinj L-mode τe

D.J. Battaglia, APS-DPP Dallas 2008

120 V 900 V Vbias = 1200 V Relaxation limit

Sufficient helicity injection is required to drive plasma to the relaxation limit

• All three discharges have the same Iinj and Bv evolution

D.J. Battaglia, APS-DPP Dallas 2008

Several issues need to be addressed in the near term to test the simple model

• What determines λedge?

– Jedge broadening due to magnetic turbulence (edge and global), magnetic shear, gun characteristics, physical geometry, etc.

• How does τe (or τK) scale with Ip?

– χ⊥ versus χ in the presence of magnetic turbulence – Confinement will depend on degree

• f stochasticity in core plasma
• What influences Zinj?

– Vbias = Zinj Iinj – Neutral fueling – Filament path length R0 = 47 cm R0 = 47 cm R0 = 47 cm

D.J. Battaglia, APS-DPP Dallas 2008

Target plasma from point-source DC helicity injection readily coupled to OH induction

6 4 2 vloop (V), Iinj (kA) 35ms 30 25 20 time 6 4 2 vloop (v) 0.15 0.10 0.05 0.00 Ip (MA) 0.15 0.10 0.05 0.00 Ip (MA)

Ip vloop Iinj plasma gun startup OH only 41708 41536

• 80 kA target

handoff to OH drive

• Equivalent Ip with

1/2 OH flux swing

– ~ 50% flux savings

• Need to assess

target suitability for

• ther CD means

D.J. Battaglia, APS-DPP Dallas 2008

Summary

• High current (~ 0.1 MA) ST startup and current drive via point-

source DC helicity injection has been demonstrated on Pegasus

– Maximum Ip described by helicity balance and relaxation criteria

• Magnetic induction compatible with gun produced target plasmas

– PF induction provides current drive and maintains radial force balance with larger Ip plasma – Handoff to OH induction robust

• Near-term work will test proposed scaling of Ip limits

– Langmuir and magnetic probes → measure λedge directly – Increase gun area → determine effect on w & increase Kinj – Decrease Rinj & maintain outboard injection → should increase both limits – Increase Lfilament → determine effect on Zinj – Characterize plasma

• nel, PRAD, Te, impurities
• Possibly implement Thomson scattering and ion Doppler shift → Te and Ti

D.J. Battaglia, APS-DPP Dallas 2008

For more information

• JP6.00012 Numerical Simulation of MHD Relaxation during Non-Inductive Startup
• f Spherical Tokamaks T.M. Bird, et. al.
• NP6.00134 Overview of the Pegasus Toroidal Experiment A.C. Sontag, et. al.
• NP6.00135 Non-solenoidal startup in Pegasus discharges A.J. Redd, et. al.
• NP6.00136 Characterization of edge instabilities in the Pegasus Toroidal

Experiment M.W. Bongard, et. al.

• NP6.00138 Pegasus power system facility upgrades B.T. Lewicki, et. al.
• NP6.00139 Computational study of a non-ohmic flux compression startup method

for spherical tokamaks J.B. O’Bryan, et. al.

• VI2.00001 Non-solenoidal Plasma Startup in the Pegasus Toroidal Experiment

A.C. Sontag

D.J. Battaglia, APS-DPP Dallas 2008

• Current along injected

filaments perturbs the vacuum magnetic field

• Bv must be sufficiently

low for null to form

• Null formation is required,

but not sufficient for relaxation

The relaxation of filaments to a tokamak-like topology requires an inboard null region

Iφ = 0 A Iφ = 4 kA

Anode Gun 2-D force free current model