A solenoid-free current start-up scenario utilizing outer poloidal field coils and center-post46th Annual Meeting of Division of Plasma PhysicsAmerican Physical SocietyNovember 15 19, 2004Savannah, GeorgiaColumbia UComp-XGeneral AtomicsINELJohns Hopkins ULANLLLNLLodestarMITNova PhotonicsNYUORNLPPPLPSISNLUC DavisUC IrvineUCLAUCSDU MarylandU New MexicoU RochesterU WashingtonU WisconsinCulham Sci CtrHiroshima UHISTKyushu Tokai UNiigata UTsukuba UU TokyoJAERIIoffe InstTRINITIKBSIKAISTENEA, FrascatiCEA, CadaracheIPP, JlichIPP, GarchingU QuebecWonho Choe, Jayhyun Kim

Korea Advanced Institute of Science and Technology

J. Menard, Masayuki Ono, and the NSTX team

Princeton Plasma Physics Laboratory

Yuichi Takase

Tokyo University

Outer PF coil-only inductive plasma start-up Ohmic solenoid has been the work horse of fusion research for decades.

Attractive fusion CTF and power plant design requires OH eliminationCompact CTF requires elimination of OH regardless of R/a.ARIES-AT and ARIES-ST design assumes no OH.

PF coils have been used to start-up the plasmaMAST / START: Merging/compression using in-vessel PF coilsJT-60U / TST-2: ~150 kA / 10 kA obtained using strong (1 MW / 100 kW) ECH pre-ionization with ~0.02 kV/m

Plasma start-up using appropriate combination of out-board and outer PF coils, and/or using a conducting center-post.

Null field generation using out-board induction coils Small radius PF coil #1 produces a peaked BV profile. Large radius PF coil #2 produces a flat BV profile. Near-midplane trim PF coil #3 produces a follow BV profile.

Mid-plane vertical field profilesSufficient mid-plane space for blanket access (NSST)

Coil #R (m)Z (m)I (kA-turn)12.002.63+16,00023.801.90 -10,0063_13.101.20 -3,6893_23.401.00 +6,670

Flux contours Significant amount of volt-sec available for current ramp-up:~4.5 V-s at R0 = 1.75 m Generation of good quality multi-pole field null Excellent out-board access (~1.8 m vertical spacing) Suitable for the interchangeable blanket modules for CTF. Case 5 contdOuter PF Start-Up could provide multi-MA seed current for future STsNSST

592 ms596 ms600 ms604 ms608 ms612 ms Successful start-up with ETBT/BP 0.02 kV/m (Takase et al., EX/P4-34) Large 0.02 kV/m contour (~1.2 m diameter)Breakdown contours (0.1 kV/m and 0.02 kV/m) (NSST)0.02 kV/m0.1 kV/m~1.2 mTime-dependent calculationNSST

Simulation for NSTXFlux contoursBreakdown contours

Lloyds condition, with strong pre-ionization, ETBT/BP 0.1 kV/m satisfied in a significant volume. *J. Kim, W. Choe, M. Ono, Plasma Phys. Control. Fusion 46 (2004) 1647 26 ms27 ms28 msTime-dependent calculation with vacuum vessel eddy currents considered*Mod-B contours (Gauss)23 ms24 ms25 mscontours (kV/m)~ 35 cm 23 ms24 ms25 ms27 ms26 ms28 ms Successful start-up with ETBT/BP 0.02 kV/m (Takase et al., IAEA EX/P4-34)

Large 0.02 kV/m contour

Loop voltage and poloidal fluxCoil currentFlux vs time (at 1.4 m) Significant V-s is available for current ramp-up. Full ramp-up scenario will require bi-polar PF5. But, initial breakdown experiment to ~100 kA should be possible with the existing power supplies. Loop voltageNullNullNullPF2PF3PF4PF5

Force balance in the initial start-up phase needs to be checked carefully. Self-consistent code including evolution of plasma current and parametersForce balance and stability issuesVertical field for force balance Stable field structure against axis-symmetric mode

Preliminary PF-only Start-up Experiments in NSTXPre-ionize plasma near RF antenna with ECH + 400 kW HHFW, nD2 = (1 - 2)10-5 Torr

Create high-quality field-null with (5 - 15) loop-volts at antenna So far, require EfBf /BP > 0.1 kV/m over substantial plasma volume

Have created 20 kA plasmas that terminate near center-stackJ. Menard

Successful initiation:OH:112152, 4.5 kGXP433-I: 113612, 3.5 kGXP433-II:114405, 3 kGUnsuccessful initiation:XP431: H:11293, 4.5 kGXP448-I: 113609, 3.5 kGXP448-II:114484, 3 kGSuccessful initiation thus far requires a large null region

Coil current waveforms used in XP-443Similar to OH startup waveforms114405PF2PF4

Field and loop voltage114405At plasma breakdownNear maximum plasma current

Camera images and reconstructions show plasmas are born on LFS and have an inward radial trajectoryLRDFIT code used for reconstructionsIVessel 10 IP

Careful control of BZ after breakdown helped raise IP from 10kA to 20kA

More BZ evolution optimization possible

114405

Thomson measurements consistent with plasma motion and peaked pe profilesThomson Te < 35eV and camera images consistent with lack of burn-through need more plasma heating power:More HHFW power during breakdownHigher VLOOP keep plasma outboardEBW power could be very helpfulB. LeBlancC. Bush, ORNL

Solenoid-free current-start-up scenario including PF4 Try to store more poloidal flux at null region for IP ramp

Start PF2 & 3 coils with large positive biasBalanced by negative PF4Store 50-100mWb at nullNull size 1/3 of XP448

Null formation very sensitive to coil current time-history and vessel current model

113609

LRDIAG simulations predict vertical merging of X-points

Shot 113608 - C. Bush, ORNL3 ms4 ms6 ms7 msCamera Gain = 95

s113609 (N. Nishino - Hiroshima fast camera)2.442 ms2.664 ms2.886 ms3.108 ms3.330 ms3.552 ms3.774 ms3.996 ms4.218 ms4.440 ms4.662 ms4.884 ms5.106 ms5.328 ms5.550 ms5.772 ms5.994 ms6.216 ms6.438 ms6.660 ms6.882 ms7.104 ms7.326 ms7.548 ms

Magnetics bound IP to < 15kA (probably only few kA)B_L1DMPPPGU7B_L1DMPPPGL7B_L1DMPPPGU8B_L1DMPPPGL8IPF4RogowskiBright emission (Hiroshima camera) in 2.5 8 ms~50 G RF noise IPF4 diff.Ip = ~ 15 kA113611 (vac. shot)B-dot loop signal (near PF4)

ResultsHHFW pre-ionization necessaryNeed sufficient neutral density, 0-0 phasingIncrease from 0.5 MW to > 1 MW w/ more strapsEBW could be very helpful (was on TST-2)Large null required for IP initiation thus farNeed more work on finding optimal balance of stored flux vs. null size vs. initial plasma shapeGood plasma position evolution following breakdown crucial to high IPDINA modeling should be helpful here

Inductive plasma start-up scheme using a conducting center-postSingle turn TF leads to an attractive ST CTF.R = 1.2 m, a = 0.8 m

Wall Loading at Test Modules (MW/m2)1.03.0HH (ITER98pby2)1.41.8Applied toroidal field (T)2.42.2Plasma current (MA)12.611.4Normalized beta (bN)4.17.0Toroidal beta (bT, %)26.845.1n/nGW (%)1752Q (using NBI H&CD)2.45.8Fusion power (MW)72214Number of radial access ports77Radial access test area (m2)12.812.8PHeat/R (MW/m)3767Tritium burn rate (kg/full-power-year)412Total facility electrical power (MW)286272Fraction of neutron capture (%)81.681.6Local T.B.R. for self-sufficiency1.231.23Toroidal field coil current (MA)14.613.2Center post weight (ton)8989Capital cost ($B) with 40% contingency1.47

Possible as long as the poloidal flux is available at the time of the plasma start-up. Assuming the center-post radius is 50 cm for NSST (or CTF-like device), Bz ~ 0.7 T would mean the center-post may provide ~0.55 Wb. However, if we use the same coils energized in the same current direction, Bz ~ 8 T can be obtained in the center-post. ~ 6 Wb is stored in the center-post. Then, can one reverse PF #2 and #3_1 before the center-post eddy current decays away?Solid center-post envisioned for ST reactor could provide additional poloidal flux

TF column charging sequenceField null formation and plasma initiationCenter-post resistive skin timet=T1T2Vacuum vessel resistive skin time

Copper TF center-post charging sequenceAt t = T1 , All PF currents in the same polarity to maximize the chargingAt t = T2 , PF2 current reverses to create field null before center-post flux decaysCopper TF Center-post

PF1,2Outermost shellAt 2 secAt (1.0 m, 0)Center-postInduced current waveforms and poloidal fluxMid-plane

Evolution of Bz profileDriving current waveformTotal Bz profileBz by center-postBz profile of a typical solenoidPF1,2Due to vesselSolenoidCenter-post

CenterpostCentral solenoidNull regionStray field due to the charged center-post Like OH, the stray field is relatively small and uniform in the breakdown region. The stray field can be easily nulled out by compensation coils (on-going). The present method is quite compatible with the PF-only start-up concept utilizing the same hardware.

Some thoughts The center-post flux storage concept can essentially double the available flux by using the same hardware of the outer-PF-only start-up concept. ~ 10 Wb could potentially solve the CTF start-up and ramp-up issue. NSTX gets 1 MA with 0.35 Wb. So, it should be possible to reach 15 MA with twice the major radius (R = 1.7 m) with 10 Wb ( R2). The concept works better with high R/a. If we go with R/a = 2 CTF, then one can envision R = 75 cm core, which can essentially double the flux compared to R = 50 cm core. The induced loop voltage is determined by the stored flux and the flux decay rate. It can be adjusted by controlling the center-post resistivity (material, temperature) or resistance (thickness). If the inner torus radius is 4 m for example for R = 6 m advanced tokamak reactor, one can conceive a metal ring structure (a combination of vacuum vessel, shield, and support structure) of mean-radius of 3.5 m. In that case, one can store up to 230 Wb of flux assuming 7 T charging field ( x 3.52 x 7)! Even a fluctuation of that would be sufficient for start-up. Since pure metal is much more radiation resistance and simple compared to highly stressed OH solenoid, it could also be a more attractive reactor option for tokamak as well.

SummaryTwo complementary inductively-based concepts to aid the solenoid-free start-up for future ST and tokamak reactors are presented.

A combination of out-board PF coils placed outside the vacuum vessel is shown to create a good quality field null region while retaining significant volt-second capability for current ramp-up. - For NSST, 4 - 5 Wb possible for ramping the current to a few MAs.- For NSTX, ~0.12 Wb possible for Ip ~ a few hundred kA.

Inboard-side conducting material to store the magnetic flux which is initially charged up by the outboard-side outer PF coils. For ST, it is conceivable to utilize the central TF conducting post as the flux storage. - The NSST (CTF) size device can provide additional 2 - 4 Wb with this method.

Like the OH solenoid, the stray field generated by the center-post flux is relatively small which would make it suitable for the plasma start-up utilization.