Non-Solenoidal Plasma Startup in

Transcription

1 Non-Solenoidal Plasma Startup in the A.C. Sontag for the Pegasus Research Team A.C. Sontag, 5th APS-DPP, Nov. 2, 28 1

2 Point-Source DC Helicity Injection Provides Viable Non-Solenoidal Startup Technique Plasma guns provide localized, pointcurrent source at plasma edge Technique appears to be flexible & scalable to larger currents & devices Divertor injection Up to.1 MA plasma current to date I (MA) I inj Midplane injection time (ms) A.C. Sontag, 5th APS-DPP, Nov. 2, Midplane injection gun off 32 Z (m) R (m) At gun shut-off: = 94 ka R =.45 m l i =.35 κ = 1.6 β p =.22 W tot = 35 J

3 Local Plasma Current Sources + Helical Vacuum Field Give Simple DC Helicity Injection Scheme * Inject current into existing helical magnetic field high I inj & modest B current filaments follow field lines toroidal current = I inj *geometric windup high I inj & low B current-driven B θ overwhelms vac. B z relaxation via MHD to tokamak-like Taylor state w/ poloidal flux amplification reduced B z * M. Ono, et al., Phys. Rev. Lett. 59, (1987) A.C. Sontag, 5th APS-DPP, Nov. 2, 28 3

4 Pegasus is Studying Physics of Plasma Gun Startup & Using Technique to Access High-I N Critical ST issue: plasma startup and ramp-up (FESAC TAP report) Relaxation through simultaneous satisfaction of multiple constraints Plasma gun startup readily coupled to other current drive Edge current drive allows access to high-i N via j(r) manipulation Goal: develop understanding to project to fully non-solenoidal operation with RF growth & sustainment of plasma gun target A.C. Sontag, 5th APS-DPP, Nov. 2, 28 4

5 Point-Current Source Injection Allows Geometric Flexibility Operation: Guns biased relative to anode DC helicity injection rate: K inj = 2V inj B N A inj Divertor injection V inj - injector voltage B N - normal B field at gun aperture A inj - injector area maximize helicity by increasing B, A inj & V inj Advantages of plasma gun system: 1. Potentially scalable to large facilities 2. Flexible geometry possible 3. Clean operation High-Z impurities trapped inside gun Midplane injecton Anode Plasma streams 3 plasma guns V inj A.C. Sontag, 5th APS-DPP, Nov. 2, 28 5

6 Pegasus Operates at Near-Unity A to Study HighIN, βt ST Regime Equilibrium Field Coils Centerstack: High-stress Ohmic Exposing Ohmic Heating heating solenoid Solenoid (NHMFL) Experimental Parameters Parameter Achieved Goals A R (m) Ip (MA)!.18!.3 IN (MA/m-T) RBt (T-m)!.6!.1! 1.4" "3.7 #shot (s) $t (%) PHHFW (MW) Vacuum Vessel 4 cm Toroidal Field Coils A.C. Sontag, 5th APS-DPP, Nov. 2, 28!.5 > 4 1. Anode RF Heating Anntenna Ohmic Trim Coils!.2! 25.2 Plasma Limiters 6 Outer limiter Plasma guns

7 Several Constraints Must Be Satisfied for Relaxation to a Tokamak-Like State Conditions for relaxation to occur: B v : low to allow null formation B TF : high to increase helicity injection B v, B TF : tokamak equilibrium - force balance, q a B v /B TF : avoid collision with injector hardware Consistent with experimental observations divertor injection: center-post limited discharges relaxation coincides with reversal of central poloidal flux midplane injection: plasma modeled as set of axisymmetric filaments perturbed magnetic field shows null in relaxed cases Current central column flux 6kA Divertor injection I inj! pol /I inj time (s) 2628x1-3 Midplane injection 2A current multiplication A.C. Sontag, 5th APS-DPP, Nov. 2, 28 7

8 Relaxation to a Tokamak-Like State Leads to Particle Confinement & Heating Relaxation to tokamak-like state very sensitive to applied B v consistent with null formation Relaxed state appears tokamak-like current multiplication particle confinement increased decay time suggestion of higher T e subject to radial force balance.8 ms after guns off sharp edge A.C. Sontag, 5th APS-DPP, Nov. 2, 28 8 Plasma current (ka) M = / I inj!n e dl (1 19 m -2 ) B v = 62 G B v = 67 G Time (ms) geometric windup

9 The Maximum is Determined by the Balance Between Helicity Injection and Resistive Dissipation Total helicity injection rate in tokamak geometry: dk dt = " 2 % #J $ B d3 x " 2 &' &t ( " 2 V % A )B$ ds resistive dissipation AC injection (inductive drive) DC injection K AC = "2 #$ #t % = 2V loop% K DC = "2 $ #B% ds = 2V inj B & A inj A DC term is recast as effective V loop : V eff = N inj A inj B ",inj # V bias given by self-consistent confinement modeling with V loop ~ V eff + V ind standard tokamak scaling is useful in regime where perpendicular losses dominate A.C. Sontag, 5th APS-DPP, Nov. 2, 28 9

10 Taylor Relaxation Criteria Also Limits the Total Sustainable for a Given Plasma Geometry Considering force-free equilibrium: " # B = µ J = $B Current penetration via Taylor relaxation gives: " plasma < " edge µ " # µ I inj 2$R inj wb %,inj Assumptions: Force-free equilibrium: % C " p $I inj ' & 2#R inj µ w Driven edge current mixes uniformly in SOL Edge fields average to tokamak-like structure ( * ) 1/ 2 R A p Plasma area C p Plasma circumference Ψ Plasma toroidal flux w SOL width w/ j A.C. Sontag, 5th APS-DPP, Nov. 2, 28 1

11 Max Achieved When Helicity and Relaxation Criteria are Simultaneously Satisfied Estimated plasma evolution Anode max Helicity limit I TF = 288 ka V bias = 1kV V ind = 1.5 V I inj = 4 ka w = d inj L-mode τ e Plasma guns Time Relaxation limit Requires B v ramp for radial force balance & V ind A.C. Sontag, 5th APS-DPP, Nov. 2, 28 11

12 Sufficient Helicity Injection is Required to Drive Plasma to the Relaxation Limit Helicity injection rate varied by changing V bias K DC "V bias " Z inj Injector impedance controlled by neutral fueling 9 V V bias = 12 V 12 V Helicity Limited Relaxation limit increases with V bias, helicity injection rate R = 47 cm A.C. Sontag, 5th APS-DPP, Nov. 2, 28 12

13 Helicity Balance Provides One Limit on Current Max appears limited by injected helicity for divertor gun data all cases with static external fields no inductive V loop Toroidal Current [ka] Divertor Gun Data Selected for V eff study V eff V surf supports helicity conservation: V loop V surf at gun shut-off measured by center column flux loop helicity efficiently transported into plasma current drive limited by helicity injection rate A.C. Sontag, 5th APS-DPP, Nov. 2, calculated V eff (V) Average (dk/dt) Inj / I TF [Wb 2 s -1 A -1 ] x 1-6 V eff = V surf.2.4 measured V surf (V).6 1.2

14 Relaxed Gun Plasma Exhibits Increased Stored Energy and Confinement after Gun Turn-Off 3-gun array: up to ~.1 MA PF induction adds AC helicity/cd helicity input limited by power supply voltage Magnetic equilibrium reconstructions for plasma characteristics stored energy steady during compression maximum Ip when plasma fills vessel Limiters anode (MA) A,! " p q 95! A W tot " p 8V #B (Gauss) q 95 Wtot (J) reconstructed flux at gun turn-off plasma gun array l i, R (m) l i R m time (ms) A.C. Sontag, 5th APS-DPP, Nov. 2, 28 14

15 Plasma Gun Startup Provides Robust Target Plasma for Consequent Ramp-Up and Sustainment e.g.: to OH CD 3-gun target then OH drive pre-oh plasma ~8 ka Equivalent with 1/2 OH flux swing ~ 5% flux savings Develop further to optimize target suitability for other CD (MA) (MA).15 plasma gun startup OH only v loop I inj ms v loop (V), I inj (ka) v loop (v) time A.C. Sontag, 5th APS-DPP, Nov. 2, 28 15

16 I N > 12, /I TF > 2 Readily Accessed at Low-A /I TF 3 techniques used to achieve high I N ( /I TF I N /6) OH drive at low TF via plasma gun pre-ionization Non-inductive discharge formation at low TF Fast TF ramp-down during OH operation /I tf gun pre-ionization TF ramp Plasma Current (MA) Guns guns only Only Ohmic Ohmic Plasma gun startup able to achieve highest I N I N (MA/m*T) soft limit w/simple OH drive & large TM A.C. Sontag, 5th APS-DPP, Nov. 2, 28 16

17 Plasma Gun Startup Appears to Give Broad, Stabilizing Current Profiles Current injection at edge leads to hollow j(r) l i.35 at gun turn off Transient TM suppression TM returns after l i increase Create stable, full-current targets in future system exploit hollow j(r) target with RF Current (ka) I inj (ka) φ pol (mwb) Mode Amp. (au) A.C. Sontag, 5th APS-DPP, Nov. 2, Typical DC-helicity-injection discharge evolution Centerstack Fluxloop l i Plasma Current Toroidal Field Rod Current Region of Interest!B 2 of Outboard Midplane Mirnov s Time (msec) time (ms)

18 Near-Term Issues to be Addressed What determines λ edge? J edge broadening due to magnetic turbulence (edge & global) magnetic shear gun characteristics physical geometry How does T e scale with? χ vs. χ in the presence of magnetic turbulence confinement depends of degree of stocasticity of magnetic field What determines injector impedance? V bias = I inj Z inj neutral fueling filament length What are plasma properties? T e, n e, j(r), P rad, etc. A.C. Sontag, 5th APS-DPP, Nov. 2, 28 18

19 Point-Source Current Injection Potentially Provides an Attractive ST Startup & Growth Technique Point-source DC helicity injection appears to be viable startup technique simple & scalable Tests today with small 3-gun system gives Ip up to ~.1 MA limited by present hardware configuration Helicity driven discharges governed by helicity balance and Taylor relaxation limits many issues remain to be addressed (edge current profiles, gun impedance, field stocasticity, χ vs χ, geometry, plasma characteristics, etc.) Pegasus moving towards high-current startup and sustainment without solenoid A.C. Sontag, 5th APS-DPP, Nov. 2, 28 19