Advancing Toward Reactor Relevant Startup via Localized Helicity Injection at the Pegasus Toroidal Experiment E. T. Hinson J. L. Barr, M. W. Bongard, M. G. Burke, R. J. Fonck, J. M. Perry, A. J. Redd, D. J. Schlossberg Workshop on Exploratory Topics in Plasma and Fusion Research University of Wisconsin-Madison Madison, WI Aug. 7, 214 PEGASUS Toroidal Experiment
Localized Helicity Injection (LHI) is a Nonsolenoidal Startup Technique Non-solenoidal startup is relevant to several ReNeW Thrusts Thrust 16 Develop the spherical torus Thrust 18 Achieve high performance with minimal field Thrust 1 Technology of plasma-surface interactions Startup without Ohmic solenoid induction is highly desirable LHI is a relatively simple startup technique Appears scalable to MA-class startup
LHI Relies on Magnetic Relaxation and Helicity Balance for Tokamak Startup.2 I P.15 I INJ Current [MA].1.5 Local Helicity Injectors. 15 2 25 Time [ms] 3 35 65358 4 Current injected along field lines Unstable current streams relax towards Taylor minimum energy (tokamak-like) state Achieved I p consistent with helicity conservation
LHI is Accomplished with Localized Current Sources in Plasma SOL Washer-gun plasma arc source serves as cathode for injector current Plasma cathode biased with respect to vacuum vessel Anode" +" VINJ" Molybdenum Cathode Anode" Boron Nitride Washers" Molybdenum Washers" +" Varc" Molybdenum Cathode" D2 gas" IINJ ~ 2 ka, VINJ ~ 1 kv, AINJ = 2 cm2 Located within 1-2 cm of LCFS
Pursuing Development and Validation of Predictive Models for LHI Startup Hierarchy of models under development 1. Global I p limits Helicity conservation Taylor relaxation 2. Predictive I p (t) models -D Energy conservation Uses lumped parameter circuit model 1.5-D via TSC; employs confinement model 3. 3D Resistive MHD via NIMROD First principles model
1. Helicity Balance, Taylor Relaxation Determine Maximum I p Taylor Relaxation: I p! I TL = C p "I INJ 2! R INJ µ w ~ I I TF INJ w with: A p, A inj : Plasma, injector area C p : Plasma circumference Ψ: Plasma toroidal flux w: Edge current channel width Helicity Balance: I p! A p 2! R! V ind +V eff ( ) with: V eff! A injb!,inj " V inj I p determined by lower of the two limits Battaglia et al., Nucl. Fusion 51, 7329 (211)
1. Global Helicity Balance/Taylor Relaxation Limits Confirmed At Taylor Limit: I p max! I INJ, I TF, w "1/2 Limited to input rate I p < I TL if insufficient HI V inj =" 12 V" LHI LHI + OH 9 V" Taylor limit" 12 V" Helicity Limited" " Battaglia et al., Nucl. Fusion 51, 7329 (211)
2. When I p (t) < Taylor Limit -D Power Balance Model Predicts I p (t) Lumped parameter model + helicity conservation: I p!" V eff +V R +V PF +V Lp # $ = V eff : From helicity conservation V R : Resistive dissipation from assumed flat Spitzer T e (R,t) = 7eV V PF : Poloidal induction voltage V Lp : Voltage due to plasma self-inductance Inputs: R (t), a(t), Ip(t ), <η >, κ(t), l i (t) Analytic low-a descriptions of L p, B z, plasma shape Differential equation in I p (t) solved when I p (t) < I Taylor (t)
2. Energy Balance Model Testing, Calibration Against Reconstructions and Data Initial tests give reasonable I p (t) At early times I p (t)= I Taylor (t) Later times trajectory follows -D model -D model predictions vs data" Strong inductive component at high I p Approach to model test/validation Low A inductance formulas validation Obtain η model with T e (R,t) measurements Confirm early I p (t) = I Taylor (t) Test helicity-dominated regime Confirm V eff model Loop Voltage [V] Voltage terms: drive dominated by inductance at high Ip" 2. 1.5 1..5. -V IR V eff V PF + V Lp 2 25 3 35 Time [ms]
2. Confinement and Resultant T e (R,t) Model Determines I p Scaling with Device Size/V eff Model depends critically on estimate of η V eff, V geo, V PF drive vs V R dissipation Need T e (R,t) model to estimate I p V R term I p [MA] -D Energy Balance for different resistivities ".25 1 ev.2 8 ev.15 6 ev.1 5 ev.5 Initial tests invoke simple flat T e profile. 2 25 3 35 Assuming dominance of stochastic transport Time [ms] <T e >~7eV estimated from V-UV spectroscopy Initial Thomson core T e data" 25 2 T e = 72 ± 22 ev 15 [e-] 1 Thomson scattering measurements imminent 5 T e (R,t) to provide accurate η model -5 54 545 55 555 Will inform confinement model Wavelength [nm]
2. Impurity Spectroscopy Hints at Hotter Core T e, Inconsistent with Stochastic Confinement O-V core intensity >> edge intensity O-V Intensity profile vs radius and time" Possible dual confinement regime? Warm Core OH-like confinement Cool Edge Stochastic confinement O-V Intensity (a.u.) 2x1 6 15 1 5 32 cm 4 cm 47 cm 51 cm Inductive drive Low MHD Reconnection Large MHD 16 18 2 Time (ms) 22 24 Ohmic-like core! favorable scaling at higher B TF, a, R Will resolve with Thomson scattering Confinement model needed for TSC
3. NIMROD Simulations Show Current Growth Resulting from Reconnection in Edge Resistive MHD simulation of divertor injection Calculated current rings in NIMROD*" Current growth via intermittent reconnection Coherent current streams in edge Filaments reconnect to inject rings into core Poloidal flux buildup and Ip multiplication 2.91ms Injected current ring 2.93ms Intermittent plasma ringlets observed Fast camera image of ringlet" 2.94ms 2.95ms *J. OʼBryan, PhD. Thesis, UW Madison 214"
3. NIMROD Calculations reproduce bursty MHD seen in experiment NIMROD MHD bursts from reconnection seen on synthetic diagnostics Qualitative similarity with experiment PEGASUS V INJ spikes consistent with inductive response to detaching ring Timed with sharp ΔI p ~ I INJ jumps Ip [ka] Outboard Mirnov [T/s] 8 6 4 2 4 2-2 -4 16 17 18 19 2 21 22 Time [ms] [OʼBryan Phys Plas.19, 212]"
3. Edge-Localization of MHD Consistent with NIMROD Reconnection Model High frequency: n=1 1-4kHz Peaked near injector radius Toroidally asymmetric Line tied kink-like Consistent with streams Auto-Power [p(t)^2/hz] 2 15 1 5 t=18-2ms f=18. khz (8kHz Bandwidth).55.6.65.7 R [m] Injector Radius.75.8.85 Low frequency: n= <5kHz Exists throughout plasma interior Consistent with plasma motion b/bt [%] 3.5 3. 2.5 2. 1.5 1..5. b/bt versus toroidal angle (Hilbert Transform analysis) Toroidal mirnov array (Z=-17cm,R=9cm) Mirnov Probe Array (Z=cm, R=93cm) 5 1 15 2 Toroidal Angle [º] Injector Location, Orientation 25 3 59624 35
3. NIMROD Model Testing Requires Research on Multiple Fronts Consistency with magnetic probe data Search for evidence of coherent current streams in edge region Compare magnetics signal with modeling of dynamic current streams Considering soft X-ray imaging diagnostic deployment Implementation of 2 nd generation divertor injectors in planning stage Ion dynamics Reconnection (ion) heating CIII Ion Temperature (ev) 8 6 4 2 CIII Continued development of NIMROD model 16 18 2 22 24 Time (ms) 26 28 3
-D Modeling Predicts LHI System Requirements for MA startup in NSTX-U Requirements for NSTX-U and beyond predicted by -D model Required HI rate V inj A inj B TF Power systems required MA startup on NSTX looks feasible Requires V eff > V geo + V PF Occurs at I p ~.2-.3 MA in Pegasus HI-dominated transport important Requires measurements at increased V inj, B TF,inj, Δt pulse,
Tests of Various Injector Concepts Favor Smaller A INJ, High V INJ Tests of large-area passive electrodes show reduced effective A INJ Integrated metallic electrode surface Cathode spot emission Above: integrated electrode injector assembly Large area gas-effused Mo Plate Hollow cathode emission Non-uniform J INJ, effective A INJ =.15 A GEOMETRIC Conclusion: Arc-based injector system most effective path to maximize HI rate I p [ka] 25 2 15 1 Area utilization: % 15% 1% Measured I p 5 2 4 6 8 Time [ms]
Controlling Plasma-Material Interaction is Enabling High VINJ Operation Injector requirements include Large Ainj, Jinj Vinj > 1 kv Δtpulse ~ 1-1 ms Minimize PMI all adjacent to tokamak LCFS Advanced injector design enables high Vinj Cathode shaping to mitigate cathode spots Shielding of cathode, insulators to prevent injector breakdown 3x improvement in Vinj, Δtpulse Voltage of quiescently operating injectors (red) and voltage after breakdown of overdriven injectors (black) 3 VINJ - Clean VINJ - Breakdown 25 2 VINJ [V] 15 1 5 16 18 2 [ms] 22 24
Hints of Higher Ip Emerging as the HI Rate Approaches the Induction Drive Levels Injector voltages and associated Ip 15 Ip [ka] Higher injector voltage operation shows increased Ip at as Veff increases Enabled by new high VINJ injector Ip 1 5 2 VINJ Pagoda -style injectors with quiescent operation VINJ [V] 15 1 5 6956 16 18 2 22 [ms] 24 26
Predictive Understanding of Plasma Impedance Required for Projecting to Higher I p Determines feasible V INJ, A INJ, I INJ and demands on power system Governed by plasma physics of arc source and tokamak edge Low I INJ : Double layer High I INJ : Space charge neutralization of e-beam by edge plasma J INJ = n edge e 2e m e V INJ n edge dependence to be validated Assuming n edge ~ fill pressure 3/2 J INJ ~ V INJ I INJ [A] 6 5 4 3 2 1 9 8 7 6 5 Ramp-up I-V characteristics for Pegasus injectors agrees with 2-parameter model across wide range of fill pressures Beam Charge Neutrality Double Layer ~V 3/2 ~V 1/2 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 V INJ [V] I INJ =(56+.15 P) V INJ P=88 Torr P=6 P=44 P=233
Critical Issues for LHI Predictive Understanding Addressed by Pegasus-U PEGASUS Physics Issues Confinement and transport Plasma control/evolution at long pulse Injector Physics Scaling with BTF Heat loads Compatibility with divertor (and passive plates?) PEGASUS-U Proposed facility upgrades to address those issues Higher BTF (5x increase) Longer pulse (1ms) LHI startup with divertor and passive plates Diagnostics: multipoint TS; CHERS via DNB
Towards MA-Class Non-Solenoidal Startup Pegasus is developing the understanding of localized helicity injection, a non-solenoidal startup technique for NSTX-U and beyond A hierarchy of models to understand and scale LHI is being developed Helicity balance/taylor relaxation place limits on max I p -D modeling to predict I p (t) Future TSC modeling will incorporate transport model NIMROD illuminates detailed physics Better understanding of injector physics enables improved injection rate Proposed upgrades to facility are focused on testing LHI at increased HI rate, long pulse and high toroidal field Talk available at: http://pegasus.ep.wisc.edu/technical_reports/