Expanding Non-Solenoidal Startup with Local Helicity Injection

Transcription

1 Expanding Non-Solenoidal Startup with Local Helicity Injection Justin M. Perry J. L. Barr, G. M. Bodner, M. W. Bongard, M. G. Burke, R. J. Fonck, E. T. Hinson, B. T. Lewicki, J. A. Reusch, D. J. Schlossberg And the PEGASUS team 56 th Annual Meeting of the APS Division of Plasma Physics University of Wisconsin-Madison Nov. 18, 15 PEGASUS Toroidal Experiment

2 Local Helicity Injection (LHI) is a Promising Non-Solenoidal Startup Technique Ip.18 MA (Iinj = 5 ka) Injected Current Stream. Ip Ip [MA] Local Helicity Injectors. Iinj 25 3 Washer-stack arc sources inject edge current Unstable current streams form tokamak-like state via Taylor relaxation Compact, modular, and appears scalable to MA-class startup

3 'RS'(K,$). ild. e 21 4 artup.! Multi-Year Technology Development has Produced Robust, High Performance Injectors Formidable requirements: E.T. Hinson GP [ms] 35 Gas Feed VPF + VLpA Model for injector impedance 3 ~3x increase in helicity input Veff High Vinj achieved recently Injector Voltage No deleterious PMI Shield Rings cm from LCFS Arc Source High Jinj (~ 1kA/cm2).5 Vinj > 1kV Loop Vo. Cathode Shield VINJ [V] Injection Cathode Fig (a): -D model and!t($ ); (b): -D LHI drive voltages. LHI current drive ~ Vinj Fig Concave injector with PM vs. quiescent frustum shield design Fig Maximum, average &XHY in condition

4 Global Power Balance Model: I p (t) Evolution LHI drive quantified by effective loop voltage:* Power balance relation: I p V LHI A inj B ϕ,inj Ψ Subject to Taylor relaxation current limit:** I p I TL ~ V inj Z [m] Shape Evolution [ ] =. V LHI +V IR +V IND I TF I inj w *: Eidietis et al., J. Fusion Energ. 26, 43 (7) **: Battaglia et al., Nucl. Fusion 51, 7329 (11) Outboard injection à Strong radial compression. R [m] κ: 1 2 δ:.5.5 Net induction voltage dominates current drive System Voltages [V] I p [MA] Taylor Limited 24 V LHI 28 I inj 25 3 V IND -V IR 32 LHI Drive Limited <T e > = 6eV 35 36

5 Detailed Dynamical Model: NIMROD Predicts Current Stream Reconnection as Drive Mechanism Inboard injection, emphasizing LHI current drive over inductive drive 1. Streams follow field lines 2. Adjacent passes attract * J. O Bryan, et al., Physics of Plasmas, 19, 871 (12) 3. Reconnection pinches off current rings 4. Accumulation of poloidal flux; current filaments persist

6 Anomalous Ion Heating Supports Existence of Strong Reconnection Activity T i > T e during LHI T i, LHI >> T i, OH CIII T i [ev] 3 1 LHIOH Handoff T i scales with expectations from reconnection experiments: T i, ~ B2 rec n e ~ I inj V inj Emerging evidence that ion heating is localized to edge HeII T i [ev] 3 1 T perp T See M.G. Burke Poster: GP I bias *(V bias ) 1/2 4 5x1 3

7 Stream Interaction Manifests as Edge Localized MHD Burst MHD bursts accompany I p growth Localized in edge* n=1 line-tied kink structure I p [ka] NIMROD Coherent streams persist in edge, matching NIMROD predictions Auto-Power Auto-Power [p(t)^2/hz] t=18-ms Internal Measurement f=18. khz From (8kHz Bandwidth) Hall Probe R [m] R [m] Injector Radius Suggests reconnection region may be localized to edge ~b [T/s] ~b[mt] PEGASUS * E.T. Hinson, Ph.D. Thesis, UW-Madison, 15.

8 Edge Localized Reconnection and Strong V IND May Support Good Core Confinement, Favorable Scaling Thomson scattering shows strong central peaking T e and pressure profiles Central T e comparable to Ohmic L-mode Contrasts with expectations of highly stochastic field across profile May reflect different drives operating: V IND (across plasma) V LHI (edge) If good confinement persists, scaling to NSTX-U very favorable Need to understand influence of helicity injection vs. inductive drive T e [ev] t shot = 23 ms 4 Approx. R R [cm] Plasma Current [MA] 6 Effect of Confinement Scaling cm Ohmic L-mode 1 Pressure x1 6 [AU] R-R Stochastic Fixed T e =3eV Normalized Helicity Injection Approx. R R [cm] 5 6 8

9 Divertor Injection: Vary Injector Geometry to Separate Inductive and Helicity Drive Effects Fixed plasma geometry à minimal inductive drive Separates effects: edge reconnection Vs. inductive drive HFS à 3-4x increased helicity input Access to higher I p startup Increased B TF test NSTX-U relevant physics New injectors fabricated, ready for install Large aperture, high voltage design Initial tests before Jan 1. I p [ka] Divertor Injectors Projected Injector Performance Past Div Injectors 1 2 7eV 5eV 3eV New Div Injectors V norm = A inj V bias /R inj [V-m] 3 4

10 Building the Physics and Technology Basis for Scalable LHI Increased Performance Injector technology development allows reliable operation at high V inj Suppression of PMI Increased Understanding Experimental evidence supports NIMROD current drive mechanism Outboard injection: reconnection localized to edge, strong inductive drive Thomson Scattering: Ohmic-like core confinement à Favorable scaling to high current startup? Next Steps: studies of confinement and scalability Divertor injectors à increased helicity injection, reduced inductive effects NSTX-U relevant physics regime