PLASMA STUDIES AT HIGH NORMALIZED CURRENT IN THE PEGASUS EXPERIMENT for the PEGASUS team: D. Battaglia M. Bongard S. Burke N. Eideitis G. Garstka M. Kozar B. Lewicki E. Unterberg Raymond.J. Fonck presented to the 6th Annual Meeting of the APS Division of Plasma Physics Savannah, GA Nov. 15-19,
Explore plasma limits as AÆ 1 Pegasus is an extremely low-aspect ratio facility exploring quasi-spherical high-pressure plasmas with the goal of minimizing the central column while maintaining good confinement and stability Primary Pegasus Goals: Stability and confinement at high I p /I TF - Extension of tokamak-like studies Limits on b t and I N ~ 6I p /I TF (kink) as AÆ 1 - Overlap between the tokamak and the spheromak Planned Future Emphases: Support ST program movement to next stages - Noninductive startup tests - EBW tests for heating & CD (w/pppl) - Novel divertor design tests (w/ut) I p /I TF 1 tokamak-spheromak overlap region TS-3, Spheromaks q y =6 } I p /I tf = figure of merit for access to low-a physics PEGASUS NSTX, MAST START CDX-U, HIT, TST-M, Globus-M, ETE MEDUSA.1 1. 1. 1. 1.6 1.8. Aspect Ratio
Phase I demonstrated low-a characteristics A ~ 1.1 via OH - High field/stress solenoid - Very low TF (<.1 T) - HHFW available Low field, ohmic only Æ high I N & b t b t (%) 3 5 15 = Pegasus data Conventional Tokamaks b N = 6 b N = 3.5 START Resonant L-C power systems - Fixed waveform evolution - Low l i, low shear 5 6 I N = I p /(ab t ) 8 n- bar (x m -3 ) 1..8.6..... n e up to density limit n G =I p /p a Pegasus Data..6 I p /p a (MA/m ).8 1. 1 8 q q. Measured low-shear q(r) mag. only (poly.). y N. y N PHC.6.8 1.
Phase I: An I p /I TF 1 soft limit observed Large resistive MHD instabilities as TF Ø.16 low B t and fast di p /dt Æ of low-order q=m/n at low T e ultra-low A, low l i Æ fi early appearance low central shear Rapid growth of /1, 3/ tearing modes and large saturated island widths I p /I TF 1 fi q 1.5 - plus reduction of V-sec as TFfl 1 Measured low-shear q(r) 8 q.. y N..6.8 1. I p =I tf 16 Plasma Current (MA).1.8.....8 I N TF Rod Current (MA) 6 I p /I TF.1.16 ka 1 1 8 6 Ip Time (s) d B /1, 3/ modes observed island size a - - Gauss
Phase II: PEGASUS Facility has been Completely Rebuilt New Tools Enhance Study of Plasma Stability Boundaries - All coil power systems upgraded to programmable waveform control - Active shaping and position programming - Increased V-sec ( -.5x) and control - Low inductance, higher B T (3x) Toroidal Field bundle for rapid TF ramp - Divertor coils for separatrix operation - Plasma Guns for plasma startup and current drive PF1 Shaping Coil Low Inductance TF Bundle Divertor Coil Equilibrium Field Coils Ohmic Solenoid Plasma Guns Ohmic Trim Coils Outer Limiter Divertor Plates PF8 Shaping Coil
Power systems offer path to high I p /I tf, b t operation Suppress tearing modes early in discharge evolution = Transiently manipulate q during discharge: - Increased TF at startup - Variable I p and R = Reduce resistivity before low-order rationals appear - Maximize J - Increase ohmic flux - Use HHFW system Explore edge kink boundary at high field utilization - Manipulate edge shear - Decrease edge currents - Manipulate plasma shape - Manipulate current profile Begin operations with reduced OH power - 9V IGBT H-bridges for low-power OH (1 MVA) - Near-future: 7V IGCT bridges for full OH ops (13 MVA) Rod Current (KA) Loop Voltage I 13Coil (KA) Example Waveforms 3 5 15 6 5. 1 1 8 6.1.1....3 Time (s).3 Time (s).5. TF.5..3 EF OH Time (s).6.5.35
Phase II: Earlier Results Recovered; Extending Operation Space Frequency (Hz) Comparable characteristics to Phase I plasma achieved - I p ramprate, maximum I p Mode activity: similar qualities - frequency, d B I p ka 1 8 15 5 ~ 3 MA/s MHD.18 I p d B.. Time (sec). 5-5 -.6 Guass Varied I p ramprates & respective envelop functions Plasma Current (ka) Mirnov envelop function (A.U.) 1 8.5..3..1.... Time (sec) 8 ~5 MA/s ~3 MA/s ~9 MA/s ~7 MA/s ~ MA/s.3 Initial tests to lower I p ramp-rate - Envelop function of Mirnov signal decreases with decreased di p /dt
New Capability Widens Operating Space Higher TF allows extension of operation space Limited OH flux and control during shakedown - /1 mode still evident with high di p /dt - Full OH will allow challenge to high I N, I p /I TF Shot 85: Sample Plasma 5 15.8 Pixel. I p = I TF Pegasus Phase I Aug-Nov 5 Plasma Current (MA)..16.1 Plasma Current (ka) 1 8 6 5 Pixel 15 5.8.16.. Time (sec).8.....8.1.16 TF Rod Current (MA)...8
Present Status - Integrating New Capabilities towards I p /I tf > 1 Summer-Fall : Shakedown & Commissioning - 1st plasma in late May - Shakedown campaign Transient suppression and PS stabilization New facility tests and systems shakedown Effects of wall currents with new waveforms Low power startup studies Phase I operation space recovered - New power systems stabilized and working as desired Robust to major failures Pixel Shot 85: Full-sized Plasma 5 15 5 Recent upgrades to enhance operations - New diagnostics - Plasma guns installed for tests of CD and fueling - V l (t) control => di p /dt control Fall : Installation of first High-V OH power supplies Plasma Current (ka) 1 8 6.16 5 Pixel 15.. Time (sec).8 5 Campaign in Winter -5: Use New Tools - Commission new OH system for high-power ops - Tearing mode suppression - Access to I p /I tf > 1, low-q, high I N, high b t regime - Characterize ext kink limits - Introduce separatrix - Use gun for startup assist
Plasma Guns Being Tested for Startup and Fueling Use MST-style gun current sources to inject helical current in divertor region - I gun 3-6A, N e ~ m-3 Vary TF, EF fields to control current path Time-integrated plasma image Diverse potential applications - Ionized plasma fueling source in SOL - Ease OH V-sec needs - Provide PF-only startup path - Non-inductive startup path Gun installed in lower divertor region Plasma Gun Divertor Plate
Current Filaments Merge Above Threshold SN 3 Vbias = 5 V Low current => Filaments Maintained No amplification SN 38 Vbias = V High current => Filaments Merge Net amplification 3 7 Current (A) 5 15 5 5 Toroidal Current Injected Gun Current 3 Time (s) 35 x -3 Current amplification up to ~ Clear merging or reconnection(?) above a threshold in power Closed flux surfaces requires field, gun optimization Current (A) 6 5 3 Toroidal Current Injected Gun Current 5 3 Time (s) 35 x -3
Gun Startup Compatible with OH or EF Inductive Drive If scales linearly with # of guns Vloop added to gun plasma No preionization or null required via OH or def/dt If ~ 6 ka (OH) and ~ ka (EF) Closed flux surfaces next target Optimize field, Igun evolution ~ x increase needed (?) Current [A] 8 6 Gun Plasma Current 1 Gun Plasma Current Gun Multiplication Factor 1 Gun Multiplication Factor 15 5 Current Multiplication Factor Toroidal Current [A] EF Coil Current [A] 15 5 1 8 Guns + def/dt Shot 5731 Shot 575 Plasma Current [A]. 7 6 5 3..... 6.6 Time [s] Guns + doh/dt. 8 1 Loop Volt OH Loop Volt OH Loop Volt, Highest Ip.3.8.3 Time [s].3.3
Economical Tests of EBW Possible on PEGASUS EBW heating and current drive of interest for ST regime Plasma startup, sustainment Applicable to low-field, overdense plasmas Of interest to future NSTX development Basic principles tested on W-7S and CDX Need to be tested at significant power levels Pegasus good candidate for EBW development Low-cost.5 GHz technology Klystrons and waveguide available from PLT Need to demonstrate good target plasma control Working with PPPL to develop best approach Modeling Hardware Experiments (a) 1 3 5 6 (b) n 6 n upshift deposition profile (a) EBW ray tracing calculations for a 5 ka PEGASUS equilibrium, central density of ne() = 13 cm -3. (b) n along the ray path, showing the upshift depends upon launch position. (c) Power deposition profiles corresponding to the rays in (a) and (b). - - (c) 1 8 MWm -3 8 6.. 6.. 6 5 3. 5.6 rho 3.8..6 rho 1 1..8 1. 1.
SUMMARY Phase I ops up to Spring - Ip/Itf = 1.1 - b t 5% - Factors found limiting plasma current: + internal resistive modes + V-s limitations + external kinks Facility completely rebuilt and upgraded to provide increased plasma control - New switching power supplies (final OH installation now) - New divertor and shaping PF coils - New TF centerstack - etc. Phase II experiments have begun - Switching systems and infrastructure debugged - Low power OH demonstrating increased control Phase I results readily reproduced Fine control of OH, TF, and PF fields being established Modest feedback control developing - Plasma gun tests suggest non-oh startup capabilities - High power operations to challenge low-q, high-b as A -> 1
PEGASUS Poster Session: Thursday Afternoon PP1.15 Stability Studies PP1.16 Diagnostic Measurements PP1.18 Control & DAS Systems PP1.19 Facility and Power Systems PP1. Plasma Gun Experiments PP1.1 Potential EBW Experimemts [Unterberg] [Kozar] [Burke] [Lewicki] [Eidietis] [Garstka] Related - Weds morning: HP1.7 NIMROD MHD modelling [Sovinec]
Low Current/Power --> Linear Scaling; Filament Maintained Current channel follows field line - Maintains helical nature Total toroidal current ~ 5 x gun current - I p /I g constant Current (A) 35 3 5 15 5 V bias = 5 V sn 33 I_tor Igun_x5 Ratio 15 5 Current Amplification 6 8 Time (s) 3 3 3x -3
HIgh Current/Power --> Nonlinear Scaling; Filaments Merge Current channels merge/reconnect - Generates extended plasma Total toroidal current > 5 x gun current - I p /I g increases Current (A) 7 6 5 3 V bias = V sn 37 I_tor Igun_x5 Ratio 15 5 Current Amplification 6 8 Time (s) 3 3 3x -3