Abstract. , low-q operating space at near-unity aspect ratio. Plasmas are characterized by high b t ( 20% to date); very low toroidal field (B t

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1 Abstract The is exploring stability limits in the high b t, low-q operating space at near-unity aspect ratio. Plasmas are characterized by high b t ( 2% to date); very low toroidal field (B t.7 T); densities approaching the Greenwald limit; and low- l i (.2-.5). A soft limit in this operating space is observed at I p /I tf ~ 1 and is related to a combination of MHD activity in the lowshear interior and limited V-s availability. A disruptive external kink limit is observed at q 95 ~ 5, which is verified by DCON analysis of reconstructed equilibria. The experiment has been modified to avoid these limits. Internal tearing mode suppression will use plasma growth control, higher TF during startup, and increased V-s. Manipulation of the external kink limit will employ q(r) tailoring (via time-variable TF and divertor operation) and plasma shaping. Future experiments will focus on exploring extreme values of I p /I tf with an emphasis on verification of the kink stability limits in the tokamak-spheromak overlap regime. *Supported by U.S. DoE grant No. DE-FG2-96ER54375 E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

2 Outline Pegasus is a mid-size ST built to explore A Æ 1 Experimental results from phase I operations - Plasmas exhibit low-a characteristics - High b t, IN, b N via OH heating - High I p /I tf - low l i - Characteristics of High I p /I tf Operation - Large scale internal MHD limit on I p /I tf - External Kink at q 95 =5 Next phase of operations will focus on accessing high I p /I tf, b t regimes - Recent modeling has demonstrated existence and accessibility to this regime - DCON and local G-S solver for stability and equilibrium - TSC for shot simulations - Upgrades to increase access to this regime - Increased V-sec.; position and shape control; B TF versus time - Power testing has commenced E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

3 PEGASUS Facility E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

4 Mission: 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 Stability and confinement at high I p /I TF - Extension of tokamak studies Limits on b t and I p /I TF (kink) as AÆ 1 I p /I TF 1 - Overlap between the tokamak and the spheromak I p /I tf Æ 1 1 TS-3,4 Spheromaks q y =6 } PEGASUS NSTX, MAST START E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 figure of merit for access to low-a physics CDX-U, HIT, TST-M, Globus-M, ETE MEDUSA tokamak-spheromak overlap region Aspect Ratio

5 PEGASUS is a University-Scale, Mid-Sized ST Equilibrium Field Coils Centerstack: Exposing Ohmic Heating Solenoid (NHMFL) Experimental Parameters Parameter Achieved Phase II Goals Vacuum Vessel RF Heating Anntenna Toroidal Field Coils A R (m) I p (MA).16.3 I N (MA/m-T) RB t (T-m).3.1 k t shot (s).2.5 n e (1 19 m -3 ) b t (%) 2 > 4 P HHFW (MW).2 1. Ohmic Trim Coils Plasma Limiters E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

6 Diagnostic Set for Reconstructions and Mode Analysis Composite Photograph of Vessel Interior Schematic of Magnetic Diagnostics EBW antenna Centerstack 1 cm Outboard limiter HHFW antenna Extensive magnetic diagnostics Segmented divertor plates = Flux Loops = Mirnov Coils E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

7 PEGASUS Diagnostic Set Presently Operating Diagnostics Diagnostic Capability Measures Core Flux Loops (6) V L, Y pol Wall Flux loops (6) Vessel currents Int. Flux loops (2) Y pol Rogowski Coils (2) I p Diamagnetic Loop (2) F tor / b p B p, Mirnov Coils (56) B r, B z / MHD activity VUV (SPRED) central chord Impurity monitor Filterscopes central chord Oxygen, Carbon, VB, Da Interferometer single chord N e l High Res. Camera 1 fps Plasma shape/position 2-D SXR Camera Internal Shape/ j(r) Poloidal SXR Diode Array (19) MHD Activity Near-Future Diagnostics Diagnostic Capability Measures Status Tangential PHA single chord T e (t) In Development Tangential Bolometer Array ~32 chords P rad Testing Ross Filters 4 chords T e (t) Testing 2-Color X-ray 4 chords Te Testing Tangential VB Array ~2 chords Z eff (R,t), Ne(R,t) Testing DNB N e (R,t), T e (R,t), j(r) Proposed EBW Radiometer T e (t) Proposed See Poster QP1.69-S.P. Burke et al. for details E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

8 A < 1.3 Æ Ready Ohmic Access to High b t b t up to 25% and I N up to 6.5 achieved ohmically Low field Æ high I N and b t 3 25 = Pegasus data b N = 6 b N = 3.5 b t (%) Conventional Tokamaks START (Sykes, EPS 1) 5 2 E.A. Unterberg, APS-DDP, Albuquerque, N.M., October I N = I p /(ab t ) 8 1

9 Equilibrium reconstructions show low-a characteristics High-b t (Ohmic): b t > 2% High-b N (Ohmic): b N > 4 Large I p /I TF : I p /I TF ~ 1 Low-A Characteristics Natural k : k > 2 High field windup: high q y at low TF Paramagnetism: F/F vac 1.5 on axis (e b p < 1) Z (m) Flux Plot.5 R (m) Sample Reconstruction 1. Pa MA/m E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 q y N p <j> y.8 1. I p R a 154 ka.34 m.29 m A 1.15 k 1.33 F.3 T-m b t 18% W 57 J l i.54 q 1. q

10 Toroidal field utilization exhibits a soft limit around unity Maximum I p I TF Limit not disruptive or abrupt - I p saturates or rolls over.16 I p =I tf Plasma Current (MA) TF Rod Current (MA) E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

11 Two factors contribute to the I p /I TF 1 soft limit Large resistive MHD instabilities degrade plasma as TF Ø low B t and fast di p /dt Æ - fixed sine-wave loop voltage high resistivity early ultra-low A Æ fi low central shear Result: rapid growth of tearing modes and large saturated island widths - Most common modes: m/n=2/1, 3/2 - Leads to decreased C E, I p I p /I TF 1 fi q Reduced effective Volt-seconds as TF Ø reduced toroidal field Æ early appearance of low-order q=m/n delayed startup delayed startup + fixed sine V loop waveform Æ ka Frequency (Hz) /1 Ip.16 reduced effective V-s.18 Time (s) d B 3/ Gauss 2/1 E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

12 PEGASUS operates in low-l i, high-density space Visible light image Generally low l i = Pegasus Data START Data (Hender PoP 99) l i q n- bar (x 1 2 m -3 ) n e up to density limit n G =I p /p a 2 Pegasus Data I p / p a 2 (MA/m 2 ).8 1. E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 b N = Pegasus Data.1 High b N > 4l i.2.3 l i l i 4l i.7

13 High q 95 external kink limit observed Real u1 High I p plasmas often disrupt q 95 = 5 observed preceding disruption - l i =.5 at this time DCON analysis fi unstable to n=1 external kink - m=5 most unstable mode Consistent with theory expectation m=1 Poloidal mode eigenfunctions via DCON m=2 m=3 m=5 m= y N E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 MA AU Plasma Current q Free- Boundary Energy (DCON) Time (s) MHD Amplitude T/s

14 Measured q-profile indicates low central shear Tangential PHC SXR image Center column 2D soft x-ray camera gives q-profile - Images soft x-rays - Constant-intensity surfaces determined - Mapped into flux space - G-S equation with SXR constraints - Iterate solution until convergence Measured q-profile fi low central shear fi Measured Image Contours: Reconstructed 12 fi q 8 4 q mag. only (poly.) y N PHC y N E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

15 Island widths are on the order of plasma minor radius Island width estimates give w>1 cm SXRÆ large radial extent of mode Ip (ka) Frequency (Hz) w 4 d B qr ª ~ a 2 B t n dq dr d B=4 Gauss at wall Mode Spectrogram.15.2 Time (s) d B (Gauss) SXR Fluctuation (AU) SXR Chords.2 Time (s).22 E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

16 Access to High I p /I tf, b t Operation Suppression of large internal MHD modes - Vary q(y ) - Lower h before q(y ) approaches low-order rational mode surfaces Expand access to external kink modes studies - Plasma time evolution, shape - Edge conditions and edge currents Access to very high b t regime for stability analysis - OH access and HHFW heating availible E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

17 DCON: Stable Ip/Itf > 1 Equilibrium DCON modeling used to develop operating spaces for future run campaigns zero -b equilibria showing I p /I tf = 1.93 Z (m) Flux Plot.5 R (m) 1. Equil. Paramters Ip(kA) I N (MA/T-m) Ip/I tf l i q R (m) a(m) Elongation q ArcSinh D I (A.U.) ArcSinh D R (A.U) Predicted Stable with DCON DCON output D I,D R > => Unstable Mercier Criterion GGJ Resistive Criterion Newcomb Criterion, n=1 (A.U.) r q Profile y N E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

18 Ip/Itf ~ 3, I N ~ 19 Modeled High-I p /I tf, zero -b Equilibria Flux Plot Equilibria Parameters Ip(kA) I tf (ka) I N (MA/T-m) li q R (m) a(m) Elongation q ArcSinh D I (A.U.) x1-1 x1-1 ArcSinh D R (A.U) DCON Output D I,D R > => Unstable Mercier Criterion GGJ Resistive Criterion r Approaches Phase II Goals of PEGASUS 12 1 b N = 6 q Profile Newcomb Criterion, n=1 (A.U.) y N b t Conventional Tokamaks START E.A. Unterberg, APS-DDP, Albuquerque, N.M., October PEGASUS Increase I p /I TF I N = I p /ab to [MA/(m T)] b N = 3.5 Increase Aux. Heating 2

19 t TF Rampdown is tool for q(r) and b manipulation E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 p: t, q falls more slowly (on t R time scale) ervation: en, li drops Ip increase PEGASUS simulation:.3 T-m in 2 ms - Ip: up from 9 ka to 14 ka from 5% to 16% - F up from 1 to 6 mwb n 2 3 )

20 ew tools to access Ip > Itf aring modes early in discharge evolution ntly manipulate q during discharge: reased TF at startup => high Itf, low inductance TF bundle riable I p and R control => coil-current-waveform control e resistivity before low-order rationals appear ximize J => Vloop control, position & shape control rease ohmic flux => new ohmic power supply HHFW system => position control, Vloop control E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 e kink boundary at high field utilization nipulate edge shear => divertor coils & PF shape control crease edge currents => loop voltage control nipulate plasma shape => shape control nipulate current profile => Vloop control, position control

21 ades in Progress for All Coil Sets Example Phase II Phase I 4 12 turns 6 turns I TF (KA) Fast time-variable Quasi-DC 45 ka-t max 15 ka-t max.8 Time (s) 8 Programmable Half-sine Phase I I ef (KA) Bv (Tesla) E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23 Phase II Waveform Waveform V loop ±5 KA ±4 KA Time (s) Independent coils Monolithic coil set Programmability 2 Resonant banks Phase I Field Phase II Field Coil current Evolution free from startup constraints Waveform constrained by startup concerns. Divertor installed No divertor.3 Time (s). All power now driven by solid-state IGBT and IGCT solid-state high power switches - Switches arranged in an H-bridge (SPA) - Many thanks to the HIT Group for their assistance See Poster QP1.68; J.C. Quinn for details ABB IGCT 2.8kV@4kA Steady-State ~ 5 cm long

22 E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

23 Current Status Power testing has begun EF and TF coils tested to nominal currents - Full stress and cross-field testing to follow - OH cooling and cross-field tests in winter 23 Scheduled first plasma: winter 23 Rod Current (ka) 9 45 TF Rod Current Increasing Bank Voltage 25 5 Time (msec) Coil Current (ka) 18 9 EF Coil Current Increasing Bank Voltage 25 5 Time (msec) E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

24 Summary Pegasus is a mid-size ST built to explore A Æ 1 Experimental results from phase I operations - Plasmas exhibit low-a characteristics - b t ~ 2%, IN ~ 6, b N > 4 via OH heating - I p /I tf ~ 1 - l i ~.4 - Characteristics of High I p /I tf Operation - Large scale internal MHD limit on I p /I tf - External Kink at q 95 =5 Next phase of operations will focus on accessing high I p /I tf, b t regimes - Recent modeling has demonstrated existence and accessibility to this regime - DCON and local G-S solver for stability and equilibrium => modeled I p /I tf ~ TSC for shot simulations and high I p /I tf accessibility - Upgrades to increase access to this regime - Increased V-sec.; position and shape control; B TF versus time - EF and TF power testing has started; full-stress, cross-field, and OH testing to follow E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23

25 Reprints E.A. Unterberg, APS-DDP, Albuquerque, N.M., October 23