Thesis Statement(s) High Field Utilization (I p /I tf ) needed to access! t ~ 1 and High I N

Transcription

1 Thesis Statement(s) High Field Utilization (I p /I tf ) needed to access! t ~ 1 and High I N in an ultra-lowaspect-ratio torus (ULART) Operations in the PEGASUS ULART have shown the need for discharge evolution to obtain high performance Modeling & experiments confirm PEGASUS can access high I p /I tf, I N regimes Recent experments in PEGASUS have given I p /I tf ~ at I p <.1 MA

2 Outline Achievable Performance of Initial Experiments Limited to I p ~ I tf - Tearing modes (TM) due to ST physics and crude experimental control - Magnetic shear mitigates TM " external kinks were observed; all limited by hardware control Modeling Points to Access to High I p /I tf Equilibrium - TSC modelling confirms accessibility at full capability - Confinement estimates point to high-! w/ I p /I TF ~ 2 - Equilibrium & DCON calculations give stable predictions at I p /I TF ~ 2 Non-Inductive and Limited-OH Experiments " I p /I tf = Achieved - Goal: I p /I TF ~ 2-3 (I N ~ 1-2) at high I p - in present experiments: 1/3 capability; prototype non-inductive source - Non-inductive sources w/ OH drive coupling: I p /I TF ~ 2 - central heating seen during OH drive - Ultra-low-field preionization w/ OH drive: I p /I TF ~ experiments limited by flux (~ 3mV-sec; full capability = 9 mv-sec)

3 PEGASUS is a University-Scale, Mid-Sized ST Phase-II additions in red Equilibrium Field Coils PF1 Shaping Coil Divertor Coil Low Inductance TF Bundle Divertor Plates Higher-Harmonic Fast Wave Antenna Ohmic Solenoid Flux Loops Typical PEGASUS Employee ~ 6 Feet Plasma Guns Ohmic Trim Coils PF8 Shaping Coil

4 Main Mission: Studies at High-! t and I N PEGASUS Goals Stability and confinement at high I p /I TF at high I p - Extension of tokamak studies Limits on! t and I p /I TF (kink) as A#1 - Overlap between the tokamak and the spheromak 8 Troyon scaling for conventional tokamaks and other STs with predicitions for PEGASUS PEGASUS Phase-I START Conventional Tokamaks Aux. Heating (HHFW, EBW) I N = I p /(ab t ) = 6 I p /I tf

5 Phase-I: Mode Activity Dependent on Current Profile Maximum I p $ I tf limit q-profile of ST condusive to TM activity - large region of low magnetic shear near axis " large island widths - low TF operations " q<2 when highly resistivity q profile measured with SXR emission % N TM activity decreased with increased magnetic shear rational surface Crude manipulation of q(r) reduced mode amplitude - Increased shear, q " delay tearing onset " Access higher I p /I TF via higher q, T e, shear

6 Phase-I: External Kink Seen at Highest Performance Highest Phase-I Ip often disrupt.16 8 q 95 = 5 observed preceding disruption - l i =.5 at this time.12.8 Plasma Current q DCON analysis " unstable to n=1 external kink - m=5 most unstable mode Consistent with theory expectation & high edge current Poloidal mode eigenfunctions m= Free- Boundary Energy (DCON).2 MHD Amplitude m=4 m=1 m=2 m= % N Time (s).23

7 Facility Modifications to Access High I p /I TF Extensive modications to facility to allow access to higher I p /I TF operations -6-4 Phase-I Shot Shot Shot 2553 Approaches and tools to increase I p /I TF - Manipulate current profile - V loop control, position/shape control, B t (t) Time (sec) Reduce & before low-order rationals appear - V loop control, position/shape control, RF heating (HHFW) - Transiently increase q during startup - B t (t), V loop control Main facility modifications - Power Supplies - OH: effective V-s ' w/ increased waveform control - Coil Sets - Lower inductance TF set: 6 turns " 12 turns - PF Set: monolithic set " 8 independent sets - Divertor coil set installed Phase-I Shot 246 Shot Shot Time (msec) Phase-I Phase-II Unramped Ramped Time (s) 3

8 Programmable Power Supplies give Highly Flexible Experiment Switchyard with the 4 subsystems 25 MVA programmable power - Economical, high-power, solid-state switches - Impedence matched for each coil - Allows more effective power with less stored energy Large degree of coil arrangement flexiblity - Up to 4 independent 4 ka available - 9V # IGBT systems - 27V # IGCT systems - PWM feedback gives msec time response (U.Wash) IGBT H-Bridge (1 of 28) Allows easy integration to active PCS system - Real-time control under development with GA Devices Transient Suppression Circuitry Flashlight (for scale)

9 Discharge Control Seen with Pre-Programmable Coil Sets Large array of new capabilities developed; deployed into routine use - Pre-programmed coil currents - New wall conditioning and fueling - Variable PF configurations - Increased TF with time-variability - Divertor coils Integration of these capabilities led to discharge control Varied I p & respective!(b!!(b! ~45 MA/s ~23 MA/s ~9 MA/s ~7 MA/s ~4 MA/s I p -dot held through TM (B Time (sec) Time (s).28

10 Tools for High I p Studies: Plasma Control System Studies at high I p /I tf known to need fine discharge control - Highly shaped discharges - Possible disruption mitigation Active control system based on DIII-D PCS - GA PCS Team collaborating - System commissioning planned mid-26 Pre-Existing Component KEY New Component Information Flow Digitizer ~12 channels.1-1mhz simultaneous 12 bit Power Supply Control (PWMs) Power Supplies Coils Plasma Diagnostics Data Acquisition System Outputs 16 Analog channels Real-Time Computer Control Algorithms Digitizer 96 channels 5 khz simultaneous 16 bit

11 Modeling Confirms Access to High I p /I TF Modeling done to verify access to interesting regimes TSC simulations with full design parameters TSC: Full ohmic flux (9 mv-s) - Flux consumption calculations indicate 3 ka I p attainable - Simulations corroborate = I p /I tf = 2 = I N ~ 12 MA/m-T - Effects of resistive MHD unclear t [s] Confinement modeling: I p /I tf = 2 "! t = 4-7% - Sufficient to assess stability boundaries - Simialr calculations confirmed in Phase-I with data! t ! t Projections Phase-I I p /I tf = I p (MA).25.3

12 zero -! Equilibria DCON predicts equilibria stable

13 High-I p /I tf, zero -! Equilibria DCON Output

14 Modeling: Current & Pressure Profiles Studies Underway Goal: To determine optimal parameters for external kink stability at high I p /I tf Base cases with I p /I tf = 2 and 3 used as starting points for further study - Varying profiles and shapes studied done to determine possible stability boundaries Studies are ongoing to determine possible stability boundaries; key parameters for high I p /I tf operations - Initials indications show need for broad current profiles & high elongation for stability 1 External kink stability from DCON q 95 I p /I TF =2 I p /I TF = Unstable Unstable Stable Stable q =1 2 1

15 Phase-II: Mitigate TM Activity en route to High I p /I tf Phase-I operational space recovered and extended - Higher TF allowed: I p # ~14 ka ; I p /I TF # ~1 - m/n =2/1 mode activity observed with ~ same characteristics as Phase-I Discharge utilizing all available V-s as OH goes to full power - ~ 3 mv-s available vs. Phase-I 6 mv-s (9 mv-s full design) Tearing mode mitigation experiments are ongoing - Optimizing startup to navigate through MHD activity # I p /I TF > 1 - Approaches: variable ramp-rate; plasma shape; gas handling; impurity injection.2.15 I p = I TF Pegasus Phase I Phase II Phase-II flux consumption estimate with given V-s.1.5 Focus of experimental operational space (Winter 26) TF Rod Current (MA).25.3

16 Starting to Access I p > I tf Regime I p ~ 1.5-2I tf achieved by operation at low TF - Significantly above Phase-I operating space of I p ~ I tf Low TF operations made avaiable using plasma guns - Tokamak-like MHD characteristics quite different from OH driven discharges - Gun as preionization source: MHD similar to Phase-I Higher I p comes with higher power plasma guns and/or full power OH I p /I tf Plasma gun Tokamak-like discharge Phase-II w/ Plasma gun preionization Phase-II w/ ECH preionization Phase-I Plasma Current (MA).15

17 I p ~ 1.5I tf Attained Using Gun Preionization Source Allows OH ultra-low TF - Using PF power supplies; gives ~ 2 mv-sec - Phase-I experience: more V-sec will allow extend discharge length and magnitude I p TF Rod Current ~ R =.3 m I p ~ 1.5I tf achieved to date 2 - MHD behavior similar to high-ip; high-field discharges - High ramprates known to limit performance 1.4 Fluctuation Amplitude of Outboard Midplane Mirnov Significance: No intrinsic limit in I p /I tf - Depends on discharge evolution and current profile Time (sec).32.34

18 Plasma Guns Produce Tokamak-like Plasmas Central flux reversal indicates formation of tokamak-like plasmas Toroidal Current Evidence for relaxation to tokamak-like flux surfaces: - Increased I ) amplification Unrelaxed plasma 1 - Visual change in plasma Central flux - Strong B-dependence - Flux reversal on center column Appearance of n=1 mode Mode Spectrogram Formation of relaxed plasma 15 1 n=1 mode Time (s).28.3 Strongly relaxed

19 n=2

20 Beginning: Non-inductive startup + OH Discharges Low V loop added to end of gun discharges - Modest increase in Ip - Prototype guns: limited to very low TF SXR emission shows heating when V loop applied Full handoff from non-inductive sources " OH under study - low field " poor confinement " requires more input power Approach: increase helicity & power from guns; increased V loop - allows higher field ops - better matching of applied field to equilibrium field - added V-sec from full power OH should help V loop added to tokamak-like discharge Shot Dead Channels Time (sec) Gun line-of-sight

21 Summary High Field Utilization in PEGASUS Needed to Access High! t ; I N Regimes Modeling Confirms Access to this Regime; Modifications Give Tools - TSC & confinement calculations confirm high I p /I tf ;! t access - DCON analysis predicts stability up to I p ~ 3I tf - New tools integrated; demonstrate discharge control I p > I tf Attained Using Plasma Gun Sources - Goal: I p /I tf ~ 2-3 (I N ~ high I p - I p ~ 2I tf w/ guns as a non-inductive startup source - I p ~ 1.5I tf w/ guns as a preionization source - I p ~ I tf not an intrinsic limit

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