1 / 33

The Pegasus Toroidal Experiment: Recent Results and Future Plans

The Pegasus Toroidal Experiment: Recent Results and Future Plans. Aaron J. Redd for the Pegasus Research Team. 15 th International Spherical Torus Workshop Oct 22-24, 2009 Madison, WI USA. Pegasus is studying low-A physics and developing non- solenoidal startup techniques. Pegasus.

alexis
Télécharger la présentation

The Pegasus Toroidal Experiment: Recent Results and Future Plans

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The PegasusToroidal Experiment:Recent Results and Future Plans Aaron J. Redd for the Pegasus Research Team 15th International Spherical Torus Workshop Oct 22-24, 2009 Madison, WI USA

  2. Pegasus is studying low-A physics anddeveloping non-solenoidal startup techniques Pegasus N = 6 5 NSTX 4 3 t (%) 50 2 40 30 20 IN (MA/mT) 10 0 0 2 4 6 8 Determining limits to IN, t High IN, t accessed through j(R) manipulation and/or fast TF ramps Tokamak-spheromak overlap Peeling modes observed Driven by high (j||/B)edge Can explore ELM physics Non-inductive startup via point current sources DC helicity injection Target plasmas couple to outer-PF induction & Ohmic solenoid drive Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  3. Pegasus Overview Outline • The PegasusToroidal Experiment • Non-solenoidal startup • Peeling mode studies • Future: Guns & RF startup & growth • Summary Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  4. Pegasus is a Compact Ultralow-A ST High-stress Ohmic heating solenoid Experimental Parameters Parameter To Date A R(m) Ip (MA) IN (MA/m-T) li κ τshot (s) βt (%) PHHFW (MW) 1.15 – 1.3 0.2 – 0.45 ≤ .21 6 – 12 0.2 – 0.5 1.4 – 3.7 ≤ 0.025 ≤ 25 0.2 Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  5. High bT at A≈1 accessible at high IN • High IN requires current drive and current profile control • Developing startup and current drive techniques, to support overall ST program and enable high-b studies • Future devices need non-solenoidal current drive anyway, Pegasus simply needs current drive solutions now Gun PI TF Ramps START, NSTX Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  6. Pegasus Overview Outline • The PegasusToroidal Experiment • Non-solenoidal startup studies • Plasma gun system description • Limits on the driven toroidal current • Peeling mode studies • Future: Guns & RF startup & growth • Summary Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  7. Local Plasma Current Sources + Helical Vacuum Field Give Simple DC Helicity Injection Scheme • Current is injected into the existing helical magnetic field • High Iinj & modest B  filaments merge into current sheet • High Iinj & low B  current-driven B overwhelms vacuum Bz • Relaxation via MHD activity to tokamak-like Taylor state w/ high toroidal current multiplication Reduced Bz BT=10 mT, Bz = 5 mT Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  8. Magnetic helicity injection is current drive Magnetic helicity: linkage between magnetic fluxes K is conserved in magnetized plasmas, decaying on resistive timescales. In tokamaks, K is proportional to the product ITFIp. Increases in K correspond to increases in Ip. Driving current on open field lines is helicity injection Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  9. DC helicity injection startup on Pegasus utilizes localized washer-gun current sources Anode Plasma streams 3 plasma guns • Plasma gun(s) biased relative to anode: • Helicity injection rate: Vinj - injector voltage BN - normal B field at gun aperture Ainj - injector area • Plasma guns have geometric flexibility • Gun-based system can be scaled to larger devices, such as NSTX Divertor injection Midplane injecton Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  10. Driven helical filaments can relax to an axisymmetrictokamak-like state • Driven helical filaments are strongly unstable • Relax into the axisymmetric tokamak-like state • Tokamak-like equilibrium satisfies a set of conditions • Radial force balance • Helicity/power balance • Kink stability: edge q > 3 • Taylor relaxation current limit • Max Ip is determined by helicity injection rate and the Taylor relaxation limit, related to magnetic geometry • Scales with ITF, Ibias, and the width w of the driven layer Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  11. Ip > 170 kA non-solenoidal startup achieved with < 4 kA injected current KFIT reconstruction for #45321, a similar 150 kA plasma, near Ip peak. Raxis 0.33m a 0.28m k 2.3 li 0.48 bT 1.7% bp 0.23 Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  12. Helicity Balance Provides One Limit on Current Ip • Far below relaxation current limit: max Ip occurs when dK/dt balances resistive decay • Decay Vloop estimated with Vsurf • Veff≈ Vsurf indicates: • Injected helicity gets into the plasma • Maximum driven current limited by dK/dt This study used only static-field divertor-gun discharges (no induction drive) Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  13. Midplane-driven plasmas evolve inward Estimated plasma evolution Anode Plasma guns Forms as small circular outboard plasma As the helicity content increases, plasma expands into high-field region Maintaining radial force balance may require vertical field ramps Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  14. Taylor relaxation criteria also limits the sustainable Ip for a given magnetic geometry Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  15. Maximum Ip achieved when helicity and relaxation limits are satisfied simultaneously Estimated plasma evolution Helicity limit ITF = 288 kA Vbias = 1kV Vind = 1.5 V Iinj = 4 kA w = dinj L-mode e Ip max Anode Relaxation limit Time Plasma guns These particular parameters require a Bv ramp, both for radial force balance and some induction. Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  16. Sufficient helicity injection is required to drive the plasma up to the relaxation limit Vbias = 1200 V 900 V Relaxation limit 120 V Helicity Limited R = 47 cm High dK/dt with modest Ibias requires high impedance Zinj Max Ip increases with Vbias Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  17. Experiments confirm relaxation limit scalings with ITF and Ibias • The relaxation limit Ip scales with (ITFIbias)1/2 • Experimental plasma currents follow these scalings: Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  18. Experiments demonstrate dependence on the width of the driven current layer Anode w 3 guns • Relaxation current limit scales as w-1/2 • One-gun discharges had higher limits than corresponding three-gun cases, indicating the gun array was misaligned: Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  19. Gun array was realigned, significantly improving plasma performance Anode w 3 guns • Changing the tilt of the gun array increased the max Ip by a factor of 1.5-1.7, implying a factor-of-3 change in w. • In this configuration, have achieved Ip > 170 kA. After alignment Before alignment Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  20. Plasma gun startup provides a robust target plasma for Ohmic handoff • Handoff to Ohmic • Gun-driven target ~80 kA • Most pre-OH current is captured by the OH drive • Corresponding OH-only • Gun startup saved ~ 50% flux • Will couple high-Ip targets to double-swing OH ramp For more details, see D. Schlossberg’s talk at 2:15PM today Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  21. Pegasus Overview Outline • The PegasusToroidal Experiment • Non-solenoidal startup studies • Peeling mode studies • Characterization of the modes • Measuring JT(R) and p(R) to test theory • Future: Guns & RF startup & growth • Summary Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  22. Observed Filaments are Similar to ELMs • Filamentary, field-aligned structures • Present under conditions of high <jedge/B> • Pegasus: L-mode edge assumed • However, may still manifest same instability Maingi, Phys. Plasmas 13, 092510 ,2006 Kirk, Plas. Phys. Controlled Fus. 49, 2007 Pegasus NSTX MAST Scannell, Plas. Phys. Controlled Fus. 49, 2007 Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  23. Near-unity A Maximizes Peeling Drive *: Thomas, Phys. Plasmas 12, 056123 2005 • Pegasus operations at A → 1 lead to naturally high jedge/B • Comparable to larger machines in H-mode • However, source of peeling drive different • Large machines: H-mode p’ → jBS • Pegasus: Large dIp/dt (≤ 50 MA/s) → transient skin current • Low-A geometry enables ITER-relevant research Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  24. Accurate Stability Analysis Requires Local Measurements of JT and p’ • Comparing experiment to peeling-mode theory requires accurate edge profiles: • Need both edge p(ψ) and j(ψ) profiles • Pegasus: measurements using probes • Hall-effect array constrains j(ψ) • Langmuir array constrains p(ψ) Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  25. Initial Internally Constrained Equilibrium • KFIT* using 5 point cubic spline basis set • Better basis set is being implemented *Sontag, A., et. al., Nuclear Fusion, 48, 095006 , 2008 Ip 157 kA R0 .30 m a .24 m A 1.2 κ 2.2 ℓi .25 βp .10 βt .02 q0 6.1 q95 17 Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  26. Peeling-mode studies are in progress • Filamentary edge instabilities consistent with peeling modes • Nonlinear phase: explosive detachment and radial acceleration • Direct comparison to theory requires accurate equilibria • Probes measure Bz(R,t) and pressure in the plasma edge • Local equilibrium code KFIT is being modified, with basis functions that can capture the experimentally constrained profiles • ITER-relevant physics accessible at low cost by operating at very low aspect ratio • For more details, and progress toward testing the theory, see M. Bongard’s talk tomorrow at 10:40AM Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  27. Pegasus Overview Outline • The PegasusToroidal Experiment • Non-solenoidal startup studies • Peeling mode studies • Future: Guns & RF startup & growth • High-Ip non-solenoidal startup and growth • Implementation of RF systems (EBW, HHFW) • High-IN, high-bT studies • Summary Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  28. Further development of gun startup needed to achieve high Ip • Pegasus near-term goal is 0.3-0.4 MA • Allows characterization of confinement/dissipation in driven plasma • Enables access to high Ip/ITF, high b regimes • This requires: • Increase TF: increase Taylor limit, improve confinement • Increase gun current: increase Taylor limit • Bigger/improved plasma guns: higher helicity injection rate • Test augmenting the guns with shaped electrodes • Outstanding issues: • What sets the bias impedance Zinj? • Is the confinement/dissipation stochastic? • What sets the width w of the driven region? Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  29. Non-solenoidal startup with RF and/or helicity injection in Pegasus • Two RF systems to be available: HHFW and EBW • HHFW: Two-strap antenna, 0.8 MW, 8-18 MHz • EBW planned for next year: 0.5-1.0 MW @ 2.45 GHz • 2.45 GHz enabled by very low field at low-A • Enables comparison of non-solenoidal startup scenarios: • Helicity injection + outer-PF induction • EBW heating + outer-PF induction • Helicity injection + EBW heating + outer-PF induction • Enables non-solenoidal sustainment on Pegasus: • RF heating, possible bootstrap overdrive • Intermittent helicity injection Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  30. Midplane-launched EBW damps near magnetic axis and gives optimal heating Ray-tracing and power deposition calculations by S. Diem using GENRAY and CQL3D. Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  31. Proposed Pegasus Facility Modifications • Magnetic and power systems reconfiguration • Increase TF by factor of 2 • Activate divertor coils • Deploy existing PCS • Helicity injection supply development • Internal hardware modifications • Install passive conducting plates • Optimize gun geometry • Install enhanced guns and/or electrodes • Install internal radial position coils • Improved core and edge diagnostics • Multi-point Thomson scattering • Poloidal SXR array • Ion spectroscopy: flows and Ti • Visible brehmsstrahlung Present Configuration Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  32. Near-term planned physics campaigns • Continue and extend non-solenoidal current drive studies • Test understanding of relaxation current limit • Study confinement/dissipation, bias impedance, driven layer width • Use additional tools as they become available: RF, increased TF, etc • Target is 0.3-0.4 MA non-solenoidal plasma current • Rigorously test peeling-mode theory • Use current-profile and pressure-profile constraints • Use divertor coils to add shear • Explore non-solenoidal sustainment • Enables high-IN, high-bT studies Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

  33. Pegasus Summary: Creating High-Ip Non-Solenoidal Discharges & Studying Edge Stability • Exploration of high IN, t space facilitated by j(r) tools • Ip/ITF > 2, IN > 14 achieved; extend operation to high Ip, nefor high bt • Making progress with non-solenoidal startup • Ip ~ 170 kA using helicity injection and outer-PF rampup • Using understanding of helicity balance and relaxation current limit to guide hardware and operational changes • Adding RF systems: develop startup and sustainment scenarios • Outstanding physics questions: ledge, Zinj, confinement, etc. • Ultimate goal is 0.3-0.4 MA non-solenoidal current • Able to rigorously test Peeling-Ballooning theory • Edge measurements constrain equilibrium reconstructions • Can compare stability calculations to experimental observations Aaron J. Redd, 2009 International ST Workshop, Madison, WI USA October 22-24, 2009

More Related