STATUS OF THE HIT-II EXPERIMENTAL PROGRAM Roger J. Smith and the HIT-II Team Plasma Dynamics Group University of Washington, Seattle, Washington
HIT-II Team Faculty/Staff Support Staff Graduate Students Thomas R. Jarboe Daniel E. Lotz William T. Hamp Brian A Nelson Dennis Peterson Griff O Neill Roger Raman Dzung Tran Valerie A. Izzo Aaron J. Redd Matthew Fishburn Paul E. Sieck John A. Rogers Susan Griffith George R. Andexler Collaborating Scientists: Dennis Mueller, NSTX, Princeton Undergraduates Rorm Arestun James Newman Aaron Siirila Annamarie Askren Edwin Penniman A. J. Sligar Ellen Griffin Chris Pihl Jedediah Smith Rabih A. Hosn Johnathan Setiawan
ABSTRACT The HIT-II device is a low aspect ratio (R o =.3m, a =.2m) torus with B t =.5T on axis. A wide range of plasma operations are possible on HIT-II by virtue of a 6 mm thick stainless steel vacuum vessel with 28 close fitting poloidal field coils that maintain a prescribed boundary flux history and a coaxial injector region. Three operational regimes for the HIT device are being actively studied at present: ohmic operation produces conventional inductively driven ST plasmas; CHI operation generates and sustains plasmas by Coaxial Helicity Injection; and, CHI initiated ohmic combines the two methods by using an initial CHI discharge which is subsequently sustained using induction. Toroidal currents of over 2 ka have been produced in ohmic and CHI modes and plasma currents of 27kA have been obtained in the CHI initiated ohmic mode. An overview of these three operational regimes is given in this paper and the present status of the HIT experiment is reviewed from an operational and a diagnostic perspective. The diagnostic complement includes: external magnetic probe measurements for equilibrium reconstruction using EFIT and recording MHD activity; internal magnetic probe array providing internal field measurements; multipoint Thomson diagnostic measures electron temperature; multi-chord FIR tangentially viewing interferometer providing density profile measurements and density fluctuation measurements associated with MHD activity; tangentially viewing 16 channel IDS diagnostic for measuring ion flow distributions; Langmuir probe measurements of edge density and temperature; several visible light diagnostics (Halpha, bolometer, SPRED, VUV spectrometer) and a soft X-ray pinhole camera as a monitor of MHD activity and electron temperature.
Contents 1. HIT-II Program Goals and Achievements to date. 2. Description of HIT-II device and diagnostics overview. 3. Program studies and results CHI only operation [QP1.6] "Parametric Scalings for CHI in the HIT-II ST, A.J.Redd, et al. CHI/Ohmic mixing experiments CHI Startup of Ohmic plasmas Ohmic only operation 4. Summary 5. Future Work
HIT-II Program Goals Develop Coaxial Helicity Injection (CHI) Techniques Characterize CHI driven plasmas Sustainment of ohmically driven plasmas CHI startup of Ohmic plasmas (Non-solenoidal plasma initiation) Explore HIT-II as an ohmically driven ST device Diagnostic Development (selected list) Internal measurement of rotating n=1 mode in CHI discharges Multi-point TS density and temperature profile measurements Ion velocity profile measurements using Ion Doppler Spectroscopy EFIT reconstruction of CHI and Ohmic Equilibria Edge floating potential fluctuation and ne/te measurements from the Langmuir probe array
Recent Program Achievements Exploration of ST Ohmic Operations Limited, Single and Double Null Divertor Operations Record: PEAK CURRENT 25 ka (2kA last year) Development of CHI techniques CHI Startup of Ohmic Plasmas 4kA non-solenoidal seed equilibrium current established CHI Plasmas: Record: PEAK CURRENT 29kA CHI/Ohmic Mixing experiments Using capacitor bank sustainment circuit Record: PEAK CURRENT 15/2kA Initial use of controllable IGBT power supplies for Sustainment Diagnostic development New Langmuir Probe Spatial/Temporal Correlation diagnostic 3 Probes: 1 triple probe and 2 floating probes New soft X-ray pinhole camera diagnostic 16 chords, AXUV Photodiode array with filter carousel
Aspect Ratio, A = 1.5, R o =.3m Magnetic Field on Axis, B o =.5T Elongation κ = 1.8 The HIT-II Device MACHINE GEOMETRY OHMIC CURRENT DRIVE Central Solenoid and Poloidal coils 6mWb, 4 Loop Volts max, 25 millisecond ramp Feedback control to prescribed Flux Boundary History using 1MW IGBT feedback controllable power supplies. COAXIAL HELICITY INJECTION (CHI) CURRENT DRIVE Sustainment Bank Power Supplies Formation Bank: 2kJ,.5 milliseconds Sustainment Bank: 1kV, 25kA for 8 milliseconds IGBT sustainment bank: 1kV, 3.2kA
Present Diagnostics External Magnetic Probes Rogowski coils, flux loops, pickup coils Multi-point Thomson Scattering (University of Texas, PPPL) Two Chord FIR Scannable Shot-to-Shot Interferometer (LANL) Visible - Hα detectors, SPRED, X-ray detectors Internal Magnetic Probe Array Ion Doppler Spectrometer, 16 channel, one chord (Hemeji I.T.) Symmetrically Viewing VUV Spectrometers Bolometer (AXUV Photodiode) Z eff Diagnostic, one chord Diagnostics Nearing Completion 16 Chord Soft X-ray/Bolometer Pinhole Camera (AXUV array) (Culham,UK) Langmuir Triple Probe edge (ne, Te) and spatial correlation of floating potentials
SCHEMATIC OF HIT-II DEVICE (Magnetic Diagnostics) 2cm 1 meter
CHI Startup Ohmic Discharge Details Highest Peak Current 29kA I p (ka) V LOOP (V), Ψ INJ (mwb) I INJ (ka) V INJ (kv) P (kw) HIT-II CHI Startup + Ohmic Shot #2753 3 Toroidal Plasma Current Both SSI guns, Ti getterin g 2 1 1 Loop Voltage 8 Injector Flux (magnitude) 6 4 2 2 Injector Current 15 1 5 1.5 Injector Voltage (inverted) 1..5. 8 6 4 Radiated Power Applied Ohmic Power 2 1 B p Fluctuations (S6P18) δb p /B T (%) -1 1 2 3 4 Time (ms)
Midplane Diagnostics
Overview of Coaxial Helicity Injection DYNAMIC FORMATION JXB FORCE PRODUCES BUBBLE BURST STEADY STATE INJECTION EDGE CURRENT RELAXES TOWARD MAGNETIC AXIS I dk dt λ inj inj = 2 Ψ inj Vinj µ I ψ 2 ψinj 2 2 µ o d I Itoroidal Iinj CHI RELEVANT RELATIONS HELICITY INJECTION RATE = o inj (J/B), LAMBDA OF THE INJECTOR inj ψ ψ Tor Tor inj ψ inj AND THE INTER-ELECTRODE DISTANCE (d) DETERMINE I ing WIND-UP FACTOR [QP1.6] "Parametric Scalings for CHI in the HIT-II ST, A.J.Redd, et al.
5 Ion Doppler Spectrometer Showing V i, T i and Intensity Vs Radius at Edge OV Ion Velocity and Temperature at the Edge of CHI Plasmas Line Averaged Temperature (OV) shots: 23138, 23133, 23147 Temperature Plasma Current (2kA Peak Current) 6 Line Averaged Velocity Shots: 23138(b),23133(g),23147(r), 5 4 4 kt (ev) 3 2 km/s 3 2 1 1 1 2 3 4 5 6 Time (s) x 1-3 -1 1 2 3 4 5 6 7 Time(s) x 1-3 Intensity [arb. units] 9 8 7 6 5 4 3 2 1 OV Ion Emissivity at the Edge and Line Avgd Temperature(Radial Chord) Line Averaged Emissivity (OV) shots: 23138, 23133, 23147 R =44cm R= 41 cm R=39cm 1 2 3 4 5 6 Time(s) x 1-3 kt (ev) 9 8 7 6 5 4 3 2 1 Line Averaged Temperature for Radial Chord shot: 2313 Temperature Plasma Current (~2kA Peak) 1 2 3 4 5 6 7 Time (s) x 1-3 [QP1.61] Far Infrared Interferometry and Ion Doppler Spectroscopy on HIT-II and HIT-SI, Griff O'Neill, et al.
Ohmic and Ohmic with CHI Startup Results CHI Only Experimental Results CHI/Ohmic Mixing Results Diagnostic Development
Langmuir Probe Array Measures Floating Potentials at Three Locations and ne/te Triple Probe at One Location Three tips for Triple Langmuir Probe 12cm toroidal separation 4cm poloidal separation 5cm depth Jedediah Smith, Aaron Siirila, Rabih Hosn
Exploded View of Soft X-ray Camera Showing Carousel and Detector 16 Channel AXUV Photodiode Array Octagonal Filter Carousel Carousel allows selection of filter on Shot-to-Shot basis (Choice of three filters or multi-chord bolometry) Annamarie Askren, Edwin Penniman
Summary The HIT-II device is steadily pushing parameters in Ohmic discharges. The peak current has gone from 2 to 25kA in the past year. CHI startup, a new operational regime for HIT-II has been developed which holds promise for the NSTX ST at Princeton. It is a non-solenoidal method of creating a seed equilibrium that couples to Ohmically driven plasmas. This technique has allowed the peak current of 29kA and more robust operating conditions. CHI/Ohmic mixing experiments are becoming more successful at coupling injector driven current (CHI) to the inductively driven currents at higher peak currents (15/2kA) and for longer times (7 milliseconds).
Future Work The Langmuir probe array will add edge ne and Te measurements of CHI and Ohmic plasmas to depths of 5cm inside the shell. The Thomson scattering diagnostic has been reconfigured to significantly extend its range beyond its present 15eV limit. Allowing temperature measurements of the high performance Ohmic and CHI startup plasmas. The new soft X-ray camera diagnostic, together with TS, will be used to optimize the performance of plasmas by determining the time of peak temperature and also to study IREs, sawteeth, snakes and other reconnection phenomena present in HIT- II plasmas. The research program will now emphasize optimization of present operating modes and obtaining as complete a data set as possible with the present diagnostic complement.
5 x 119 Density Profile Inversion Compares Well with TS Density Measurement July 3, 23 Shots: 26686-2673 4 Density [1 19 m -3 ] 3 2 1 Thomson scattering measurement Two discharges FIR Density Profile time (ms): 17.5-1.25.3.35.4.45.5 Major Radius [m] Two Chord FIR Data Showing a Snake-Like Instability Localized near the Mag. Axis 15 Shot:23343Plasma Current Current [ka] 1 5 Density [1 19 m -3 ] 6 4 2 6.5.1.15.2.25.3.35.4.45 Shot:23343Line Average Density Impact Par: 31cm Impact Par: 39cm.5.1.15.2.25.3.35.4.45 Density [1 19 m -3 ] 4 2 Expanded Time Window.23.24.25.26.27.28.29.3 Time [Sec]
CHI/Ohmic Mixing using IGBT Bank Shows Little, if any, Current Enhancement Due to Low λ inj compared to λ Tokamak [LP1.23] Investigation of current drive with CHI during an ohmic discharge in ST s, Dennis Mueller, et al.
Thomson Scattering ne and Te profiles for an Ohmic Discharge [QP1.62] Electron Temperature and Density Profiles of Transient CHI Initiated Discharges in the HIT-II Experiment, W.T. Hamp, et al. Langmuir Probe: Floating Potential (Three spatially separated probes) Shot: 26643 Insertion Distance=2.5cm Voltage [V] -5-1 -15 5 1 15 2 25 Shot: 26644 Insertion Distance=1.3cm Voltage [V] -5-1 -15 5 1 15 2 25 Time [milliseconds] Aaron Siirila
CHI Startup of Ohmically Driven Plasmas A short (5 µsec) CHI pulse produces a toroidal current in the main chamber which couples to an Ohmically driven SN plasma. The seed equilibrium is generated using CHI without the use of the solenoid. An additive hand off current of 4-5kA is maintained throughout the Ohmic discharge Volt-seconds are saved and plasma performance is enhanced above Ohmic drive operation alone. CHI startup does not rely on an injector flux ramp-down [CI2.1] Demonstration of plasma start-up by Coaxial Helicity Injection, Roger Raman
Current Enhancement is Shown from CHI/Ohmic Mixing using Capacitor Sustainment Bank [LP1.23] Investigation of current drive with CHI during an ohmic discharge in ST s, Dennis Mueller, et al.
CHI Operation: n=1, Rotating Mode Structure (hp filtered) Shot 267: CHI discharge, I P = 15 ka Reversal of Poloidal Field Across the Mode (Red) Sharp Asymmetric Features in the Radial Field are Observed (Blue) Radial Extent of Mode is Typically ~5cm.