Current Drive Experiments in the Helicity Injected Torus (HIT II)

Transcription

1 Current Drive Experiments in the Helicity Injected Torus (HIT II) A. J. Redd, T. R. Jarboe, P. Gu, W. T. Hamp, V. A. Izzo, B. A. Nelson, R. G. O Neill, R. Raman, J. A. Rogers, P. E. Sieck and R. J. Smith Aerospace & Energetics Research University of Washington Seattle, Washington USA M. Nagata and T. Uyama Himeji Institute of Technology Himeji, JAPAN Poster 2/2.4 Presented at the 22 Innovative Confinement Concepts Workshop College Park, Maryland January 22 24, 22

2 Abstract The Helicity Injected Torus (HIT II) is a low-aspect-ratio tokamak capable of both inductive (Ohmic) and Coaxial Helicity Injection (CHI) current drive. HIT II has demonstrated 2 ka of toroidal plasma current, using either CHI or induction separately. The loop voltage, boundary flux and plasma equilibrium are controlled in HIT II by a real-time flux feedback system. HIT II Ohmic plasmas exhibit reconnection events during both the current ramp-up and decay, events which relax the current profile while conserving the magnetic helicity. A new operating regime for CHI plasmas, using a double-null divertor (DND) boundary flux, has been explored. DND CHI plasmas exhibit: good shot-to-shot reproducibility low impurity content minimal shorting current in the absorber region EFIT-reconstructed equilibria consistent with significant closed-flux core regions continuous n=1 mode at the outer midplane, a mode which has been correlated experimentally with current-profile relaxation A detailed explanation of helicity injection current drive has been developed which is consistent with experimental observations of HIT and HIT II discharges. According to this mechanism, asymmetric distortion of the n=1 mode structure generates current drive in the core plasma by dynamo action, relaxing the CHI-driven current profile This relaxation can also generate differential rotation between the core plasma and the driven edge region, a result that is observed experimentally

3 The HIT-II Spherical Torus

4 The HIT-II Spherical Torus HIT-II Engineering Parameters: Major Radius R.3m Minor Radius a.2m Aspect Ratio A 1.5 Elongation κ 1.75 Ohmic Flux Available 6 mwb HIT-II Achieved Plasma Parameters: Parameter Ohmic CHI Pulse Length 6 ms 25 ms Peak Current 2 ka 2 ka Density n e m m 3 HIT diagnostics include: Multi-point Thomson Scattering Scannable two-chord FIR interferometer Scannable single-chord 16-channel Ion Doppler Spectrometer Pair of vacuum-uv (VUV) spectrometers (OVI/OV ratio) Single-chord average-z eff measurement H-α visible light detectors Surface magnetic triple probes Bolometer (total radiation emission) SPRED Multi-chord soft X-ray (SXR) camera Internal Langmuir and magnetic probes

5 HIT Diagnostic Systems Enable the Study of Important ST Physics MPTS FIR IDS VUV Zeff H-α Surface probes Bolometer SPRED SXR Langmuir probe Internal B-probe Study Plasma control and optimization * * * * * * * * * * Magnetic activity, relaxation and * * * * * * * * * * reconnection events Impurity control * * * * * * * Edge physics * * * * * * * * * MHD Equilibria (with EFIT) and * * * * * * * * * stability analysis Power balance * * * * * * * Particle and energy confinement and * * * * * * * transport (BALDUR) TIP

6 Overview of HIT Midplane Diagnostics

7 Two-Chord FIR Interferometry In HIT II Ohmic Discharges Above left, two-chord interferometry data consistent with an approximately flat electron density profile in Ohmic HIT II discharge # Above right, interferometry results consistent with a peaked electron density profile in Ohmic HIT II discharge #21133; note the sawtoothing density evident on the innermost chord (in blue). In both discharges, the line-averaged densities exhibit sharp drops, correlated with currentprofile relaxation events (also called Internal Reconnection Events or IREs). Above left, two-chord FIR data for Ohmic HIT II discharge # Above right, detail of the two-chord

8 Multi-Point Thomson Data From HIT II Ohmic Discharge 15 HIT-II #21311 at 19. ms - Limited Ohmic 1 T e (ev) 5 12 Electron Temperature (ev) From Thomson Scattering n e (arb) 8 4 Electron Density (unscaled) From Thomson Scattering Major Radius (cm)

9 I p (ka) V LOOP (V) HIT-II # Limited Ohmic MPTS Time Plasma Current 2 Imposed Loop Voltage Time (ms) 75 I p (ka) Φ TOR (mwb) δb P /B P (%) Plasma Current Paramagnetic Flux Poloidal Field Fluctuations (S7P18) Time(ms)

10 HIT II CHI Plasmas Peak currents of up to 2 ka Continuous high-frequency n=1 mode observed in all high-performance discharges Measured ion flows consistent with dynamo model for helicity injection current drive Plasmas can be generated at a relatively low density (n e m 3 ) Unbalanced double-null divertor boundary improves the plasma performance, compared to single-null: Less shorting current across absorber gap Better shot-to-shot reproducibility Relatively low impurity radiation Also, injector current scales as injector flux squared (see Jarboe, Fusion Technology 15, 7 (1989))

11 Difficult to Prove Closed Flux in CHI Plasmas Ohmic plasmas have a B p structure that obviously indicates closed flux: Up-down symmetric Hoop force pushes the magnetic axis outward High-performance CHI plasmas (in HIT and HIT-II) have B p structure consistent with closed flux. When a continuous n=1 mode is present: Absorber shorting current drops to nearly zero (HIT and HIT II) Spectroscopy shows increasing temperature (HIT, HIT II, NSTX) T e 1 ev (HIT single-point Thomson) EFIT converges (HIT and HIT II) IDS measurements of core and edge ion flows provide some evidence for a closed flux core Need additional evidence: Multi-point Thomson and more detailed IDS measurements

12 δb T-RG2 /B T (%) I inj (ka) V inj (kv) I p (ka) HIT-II # Single-Null CHI Plasma Current Injector Voltage Injector Current -.4 Toroidal Field Fluctuations Time(ms)

13 δb n e (1 19 m -3 T-RG2 /B T (%) ) I inj (ka) V inj (kv) I p (ka) HIT-II # Double-Null CHI Plasma Current Injector Voltage Injector Current Electron Density Toroidal Field Fluctuations Time(ms)

14 HIT-II DND Flux Configuration (from EFIT) For CHI Discharge 4.2 ms

15 EFIT Reconstruction of HIT-II CHI Discharges EFIT reconstructions of HIT-II CHI discharges are undergoing continual improvement. Presently, HIT-II EFITs utilize: Toroidal plasma current, diamagnetic flux and current on the driven flux (I inj ) 24 poloidal flux loops 24 F-coil currents (code is being verified) 18 Rogowski segments measuring poloidal field 7 poloidal magnetic surface probes Realistic estimates for measurement errors The fit functions are of the following form: F F 1(y) = γ 1 (below the X-point) F F 2(y) = γ 2 + γ 21 y (above the X-point) p(y) = α (1 y) where y ψ ψ axis ψ abs ψ axis and ψ abs is the poloidal flux in the absorber region Qualitatively, reconstructions fit measurements acceptably Quantitatively, overall error 1 4 but χ 2 1 In the near future, HIT-II EFITs will also utilize: 12 more poloidal surface probes 12 surface I inj (ψ)-probes Multi-point Thomson scattering data

16 A Helicity Injection Current Drive Mechanism A tokamak with sufficiently hollow current profile and edge vacuum region containing a rational-q surface is unstable to an n=1 external kink mode (PEST and PEST-3 result; for details, see D. Orvis, PhD Dissertation, Univ. of Washington (1997)) Dynamo action : This n=1 mode dissipates the free energy of the hollow current profile by driving toroidal current in the plasma core, flattening J/B Empirically, the presence of a continuous n=1 mode is required for high-performance CHI plasmas Current-profile relaxation and core current drive may not require either magnetic reconnection or multiple modes Differential rotation of the electron fluid causes the asymmetry of the n=1 mode structure necessary for dynamo action (K. McCollam, PhD Diss., Univ. of Wash. (2))

17 CHI Tokamak Observations Consistent with Dynamo Model Experimental results are obtained by reversing the electrode polarity from cathode central-column (CC) to anode central-column, and producing comparable CHI plasmas. n=1 mode propagates in the E B direction Edge plasma rotates in E B direction, but plasma core rotates opposite to toroidal current I p Cathode CC has larger n=1 amplitude than anode CC Cathode CC has lower n=1 frequency than anode CC Cathode CC plasmas exhibit more relaxed current density profiles than anode CC plasmas Cathode CC has more ion heating than anode CC

18 δb T-RG2 /B T (%) I inj (ka) V inj (kv) I p (ka) HIT-II CHI Shot (Cathode CC) Plasma Current Injector Voltage Injector Current -.5 Toroidal Field Fluctuations Time(ms)

19 δb T-RG2 /B T (%) I inj (ka) V inj (kv) I p (ka) HIT-II CHI Shot 1737 (Anode CC) Plasma Current Injector Voltage Injector Current -.1 Toroidal Field Fluctuations Time(ms)

20 Edge Ion Spin-Up Follows E B Cathode CC Opposite to Anode CC Line-averaged toroidal velocities from IDS Edge chord: impact parameter = 41.4 cm CIII emission line: λ = nm Cathode CC shot was #16693, Anode CC was #1736

21 Core Ion Spin-Up Opposite to I p Cathode CC Parallel to Anode CC Flow directions consistent with dynamo current drive and the formation of a closed-flux plasma core Line-averaged toroidal velocities from IDS Core-plasma chord: impact parameter = 38.5 cm OV emission line: λ = nm Cathode CC shots were #19716 and #19717 Anode CC were #1987 and #19872

22 Cathode CC Has More Relaxed Current Profile Than Anode CC

23 Cathode CC Has More Ion Heating Than Anode CC

24 LEFT: IDS-measured toroidal velocity v TOR versus time for the same chords and discharges. 22 ICC Workshop College Park, MD, Jan 22 24, 22 DND CHI Exhibits Significant T i and Rigid Rotor Ion Flows LEFT: IDS-measured ion temperature T i versus time for four chords in a set of repeatable DND HIT II CHI discharges. The IDS instrument was tuned to OV emission at nm. The shots and chords are as follows: #1924 and #1922 for chord #2 (impact parameter of 17.6 cm, black curve); #1928 and #1921 for chord #3 (21.4 cm, green curve); #19215 for chord #6 (32.1 cm, blue curve); and, #19212 and #19213 for chord #9 (41.4 cm, red curve). RIGHT: Average T i (from 3 ms to 6 ms) for these four chords, plotted versus the chord impact parameter.

25 HIT II Ohmic Plasmas Peak currents of 2 ka (limited or diverted) Reconnection events observed during discharges, usually during current decay phase. These helicity-conserving events are characterized by: Growing low-n precursor modes Degradation of plasma confinement Rapid change in equilibrium (< 1 µsec) Current profile relaxes to more uniform J/B Preliminary Thomson scattering shows T e > 1eV Average n e is significant fraction of Greenwald IDS shows T i 5 1 ev

26 I p (ka) V LOOP (V) HIT-II # Limited Ohmic Plasma Current 6 Imposed Loop Voltage Time (ms) 145 I p (ka) Φ TOR (mwb) δb P /B P (%) Plasma Current Poloidal Field Fluctuations (S7P18) Paramagnetic Flux Time(ms)

27 δb n e (1 19 m -3 T-RG3 /B T (%) ) V LOOP (V) I p (ka) HIT-II Ohmic Shot (HIT-like Divertor) Plasma Current Loop Voltage Electron Density Toroidal Field Fluctuations Time(ms)

28 δb n e (1 19 m -3 T-RG3 /B T (%) ) V LOOP (V) I p (ka) HIT-II Ohmic Shot (HIT-like divertor) Plasma Current Loop Voltage Electron Density Toroidal Field Fluctuations Time(ms)

29 B p /B p-mid B p /B p-mid B p /B p-mid B p /B p-mid HIT-II #2651 Normalized Poloidal Fields Before RE at 11.2 ms After RE Before RE at 13.6 ms After RE Before RE at 33.3 ms After RE Before RE at 35.3 ms After RE -5 5 Poloidal Angle (degrees)

30 Ohmic Discharges Exhibit Ion Flows During Current Rise and Decay I p (ka) HIT-II # Limited Ohmic Discharge Plasma Current Time (ms) ABOVE: Toroidal plasma current versus time for HIT II Ohmic shot #2555. LOWER LEFT: IDS-measured toroidal ion velocity versus time for two opposing chords (impact parameter of 21.4 cm) in the HIT II midplane, measured during matching Ohmic discharges (IDS tuned to Boron-IV emission at 282 nm). The upper-port shot is #2558 (black), and lower-port shot is #2555 (blue). Note that the flow is approximately zero during the current flattop, but is nonzero during the current rise and decay; the measured flow opposes the toroidal plasma current.

31 In the Ohmic discharges, the rotation is also understood by the application of the concept that relaxation current drive only drives the electrons, and the electron drag in turn accelerates the ions. With no E B present, the ions flow to counter the relaxation current drive. During the Ohmic current rise, relaxation drive is observed on the inside (near the magnetic axis) and relaxation anti-current drive near the edge. During the current fall, relaxation current drive is observed in the edge and anti-current drive on the inside. The BIV flows are consistent with electron drag spin up if BIV is only at the edge. 22 ICC Workshop College Park, MD, Jan 22 24, 22 Discussion of Results As described previously, the n=1 mode is destabilized by the hollow current profile and the presence of a mode resonant surface in the vacuum edge region. The n=1 mode exists primarily on open field lines and its propagation is always in the E B direction, where the electric field E is determined by the applied voltage. Experiments have shown that the propagation direction of the n=1 mode reverses when the polarity of the applied voltage reverses. An asymmetric driven n=1 mode gives nonvanishing < δv e δb > current drive on closed flux. Closed flux plasma rotation is driven by resistive drag of ions by electrons. Thus, the direction of this core rotation does not depend on the edge E B direction, but depends only on the direction of plasma current. On closed flux, the rotation is in the direction of electron flow. A transformer applied electric field does not cause a net force on the ions, as the electric force on the ions cancels the electron drag. Experiments have shown, using IDS, that the flow of OV ions is in the direction of the electron drift and is not influenced by the direction of E B. One interpretation is that the confinement time needed to ionize neutral oxygen to OV requires closed flux, so the OV emission is from the closed flux core, and therefore always rotates in the electron drag direction. It has also been shown experimentally that the cathode polarity drives closed flux current more effectively. In this case, the E B direction in the open flux is also the electron drift direction of the closed flux, so that rotating magnetic field current drive mechanisms can operate via the n=1 mode to drive current in the closed flux core.

32 Summary Ohmic plasmas driven with I p up to 2 ka and core electron temperatures above 1 ev. Reconnection events observed during current rise and decay, which are understood as current-profile relaxation events (flattening J/B). CHI plasmas generated with I p up to 2 ka and densities as low as m 3. Continuous n=1 mode in high-performance plasmas Performance improved by using DND boundary A detailed mechanism proposed for helicity injection current drive. This single-mode model is consistent with existing HIT and HIT II experimental results, and does not require reconnection.

33 Near-Term HIT II Physics Studies Confirm that relaxation-generated ion flows exist in HIT II Ohmic discharges BIV (edge emission only?) CV OVI and/or OVII Note that there are now more chords available for these IDS flow/temperature studies than ever before Determine the intensity profile for OV emission in DND CHI discharges: Is the apparent rigid rotor ion motion simply emission from a thin edge layer? Also, measure higher-charge-state emissions (e.g., OVI and OVII) Remeasure the edge and core ion flows in Cathode- CC and Anode-CC CHI discharges, and correlate the differential rotation layer with the edge of a closed-flux region from EFIT

34 HIT-II Active Flux Control System The HIT-II passive flux conserver is: 6mm Stainless-Steel shell (L/R 1.2ms) 3.5mm S-S central column, covered by 12mm of graphite For timescales longer than the L/R time, HIT-II boundary fluxes are controlled by an active feedback system: 24 flux loops (14 in the center-column, 1 on the shell) Measured fluxes compared to pre-programmed flux demands Currents in 28 Poloidal Field Coils (PFCs) are adjusted to correct differences between measured fluxes and demands The flux demand functions can be used to specify the following throughout a discharge: Loop voltage Divertor flux (single- or double-null) Vertical field Feedback system can respond to any axisymmetric displacement, and controls the plasma position and shape in real time

35 Magnetic Helicity Magnetic helicity is a measure of flux linkage. The helicity K is defined by: K (A B) dv = (A dl) (B ds) For two flux tubes, φ 1 and φ 2 : K = 1 (A 1 dl 1 ) (B 1 ds 1 )+ (A 2 dl 2 ) (B 2 ds 2 ) 2 That is, In a tokamak, K = φ 2 φ 1 + φ 1 φ 2 = 2φ 1 φ 2 K I p I TF

36 Helicity and Ohmic Current Drive Ohmic current drive injects helicity by adding poloidal flux which links toroidal flux V LOOP injects ψ POL which completely links φ TOR The helicity injection rate, K, is: K = 2V LOOP φ TOR

37 Helicity and CHI Current Drive Coaxial Helicity Injection (CHI) injects toroidal flux φ TOR which links the poloidal flux ψ inj Insulator I tf ψ inj V inj V inj injects φ TOR which links ψ inj