RF Physics: Status and Plans

Transcription

1 RF Physics: Status and Plans Program Advisory Committee meeting February 6-7, 2002 S. J. Wukitch Outline: 1. Overview of RF Physics issues 2. Review of antenna performance and near term modifications. 3. Discuss RF physics and near term plans. 4. Present RF 5 year plan

2 Major RF Research Themes RF coupling, propagation, and absorption. Reliable antenna operation with minimum negative impact on plasma. Investigate D( 3 He) absorption. Investigate lower hybrid wave propagation near density limit. Heat and control transport barriers. Investigate RF flow drive thru mode converted ion Bernstein waves. Heat and possibly control internal transport barrier modes via localized heating/current drive. Improvement of H-mode confinement when injected power exceeds threshold by 2-3. Investigate the physics of flexible, efficient RF current profile control. Examine mode conversion current drive. Investigate lower hybrid current drive. Comparison of modeling with experimental results. Comparing detailed PCI measurements with TORIC in the mode conversion region. LHCD modeling with ACCOME/future full-wave code (Sci-Dac)

3 D and E-Port Fast Wave Antennas Two 2-strap, fast wave antennas couple power at 80 MHz. 3.5 MW into plasma.» routinely inject 3 MW into EDA H-modes. Operated with field aligned Faraday screens.» ~27% transparent Operated with strap currents out of phase (0,π). Peak n φ = 10 in vacuum spectrum. Voltage handling capability 50 kv.

4 4-strap, fast wave antenna couples power through single horizontal port. 3 MW into plasma.» routinely inject 2 MW into EDA H-modes. Faraday screen is aligned to B T -field.» 50% transparent Operated with strap currents out of phase. Peak n φ = 13 in vacuum spectrum. J-port Antenna Operating 25 kv in plasma.» Tested up to 40 kv in vacuum on C-Mod.

5 Antenna Heating Efficiencies are Comparable J-port antenna has unique features compared to other antennas. J-port has a folded strap with a single tap. Antenna box and Faraday screen are more open. Faraday screen elements are aligned with the toroidal B-field. Substantially higher loading than D and E-port antennas. J-port is Ω or Ω /m D and E-port is 4-5 Ω or Ω /m E J D P RF [MW] W plasma [MJ] T e0 [kev] R neut [x10 12 s -1 ] P rad [MW] B T =5.2 T I P =0.8 MA Time [sec]

6 Arcing Limited Operating Power Previous Design Present Design B B Modified antenna feeds to reduce E-field where E B. Orient feeds so E B. Limit E<15 kv/cm where E B. Reduce peak electric fields. Increased radii of strip lines and back plate. Increased electrode spacing from 1 cm to 1.5 cm.

7 Modification was Successful in Preventing Arcing Previous Design Present Design Arc damage was where E B. From measurements, E-field was estimated to be 15 kv/cm in this region. Similar limit reported at RF conference in Maximum operating voltage was increased from 17 kv to 25 kv into plasma at 78 MHz. Conditioned to max voltage of 40 kv in vacuum.

8 Maximum Voltage Limited by Current Strap Design Near bridge, a moderately high voltage point passes near a ground where E B. Estimated voltage at location for 25 kv maximum voltage is ~18 kv (78 MHz). Gap is ~1.4 cm. Arc Damage E-field is ~13 kv/cm. Observed change in maximum voltage consistent with frequency change from 78 to 70 MHz. V max = 25 kv for 78 MHz. V max = 30 kv for 70 MHz.

9 J-port Strap Modification Reduce both average E-field and peak E- field along B-field. Increase the gap to 2.0 cm from 1.4 cm. Compound radius reduces peaking. Average E-field is reduced ~43%. Peak E-field is reduced and not aligned with B-field.» Should be obtain ~36 kv.» Should allow for doubling of the power. Previous New 0.24 cm 0.76 cm 25/64" [10.01mm] 3/8" [9.6mm] 23/64" [9.12mm] 35/64" [14.05mm] 7/16" [11.22mm] 33/64" [13mm] 3 5/32" [80.01mm] Ground plane

10 RF-Plasma Interaction is Reduced Ip =0.8 MA Previous operation was limited to P inj ~2 MW. Injections would result in disruptions.

11 Injections from BN-Metal Interface Disrupted Discharges Injections occurred at high power or high target density discharges. Camera images captured several injections. All antennas had evidence of melting at BN-metal interface. Melted fastener Since BN installation, H/D ratio has remained higher than previous campaigns. May need to improve bakeout capability of the tiles.

12 J-port BN Tile Design BN front tile fasteners are covered by the BN end tile. BN end tile fastened upon a double rail with back stop. Inconel endplate SS shorting strap Mo front tile BN end tile BN front tile Invessel assembly will require: Placing the BN end tile on the endplate without the stop, Securing the BN front tile, and Installing the end tile back stop

13 Shield BN-metal Interface from Plasma Remove metal triangular piece from FS. Original purpose was shield radial electric field. Modification does not compromise this function. 1.7" 3/8" 1.25" Tile fasteners shielded from plasma. Tiles are back to original position.

14 Investigate Wave Propagation by Phase Contrast Imaging Use a heterodyned technique to detect RF driven density fluctuations. CO 2 laser beam is modulated near the RF frequency. f~ khz. RF density fluctuations are detected at the beat frequency f. System characteristics are: 2 < f < 500 khz. 0.5 < k R < 10 cm < R < 79 cm. RF ANTENNA CO 2 LASER

15 Observed Mode Converted Ion Bernstein Wave Measured wavenumber is in agreement with IBW dispersion relation. Observed IBW is a backwards wave as expected. Signal level scales square root of power. f [khz] k R, cm -1 IBW phase velocity away from antenna phase velocity towards antenna IBW f RF [MHz] FWrefl FWtrans FWfor

16 PCI Data Structure Raises Questions Signal has maximums separated by ~1.5 cm. Too long for IBW and too short for Fast wave.

17 Structure Could be Result of Line Integration ω ch Z [cm] ω che3 R-R 0 [cm] E- shows ion Bernstein wave fronts. Fluctuations are dominated by E contribution. Chord integration results in structure that is 1-2 cm. Suggests PCI could measure region near mode conversion.

18 Code Algorithm Improvements Allow more Poloidal Modes TORIC is a full wave, spectral code (M. Brambilla). Requires high poloidal mode numbers to resolve IBW ( ). Spurious off-axis 3 He absorption is suppressed as N m is increased Demonstrates convergence of poloidal mode expansion

19 Near Term RF Experimental Plans Evaluate D, E, and J-port antennas performance with new modifications. Investigate J-port power and voltage limits. Examine impurity production for D, E and J-port. Requires both 70 and 78 MHz operation. Begin investigation of MCIBW flow drive. Measure RF power deposition profile via power modulation, RF driven density fluctuations with PCI, poloidal flow with HIREX, and affect on transport via sawtooth heat pulse propagation. (s)time Chordal d v θ [ v θ - < v θ ( s)> ] 2 MW off-axis MC MC radius Major Radius (m) Ω3He (km/s) 0.5 C.K. Phillips et al., NF 40,

20 Near Term RF Experimental Plans D( 3 He) Absorption Studies at high power. Absorption was ~75% compared to 90% for D(H) More sensitive to 3 He concentration than expected. Investigate effect of higher power density and plasma temperature. Evaluate J-port antenna CD and (0,π,π,0) phasing operation. Examine voltage and power limits. Investigate impurity production. Use both D(H) and D( 3 He). Begin to investigate MCCD. Effect on instabilities. Loop voltage analysis. Wukitch, 17 th IAEA, Yokohama 1998.

21 RF 5 Year Plan RF coupling, propagation, and absorption. Reliable antenna operation with minimum negative impact on plasma.» Minimize impurity production with ICRF» Fast electron production in LH near field» Advanced modeling of antennas in plasmas Investigate D(H) and D( 3 He) absorption» Mode conversion physics PCI measurements and code comparison. Investigate lower hybrid wave propagation near parametric decay instability density limit. Investigate interaction of MCIBW with LH waves.

22 5 Year Plan Heat and control transport barriers. Investigate MCIBW flow drive.» Evaluate MCIBW flow drive.» Understand underlying physics ponderomotive or Reynolds stress induced flow testing of numerical simulations. Investigate RF driven toroidal flow.» Examine role of resonant ions.» Investigate antenna phasing in both ion and electron dominated absorption regimes.

23 Five Year Plan RF current drive and current profile control Evaluate MCCD. Evaluate LHCD near density limit. Influence of compound spectrum on LHCD efficiency. Investigate MCCD and LHCD interaction. LH startup experiments. NTM stabilization with LHCD/MCCD Sawtooth stabilization using LH/MCCD Evaluate FWCD where ω< ω ci Investigate ICCD for local current profile tailoring. Comparison of modeling with experimental results Sci-Dac initiative Comparing detailed PCI measurements with TORIC in the mode conversion region. LHCD modeling with ACCOME and full-wave modeling.

24 RF 5 Year Hardware Initiatives Second compact 4-strap ICRF antenna to replace D and E- port antennas. ( 05) Second LH grill to enable reliable full power operation (4 MW) and compound spectrum for improved efficiency and current profile control. ( 05) Upgrade PCI: to enhance number of channels and resolution And to allow localization of signal. Current profile diagnostic and x-ray camera are crucial to understanding physics many of the proposed experiments. Local edge density measurements and thermal imaging of antennas would greatly improve investigative tools.