Overview of ICRF Experiments on Alcator C-Mod*

Transcription

1 49 th annual APS-DPP meeting, Orlando, FL, Nov Overview of ICRF Experiments on Alcator C-Mod* Y. Lin, S. J. Wukitch, W. Beck, A. Binus, P. Koert, A. Parisot, M. Reinke and the Alcator C-Mod team MIT, Plasma Science and Fusion Center Cambridge, MA 02139, USA *Work supported by US DoE Cooperative agreement DE-FC02-99ER Contact info: Yijun Lin, 1

2 Part I: Real-time Fast Ferrite Tuning System 2

3 Stub Tuner/Phase Shifter System Goal: zero reflection on the matched side at all time. Stub tuner and phase shifter are adjusted between-shot based on anticipated average antenna loading. Difficult maintaining the match when the antenna loading changes significantly during a discharge. Difficult finding ideal ST/PS positions in experiments with large plasma parameter variation shot-by-shot (e.g., density scan). 3

4 Real-time Matching Techniques 1. Frequency modulation: Varying RF source frequency to change impedance Fast (milliseconds) Requires a long transmission line and a wide-band response of the transmitters Tested on JET and LHD (Not applicable on Alcator C-Mod) 2. Dielectric liquid: Pumping/filling dielectric liquid in stubs and/or phase shifters to change electrical length Slow (seconds), but good for long pulse operation Installed on LHD (Not applicable on Alcator C-Mod). 3. Ferrite material: Varying magnetic field on ferrite material to change effective electrical length Fast (milliseconds) Limited range of electrical length variation Tested on ASDEX-Upgrade (Successfully implemented on Alcator C-Mod) 4

5 Fast Ferrite Tuning (FFT) System Tuner Detail The equivalent electrical length of the tuners (at 80 MHz) can vary 35 cm for coil current from -150 A to 150 A. System Layout Real-time digital feedback control to match the time-varying antenna loading. Computation cycle 200 μs. 5

6 FFT System (Top View) 9-inch 50 Ω line from transmitter Tuner #1 L 1 =1.60 m 9-inch 50 Ω line to antenna L 12 = 3/8λ DC1 Tuner #2 L 2 =.99 m DC2 6

7 FFT System (Side View) Tuner #1 SF6 Gas Tuner #2 Arc detection fiber Coil current Cooling water 7

8 Control Hardware Linux server (Intel Xeon 3.2 GHz with 2 GB memory) Compact-PCI controller (32-input, 16-output) PLC for remote control Power supply for tuner #1 RF demodulators for DC1 and DC2 8

9 FFT at Beginning of RF Pulse The FFT system lowered the power reflection coefficient on DC1 from ~ 30% to less than 3% in less than 2 ms. The power reflection was maintained < 3% under feedback control for the entire RF pulse. <2 ms 9

10 FFT during L-H L H Transitions and ELMs With the real-time matching, the power reflection coefficient did not exceed 15% during L- H-L transitions and ELMs. The response time was generally limited by the power supply capability, but not the computer cycle. Ran successfully in both L modes and H-modes. Average density x10 20 m -3, I p = MA. Successful up to 1.1 MW forward power in H-mode (27 kv, 3.3 MW circulating power). 10

11 Part II: ICRF Sheath Study 11

12 Boronization on Alcator C-Mod Boronization is essential in obtaining high performance on C-Mod. However, an overnight boronization (~200 nm layer) can only last about 50 MJ RF energy. RF heated plasmas show much faster (~ 5 times) boron erosion, Joule to Joule, compared to Ohmic plasmas. Boron erosion is preferential in a rather small area P RF [MW] H ITER-89P Time [s] P rad [MW] , Blue: unboronized Red: boronized 12

13 RF Induced Boron Erosion is Localized st Discharge Ant 1 2nd Discharge Ant 1 Radiate Power (MW) H-mode Time (s) 1st Discharge Ant 1 2nd Discharge Ant 2 Radiate Power (MW) H-mode Time (s) After a between-shot boronization, running the same antenna in consecutive plasma discharges helped to raise the radiated power. Running a different antenna in the 2 nd discharge showed no increase in the radiated power. Different antennas were eroding the boron layer at different locations. 13

14 RF Sheath Potential Ant 1 D E J Ant 2 Probe Plasma potentials measured by an emissive probe were higher due to RF sheath when the linked antenna (Dport antenna) was powered. RF sheath is thought to be the major cause of the enhanced boron erosion (significant sputtering if greater than 30 V) compared to Ohmic plasmas. 14

15 Sheath Potential Scales with RF Power Sheath potential is higher at higher RF power. Higher in H-mode (~100 V at 1.5 MW) than in L-mode (~50 V at 1.5 MW). In H-mode after boronization, V ~ P 0.5. In unboronized H- mode, V ~ P. The difference of sheath potential vs. wall conditions and plasma confinement is not yet understood. Impurity generation (as seen in Prad) was similar in D(H) and D(He3) heating scenarios. Plasma Potential [V] unboronized boronized Sqrt(RF Power) [MW 1/2 ] 1.4 We don t know whether and how the sheath affects impurity penetration and transport , ,7,9 15

16 B T ErxB T Simple Sheath Model Open field lines connect conducting surfaces and enclose RF flux. Electrons respond to oscillating RF voltage and are lost preferentially. Field lines are charged positively and most voltage drop appears across the sheath. E r Antenna with boron nitride tiles ( ) Adding boron-nitride tiles on the antenna was expected to break the circuit. Surprisingly, it didn t. 16

17 BN Tiles Didn t Eliminate the Sheath 1 RF Power [MW] 1.5 RF Power [MW] with BN tiles with metallic tiles 0.5 H-mode Plasma Potential [V] Time (s) , Plasma Potential [V] Time (s) Sheath potentials at similar power level were similar with and without BN tiles. With BN tiles, the sheath potential was still as high as 100 V in H-mode. What are missing in the sheath model? 17

18 Research Directions and Impacts Marked tiles to identify the exact sputtering locations due to the RF sheath, and compare normal and reversed field plasmas to see whether and how the sheath affects impurity penetration and transport. On the new 4-strap antenna design (See Part III of this poster), we have made careful consideration to reduce poloidal and radial B field near the end of the straps lower sheath voltage and possibly lower impurity penetration. 18

19 Part III: New 4-strap 4 Antenna 19

20 Existing D-D and E-port E Antennas Operated routinely at high power (> 3 MW total), high standing voltage and over a wide density range. Easy to install and robust toward disruptions. Short vacuum transmission line and minimized E B regions. No spectrum control (2-strap, dipole) Impurity generation seems to be higher than J-port antenna at the same power density. Prone to arcing at E B locations. 20

21 Existing J-port J 4-strap 4 Antenna More transparent Faraday screen, higher antenna loading than D- and E-antennas Phase controlled for both heating and current drive. Improved spectrum (4-strap vs. two 2-strap). Good for a wide frequency range (operated at 50, 70 and 78 MHz). Limited at 3 MW total power. Having difficulties coupling to high neutral pressure plasmas (possibly due to a long vacuum transmission line). Time-consuming in-vessel assembly (~2 people for 2+ days). 21

22 Requirements for the New Antenna To free up a horizontal port for the 2 nd LH launcher but not comprising total ICRF power (FY2009). > 3 MW into high performance plasmas with minimum impurity production. Frequency range MHz. Pulse length up to 5 sec with 20 minutes betweenshot (inertial cooling). Withstand thermal loads 12 MW/m 2, and disruption loads of 1 T/ms at 9 T. Adequate diagnostics. Simple in-vessel installation. 22

23 Straps, Feedthrus & Transmission Lines Folded strap concept and strap separation (Similar to J-port antenna). Re-entrant 5 feedthrus (larger than the existing 4 feedthrus for higher power handling capability). Short parallel plate transmission line to reduce impact of neutrals. 23

24 E Field Limits and Radial Feeders Limit peak E < 10 kv/cm, and avoid E B wherever possible (Universal empirical observation). Limit max E B to 35 kv/cm. Shield radial feeders entirely to limit RF sheath formation (See Part II of this poster). 24

25 Faraday Screen Horizontal Faraday screen rods (same as J-port antenna). ~50% transparency (like J-port antenna) Molybdenum rods, electrically insulated at one end (~0.1 Ω to ground) to eliminate disruption induced currents (similar to J-port antenna). Modular design for simple assembly (similar to D- and E-port antennas) 25

26 Additional Features Diagnostics: GHz frequency swept X-mode reflectometer for edge density profile (poster by C. Lau, this session). Optical arc monitors. Voltage and current probes for RF pattern. External loop and matching: Flexible phasing, both heating phase and current drive phase. Standard resonant loop configuration. FFT system for real time matching (future). Assembly: Allows removing feedthrus without full antenna disassembly. 26

27 Part IV: Summary and Plans 27

28 Summary A fast ferrite tuning system has been successfully implemented on the E-port antenna. ICRF sheath study shows that the RF sheath is responsible for enhanced local boron erosion. The result also calls for more theoretical/modeling work to understand the sheath problem. A new 4-strap antenna has been designed. The design has incorporated the experience on the existing antennas, and also the recent RF sheath study result. 28

29 Plans The fast ferrite system is being upgraded to have higher voltage handling. We plan to develop and install similar systems on all transmitters/antennas in the future. We plan to do more experimental studies on the RF sheath problem, for example, marked tiles, reversed field plasmas, etc. We plan to install more emissive probes for more sheath monitoring. The new 4-strap antenna is scheduled to be fabricated and installed on the tokamak in FY