Status of the rf Current Drive Systemson MST

John A. Goetzfor

A. Almagri, J.K. Anderson, D.R. Burke, M.M. Clark, W.A. Cox, C.B.Forest, R. Ganch, M.C. Kaufman, J.G. Kulpin, P. Nonn, R.

O’Connell, S.P. Oliva, S.C. Prager, and the MST team

12th IEA RFP Workshop • Kyoto, Japan • March 26-28, 2007

Summary of rf experiments on MST

• Two experiments to drive current for fluctuation suppression– Lower Hybrid wave injection at 800 MHz– Electron Bernstein wave injection at 3.6 GHz

• Each system has operated successfully at the ~ 100 kW power level– Coupling studies performed

• Each system has been upgraded to the ~ 250 kW power level– Lower hybrid successfully operates at 220 kW

» Good plasma loading» HXR generation observed

– EBW antenna conditioning in progress

The RFP can benefit from current profile control.

• ad hoc force near reversal surface to mimic parallel current

• healed flux surfaces appear in the core

• inductive current profile control (PPCD) shows increased Te, χe,decreased fluctuations, and > 100 keV x-rays

• rf waves (LH and EB) are accessible, despite “overdense” plasma (ωpe2 / ωce

2 >> 1)

• Resistive MHD computation (DEBs code) shows reduced tearing

LH waves can be used for current drive in MST.

• Ray tracing (GENRAY) and Fokker-Planck (CQL3D) calculations♦ deposition controlled by n||, ω, and launch location♦ predicted efficiency is relatively high

• Inboard launch of 800 MHz waves with n|| ~ 7.5 shows good absorptionin the desired region of the plasma

Electron Bernstein waves can be used for current drive in MST.

• RFP plasma is overdense (ωpe >> ωce); no EM waves in ECRF

• EBWs are opportunity for heating and current drive in the ECRF

• Ray tracing has shown that wave directionality can be controlled

The two RFCD systems on MST have complementarychallenges.

• EBW physics has to be validated fortokamaks, ST’s and the RFP– shown to work in stellarators

• Waveguide antenna can be used tolaunch the EM wave– fits in 4.5” MST port

• Lower hybrid physics is wellestablished in tokamaks– need to extend to the RFP

• Antenna design is constrained byMST vessel requirements– interdigital line has two

feedthroughs and small radialextent

The LHCD interdigital line antenna has been upgraded for higherpower capability.

• Increase power handling capability to ~ 250 kW

– larger vacuum feedthrough– longer impedance-matching section

• Achieve VSWR < 1.4

– remove need for external tuning– better directivity

• Improve instrumentation

– vector power measurement of eachantenna element with better calibration

– density measurement with Langmuir probes

New LHCD antenna installed in MST.

• Third generation antenna installed in MST in November 2005

• Power supply upgraded to provide 46 kV - 17 A - 30 ms pulse to klystron

• Antenna has handled power up to the present transmitter limit of ~220 kW

Antenna operation has been extended significantly.

• “radiated” = source - (reflected + through + Ohmic losses)

• Antenna operates well under a variety of plasma conditions

• Work to increase the transmitter output power is ongoing

MkIIantenna“radiated”

MkIIIantenna“radiated”

MkII, MkIIIsource

Measured n|| spectrum is correct.

• New vector power measurement electronics designed and built– based on IF mixing and direct digitization

• Determine the magnitude and phase of the rf power on each antenna element

• n|| spectrum is robust to plasma changes

HXR diagnostics on MST

• Individual ZnCdTe detectors– Active area of 1cm by 1cm– Energy range from 10 to 200 keV– Direct digitization of Gaussian shaped pulses

• 16 channel array of ZnCdTe detectors– Active area of 16 x (3 mm by 5 mm)– Energy range from 10 to >300 keV– Direct digitization of bipolar shaped pulses

• 0.4 mm aluminum or 2.7 mm borosilicate vacuum windows are used– Detectors and camera can be easily moved

HXR emission has been observed during LH operation.

• Emission observed within ±30° toroidally of the antenna location

• Greater intensity observed when viewing at the antenna location (90T)

• Higher energy x-rays observed when viewing antenna directly

• Generation mechanism being investigated

LHCD Plans

• Continue coupling and loading measurements at moderate power (≤ 250 kW)• antenna and wave propagation studies to confirm correct wave

• wave-plasma interaction studies (with improved x-ray detection)

• CQL3D used to help interpret results

• Extend studies to ~ 500 kW source power• antenna optimization

• estimate current drive efficiency

• density limit, fast electrons, etc.

• Design and implement 1 - 2 MW LHCD experiment to reduce fluctuations

Coupling to EBW depends sensitively on edge density.

Cutoff ( R )Upper hybrid resonance

Cutoff ( L )

EBW

• Reflection occurs from each cutoff

• Distance between layers determined by ne and B profiles

• Interference of reflected waves leads to optimized transmission

Launched EM wave couples to Bernstein mode atupper hybrid resonance

EM wave

Coupling to EBW inferred from measured reflection.

• Predicted dependence with edge density ismeasured.

• Oblique launch enhances coupling to EBW

• BN Antenna cover improves coupling– Affects local electron density gradient– Blocks plasma from entering antenna

(source of arcing)

• Coupling can be good: R/F < 10%

• Ln = n/∇n of MST PPCD plasmas is nearideal value for this coupling.

simulationdata

Ln [cm]

4-waveguide EBW antenna installed on MST.

• Twin waveguide antenna operated successfully at 125 kW– no indications of rf-plasma interaction– enhanced boron levels during operation

• Rf sources tested to long pulse (10 ms) at full power (75 kW)

• Windows, transmission lines worthy for power upgrade

• Langmuir probes added to boron nitride cover plate

Conditioning of the EBW antenna is ongoing.

• Antenna operation was hampered by arcing• Inspection revealed damage along the seams• Damage repaired by polishing the inside of the antenna

• Optical and rf arc detection systems under development• Present power limit of 25 kW achieved in two arms only

EBW Plans

• Is this design the optimal coupling structure for MST?

• Continue to operate and condition 250 kW system

• Search for rf-plasma interaction with ≥ 200 kW applied power♦ measure power deposition profile♦ infer modification of electron distribution function

• Investigate low power coupling in the C-band @ 5.65 GHz several 1.2 MW tubes are available C-band => smaller antenna; improve MST access large undertaking; pending results of present experiment

Summary

• Two rf current drive techniques are being explored on MST as ameans to suppress tearing mode fluctuations

• Third generation LHCD antenna installed and operating at 220 kW

– Good antenna-plasma loading and coupling observed

– Launched spectrum has n|| = ± 7.5 as designed

– HXR production observed

• 4-waveguide EBW antenna is installed

– Power conditioning is an ongoing endeavor