1. NSTX Reflectometer Measurements of RF Waves in the Scrape-off Layer in Front of the HHFW Antenna Array J. B. Wilgen, G. R. Hanson, P. M. Ryan, D. W. Swain Oak Ridge National Laboratory S. Bernabei, N. Greenough, S. DePasquale, C. K. Phillips, J. Hosea, J. R. Wilson Princeton Plasma Physics Laboratory 47th APS-DPP Denver, Colorado October 24-28, 2004

2. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 2 JBW APS-DPP2005 Abstract The microwave reflectometer on NSTX, in addition to its primary function of measuring edge-density profiles, has been modified to monitor RF waves in the scrape-off layer in front of the 30 MHz High Harmonic Fast Wave (HHFW) antenna array. Access to the plasma is located on the horizontal midplane, between two current straps of the HHFW array. A broadband reflectometer covers the frequency range of 6-27 GHz, probing the density range from below 1 x10 17 m -3 up to 8 x10 18 m -3. RF wave-related signals are extracted from the reflectometer using a high-pass filter and preamplifier circuit, and then digitized at 100 MHz sampling rate. The reflectometer microwave signal exhibits 30 MHz sidebands, due to the modulation of the cutoff layer by the electrostatic component of the RF wave. In addition, parametric decay waves are detected at frequencies below the heating frequency, near 28 and 26 MHz. Dependence of the RF spectra on the antenna phasing and on the reflection location within the scrape-off layer will be presented and compared with similar spectra obtained from a floating Langmuir probe located in the HHFW antenna.

3. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 3 JBW APS-DPP2005 Motivation for RF Wave Monitoring The efficiency of RF power coupling to the NSTX core plasma has been observed to depend on HHFW antenna array phasing. Doppler broadening of impurity emission lines showed phase- dependent edge ion heating (Biewer). One possible mechanism for power loss in the edge plasma was hypothesized to be the parametric decay instability (Wilson). Last year, spectrum analyzer data obtained with a floating Langmuir probe in the HHFW antenna revealed the characteristic PDI decay spectrum (S. Diem, APS-DPP2004). The reflectometer enables probing the plasma edge region ( up to the outermost flux surface & slightly inside ) for similar evidence of parametric decay activity, and anything else that might shed light on the RF coupling issues.

4. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 4 JBW APS-DPP2005 Reflectometer Probing of RF Waves on NSTX The HHFW Reflectometer has been modified to monitor RF waves in the scrape-off layer in front of the 30 MHz antenna array on NSTX Access to the plasma is located on the horizontal midplane, between two straps of the HHFW array Reflectometer signal exhibits 30 MHz sidebands, due to the modulation of the cutoff layer by the electrostatic component of the RF wave Parametric decay waves are detected at frequencies below the heating frequency, near 24, 26, & 28 MHz Spectra are compared with similar data obtained from a floating Langmuir probe that is also located within the HHFW antenna

5. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 5 JBW APS-DPP2005 Reflectometer Access Located Between 2nd and 3rd Straps of the HHFW antenna Reflectometer launchers RF Langmuir probes

6. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 6 JBW APS-DPP2005 Broadband Cylindrical Waveguide Launchers (HHFW Reflectometer) 1.5” OD Cylindrical Waveguide Antenna  Antennas are recessed 2.5 cm behind BN tiles (& Faraday screen)  Linear polarized launch couples to X- mode propagation in plasma  Launched polarization externally adjustable to match the pitch angle of the magnetic field (typically 35 degrees off vertical)

7. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 7 JBW APS-DPP2005 Extracting RF Wave-Related Information from the HHFW Reflectometer Goal: Extract RF wave-related sidebands from the reflected microwave reflectometer probe -- without affecting normal operation of the reflectometer The X-mode reflectometer scans the 6 to 27 GHz frequency range, probing the density profile of the scrape-off layer The probed density ranges from below 1x10 11 /cc up to nearly 8x10 12 /cc The I/O demodulator has an IF output frequency range of dc- 500 MHz -- RF wave-related sidebands were already present at the reflectometer outputs A 10 MHz high pass filter was used to sample one of the I/Q outputs ( normal reflectometer operation only makes use of the dc-2 MHz IF frequency range )

8. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 8 JBW APS-DPP2005 RF Wave Monitoring Circuit HHFW Reflectometer HHFW Reflectometer Instrument Enclosure I/Q Mixer 5-27 GHz IF range DC-500 MHz I Q Plasma path signal Reference path signal 10 MHz HP filter Preamp. (Mini-Circuits) 28 db gain 0.1-500 MHz 400X DC-2 MHz L6810 Digitizers 3 db Amplifier (Mini-Circuits) 21 db gain 10-500 MHz RF Langmuir Probe 100 MHz Fast Digitizer RF Wave Spectra Edge Density Profiles Density Fluctuations

9. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 9 JBW APS-DPP2005 Examining the Parameter Dependencies of Parametric Decay Spectra How do the parametric decay spectra change with experimental conditions?  Location within the scrape-off layer  Plasma Current  Antenna phasing -90 degrees (co-CD) +90 degrees (CCD) 180 degrees (heating)  Outer gap size (density in scrape-off layer?)  RF power level

10. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 10 JBW APS-DPP2005 Dependence on probing location throughout the scrape-off layer, and on plasma current For 16 shots on June 21st, the reflectometer frequency was slowly swept with a 10 Hz triangle modulation waveform - scanning the probing location periodically throughout the scrape-off layer This data set (a subset of XP 527) includes three antenna phasings (-90, +90, & 180) Plasma currents of 300, 600, & 800 kA are included Initial Result: It appears that the RF spectra are probably much the same throughout the entire scrape-off layer, and largely independent of the plasma current

11. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 11 JBW APS-DPP2005 Dependence on Antenna Phasing For 300 kA plasmas, with a fixed probing frequency of 17.5 GHz ( cutoff density 2.5x10 12 /cc ) Have data for two antenna phasings from July 20th continuation of XP527 (see below)  -90 degrees (co-CD), shot 117239  180 degrees (heating), shot 117243 Parametric decay spectra are strongest for -90 degree phasing, and practically non-existent for symmetric (180) phasing

12. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 12 JBW APS-DPP2005 Phase-Averaged Density Profiles in Front of the HHFW Antenna At 17.5 GHz, the reflectometer probes the edge at a cutoff density of about 2.5x10 12 /cc, typically about 3-4 cm in front of the HHFW antenna,,depending on the outer gap spacing

13. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 13 JBW APS-DPP2005 RF Spectra for -90 degree antenna phasing (2.0 MW Power, 4 cm gap) Reflectometer RF Spectra RF Langmuir Probe Spectra Parametric decay frequencies 30 MHz Heating Wave

14. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 14 JBW APS-DPP2005 RF Spectra for 180 degree antenna phasing (1.8 MW Power, 4 cm gap) Reflectometer RF Spectra RF Langmuir Probe Spectra High-pass filter roll-offAnti-aliasing LP filter roll-off 31 MHz LO Signal

15. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 15 JBW APS-DPP2005 Individual RF Spectra for -90 degree antenna phasing 11 spectra from 240 to 260 msec 11 spectra from 260 to 280 msec Parametric features are intermittent, whereas 30 MHz is always present Amplitudes of parametric features are nearly as large as the 30 MHz component

16. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 16 JBW APS-DPP2005 Individual RF Spectra for 180 degree antenna phasing 11 spectra from 240 to 260 msec 11 spectra from 260 to 280 msec Note the reduced amplitude of the 30 MHz feature, and the absence of parametric frequencies

17. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 17 JBW APS-DPP2005 Dependence on Outer Gap Spacing (plasma density in front of the antenna?) For 300 kA plasmas, with a fixed probing frequency of 17.5 GHz (cutoff density 2.5x10 12 /cc) Have data for various outer gaps (see below)  5 cm gap, shot 117238  3 cm gap, shot 117240  8 cm gap, shot 117250 Parametric decay spectra are most prominent for the largest (8 cm) gap spacing, with lowest density at the antenna

18. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 18 JBW APS-DPP2005 RF Spectra with medium gap, -90 phasing (nominal outer gap: 5 cm, 2 MW RF power) Reflectometer RF Spectra RF Langmuir Probe Spectra

19. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 19 JBW APS-DPP2005 RF Spectra with small gap, -90 phasing (nominal outer gap: 3 cm, 2 MW RF power) Reflectometer RF Spectra RF Langmuir Probe Spectra

20. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 20 JBW APS-DPP2005 RF Spectra with large gap, -90 phasing (nominal outer gap: 8 cm, 2 MW RF power) Reflectometer RF Spectra RF Langmuir Probe Spectra Note the intermittency of the parametric features in the reflectometer spectra

21. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 21 JBW APS-DPP2005 Individual RF Spectra for -90 degree antenna phasing, 3 cm gap 11 spectra from 240 to 260 msec 11 spectra from 260 to 280 msec Parametric features are intermittent, whereas 30 MHz is always present Amplitudes of parametric features can be nearly as large as the 30 MHz component

22. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 22 JBW APS-DPP2005 Individual RF Spectra for -90 degree antenna phasing, 8 cm gap 11 spectra from 240 to 260 msec 11 spectra from 260 to 280 msec Parametric features are intermittent, whereas 30 MHz is always present Parametric features evident on high frequency side, above the 30 MHz component

23. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 23 JBW APS-DPP2005 Dependence on RF power level (incidental evidence) Don’t have a systematic power scan - look at power variations within the standard power waveform Examine power ramp at beginning of RF power waveform - power ramps up over a 20 msec interval  During power ramp-up, 30 MHz appears first  Parametric decay products don’t appear immediately  Parametric features appear first at 28 MHz, followed by 26 & 24 MHz  Onset timing is indicative of power threshold During power turn-down from 300-330 msec, power is reduced by about 100x, from 2 MW to about 20 kW, reducing the fields by 10X  Still see evidence for 30 MHz signal during this period  Don’t see any indication of parametric decay frequencies  Indicates power threshold of > 20 kW

24. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 24 JBW APS-DPP2005 Details of the RF power waveform, the outer gap spacing, and the edge-density profile Contour plot of edge- density profile in front of the HHFW antenna (for 120 degree phasing) For this gap spacing, the edge profile measurement also extends 3-5 cm inside the outermost flux surface Also displays the time dependence of the outer gap spacing (see black diamonds) Note the gradual ramp-up of the RF power starting at 200 msec, and the power reduction at 300 msec Density Contours (x10 13 /cc) Note: Steep density profiles at t=340 msec is not an RF effect, it is due to plasma hitting the antenna

25. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 25 JBW APS-DPP2005 Expanded view of reflectometer spectra with a large 8 cm gap, -90 phasing Parametric decay products are delayed, appearing 5-8 msec into the 20 msec power ramp which starts at t = 200 msec Indicates power threshold of 100-300 kW

26. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 26 JBW APS-DPP2005 Expanded view of reflectometer spectra with a smaller gap, -90 phasing Parametric decay products are delayed, appearing 8-10 msec into the 20 msec power ramp which starts at t = 200 msec Indicates power threshold of about 300-400 kW

27. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 27 JBW APS-DPP2005 Summary: Monitoring RF waves in front of the HHFW antenna using the edge reflectometer  Have only looked at a fraction of the RF wave data - typically 20 usec snapshots every 2-10 msec.  Find evidence of systematic variations of parametric decay spectra with antenna phasing  See indications of a power threshold in the 100-400 kW range for co-CD phasing, depending on outer gap  Suspect a dependence on outer gap spacing, suggesting dependence on plasma parameters there  Have not yet seen any indication of systematic changes with probing location within the scrape-off layer  Have not yet seen a dependence on plasma current

28. O AK R IDGE N ATIONAL L ABORATORY U. S. D EPARTMENT OF E NERGY 28 JBW APS-DPP2005 Expanded view of reflectometer spectra with a large 8 cm gap, -90 phasing Parametric decay products are delayed, appearing later in the 20 msec power ramp which starts at t = 200 msec Only the 30 MHz signal is evident during the reduced power interval of 300-330 msec, suggesting a power threshold > 20 kW