1. NSTX Boundary Density Fluctuation Measurement by FIReTIP System with USN/LSN Configurations K.C. Lee, C.W. Domier, M. Johnson, N.C. Luhmann, Jr. University of California at Davis H. K. Park, S. Zweben PPPL, Princeton University NSTX Research team Princeton Plasma Physics Laboratory

2. Abstract The outermost edge channel of the Far Infra Red Tangential Interferometry and Polarimetry (FIReTIP) system provides absolutely calibrated density fluctuation information in the edge region of the plasmas including the scrape-off region. Recently upgraded time resolution up to 500 kHz and a visible interferometer system, enable measurement of semi-localized electron density fluctuations as well as the corresponding chordal density. The study of a higher fluctuation level when the ion grad B drift direction is away from the X-point due to the boundary turbulence modes such as the X-point mode claimed by BOUT code has been proposed via two different configurations (USN and LSN) on NSTX. In this paper, the edge channel FIReTIP measurement of density fluctuations together with the comparison of GPI data including L/H transition and ELM period of NSTX plasmas will be discussed. This work was supported by U.S. DoE Contracts DE-FG02- 99ER54518 and DE-AC02-76CH03073

3. Contents n Overview of FIReTIP system o Introduction o Upgrades of 2004 n Visible Interferometry n Boundary Density fluctuation Measurement o USN vs. LSN Configuration o Comparison with GPI data o L/H Transition and ELMs n Summary

4. 3-D CAD of Far Infra Red Tangential Interferometer Polarimeter

5. Principle of FIReTIP system on NSTX w LO – w 1 = 4.0±1.0 MHz w LO – w 2 = 6.0±1.0 MHz Rommers & Howard 3-laser system [K.C. Lee et.al., IEEE Trans. on Plasma 32,1721(2004)]

6. FIReTIP channels (ch1, ch2, ch3, ch7 for 2004) New for 2004 n Time resolution is upgraded to 500 kHz => ~1 MHz (2005) n Vibration compensation for CH7 with visible interferometer

7. n Time Resolution of FIReTIP can be up to ~MHz by using Stark-tuned laser [K.C. Lee et.al., RSI 74,1621(2003)] n Time response of system was limited by the digitizer (100kHz, 2003) n Temporal resolution upgrade (500 kHz, 2004) Upgrade of FIReTIP Digitizer FIReTIP signals D-TACQ ACQ16 Digitizer Board Linux OS Installed PC MDSplus Software Tool Set NSTX Data Base Maximum Sampling rate : 1.8 MHz No. of Sampling channel : 16 ( simultaneous) Data resolution : 14 bit

8. Visible interferometer Compensation HeNe laser detector beam splitter retro-reflector FIR beam line  Ch7 (R T =150cm) has 40 times smaller signal to noise ratio than other channels due to short beam path and extremely low electron density => require two color compensation  A visible homodyne interferometer (632.8 nm) was installed => upgrade to heterodyne (2005) ch7

9. Result of the Vibration Compensation for Edge Channel by Visible Interferometer  Raw data of Ch7 (R T =150cm) have low frequency vibration noises ~ 10 13 (/cm 3 )  Remained uncertainties : (1) fringe direction due to homodyne system (2) beam path difference of visible interferometer

10. Edge Density Fluctuation Measurement  (c) : fluctuation ( ) spectra of edge density measurement covering full discharge without loss of fringe.  #111292 was an ohmic H-mode shot  Edge channel (R T = 150 cm) showed 50 ~ 100 times higher fluctuation level than other channels  Apparent reduction of fluctuation level at L\H transition [K.C. Lee et.al., RSI 75,3433(2004)]

11. BOUT code prediction (a) turbulent particle flux in the radial-poloidal plane; (b) flux surface averaged turbulent particle flux with variation of the ion grad B drift direction related to X point. Ion ▼B drift directionaway from X-point (USN) toward X-point (LSN) turbulent particle flux & P th highlow E r XB T velocity shear(DIIID)lowhigh toroidal velocity(C-Mod result)counter-currentco-current BOUT predictions [Xu et al. Nucl.Fusion 42,pp.21-27 (2002)] LSN vs. USN study

12. FIReTIP Edge Density Fluctuation Measurement ch7 LCFS locations ■ Typical density profile of NSTX plasma at the edge and the FIReTIP channel configuration (the location of LCFS is between Rt=146 cm and Rt=150 cm).

13. Edge Density Fluctuation Measurement with LCFS Scanning LSN cases #112825, 36,38,39,40 ■ RMS value of ∂n e increased with LCFS moving out ■ Ave R-mid-out ↑  plasma moving out  Integral area of CH7 measurement Increase In SOL where n e is constant ■ Result is agreed to prediction of BOUT simulation ■ Comparison with USN cases will be done in 2005

14. Result of LSN vs. USN study (2004) LSN USN  Edge density fluctuation shows no clear difference for  USN cases had no coherent MHD oscillations BOUT simulation X-point modes, Ion Grad B drift FIReTIP density fluctuations (R T = 150 cm)

15. TAE/f.b.s Measurement  Upgrade of the time resolution made feasible to measure frequencies up to TAE range  Four channel data => including TAE/f.b.s in the core of the plasma

16. Continue  Density fluctuation spectra of TAE/f.b.s measured by FIReTIP  Spectra of density fluctuation ( ) up to Nyquist frequency limit of 500 kHz

17. Electron density fluctuation during the ELMs ■ Example of electron density fluctuation during the ELMs measured by FIReTIP edge channel which shows high frequency ELMs (type III). ■ (d) is magnified detail of (b) and (e) is detailed spectrum of (c) where agreed peaks were indicated for each individual ELM by both D-alpha emission and FIReTIP density measurements. ■ during ELMs spectrum becomes more like L-mode than quiet H-mode period (e) Spectrum of FIReTIP ∂n e (kHz)

18. Faraday Rotation Measurement  CH1 & CH2 measures Faraday rotation angle correlated with three giant ELMs indicated in the CH1 calculated values  Strong para-magnetic in CH3 : more effective channel for plasma induced magnetic field measurement  Smoothing was taken at 125 Hz for signal (0~125Hz)  Removal of additional noise is in progress: New ICQ fringe counter & enforced vibration isolation

19. Comparison with GPI measurement  FIReTIP electron density data have phase shift compared with the GPI data  Examples of High-n modes : blobs (#113487) and periodic oscillations (#113525) cases FIReTIP beam path GPI data from S. Zweben

20. n FIReTIP system upgrade o Four channel operation in ’04 (ch1, ch2, ch3, ch7) => ’05 (ch1, ch2, ch3, ch4, ch5, ch7) : 6 channel system o Temporal resolution is increased up to 500 kHz by PC based DAQ => high time resolution electron density fluctuation study ’05 ( 1 MHz : by upgrade of signal processing electronics) o Vibration compensation by a visible interferometer => absolute value of edge electron density 2004 measurement o FIReTIP measured edge fluctuation of H-mode plasma including L/H transition and ELMs o LSN vs. USL study in conjunction with BOUT simulation o Density fluctuation spectra up to 500 kHz covering TAE/f.b.s o Faraday rotation measurement of the core channels o Comparison with GPI measurement Summary and Future Plan