ECH Density Pumpout and Small Scale Turbulence in DIII-D

Transcription

1 ECH Density Pumpout and Small Scale Turbulence in DIII-D By K.L. Wong, T.L. Rhodes, R. Prater, R. Jayakumar, R. Budny, C.C. Petty, R. Nazikian, and W.A. Peebles

2 Background It has been known for more than 3 decades that ECH at the core of tokamaks or stellarators can lead to density pumpout - see Erckmann and Gasparino, PPCF(1994) Recent theory based on the different effects of ITG & TEM turbulence on thermodiffusion was used to explain the ASDEX-U results -Angioni et al., NF(2004) This motivates our search for ITG/TEM during ECH experiments in DIII-D

3 Experimental Parameters I p = 1.3 MA, B t = 1.83 T, R 0 = 1.77 m, a = 0.56 m, q 95 = 4.2, 1.6 > q(0) > normal shear P ech up to 3 MW at 110 GHz,! res ~ QH mode plasma - no ELMs Counter- injected neutral beams Deuterium plasma with deuterium beams Single null upper diverter

4 Discharge Waveforms Ip, Bt, beams, ECHPWR, density

5 FIR scattering system schematics FIR scattering system makes simultaneous low and intermediate-k measurements 288GHz forward scattering system (O-mode) Intermediate-k mixer 8-15 cm -1 Low k mixer 0-2 cm -1

6 FIR scattering system instrument function FIR scattering system instrument function Low k (1 cm -1 ) is chord average along midplane For this value of intermediate k (7 cm -1 ) instrument function is centered near edge on high Þeld side

7 FIR Scattering Data K! ~ 1 cm-1 and 7 cm-1 # (homodyne) and # (heterodyne): freq & direction of k in plasma frame? Location?

8 Density Profile Evolution (pumpout)

9 Details of n e (r) variation: n e (0) rises after 2.3 s as the 7 cm -1 signal drops

10 When no pumpout, no small scale fluctuations at 7cm-1 # pumpout at 3 MW No pumpout shots at MW: (QH-mode) (L-mode) (H-mode) (H-mode)

11 Results from TRANSP electron convection dominates electron energy transport in plasma core

12 Results from TRANSP electron conduction dominates electron energy transport near and outside the EC resonance layer

13 Heat pinch? (! e <0 at "<" res )

14 Expected Fluctuations during ECH Localized electron heating produces steep Te gradient, and destabilizes ETG modes Unlike ITG turbulence, ETG turbulence is not isotropic, can generate streamers, although the transport enhancement is still a subject of debate [see Jenko et al., PoP(2000), Diamond et al., PPCF(2005), Lin et al., PoP(2005)]

15 Theoretical Characteristics of ETG Streamers Broad k! -spectrum k! > k r on the low-field side of the torus where ETG turbulence peaks - standing wave in radial direction - scatt. system most sensitive to k r ~0 spectrum peaks around 2"/#! where #! is the average poloidal spacing of the streamers #! ~ 10 s of $ e (Lin-private communic.) ~ 30 $ e [Jenko et al., PoP(2000)] ~ c/% pe if #! = 60 $ e ~ 7 cm -1 if #! = 90 $ e

16 ! e is higher during ECH

17 Is it possible that the 7cm -1 signal is due to scattering from streamers? K. L. Wong: Yes, if gyrokinetic simulation yields a k! spectrum that embraces 7 cm -1. T. L. Rhodes: No, because the scattered waves miss the receiver due to k r of the streamers. - explanation follows-

18 During ECH T e increases but n e and T i decreases

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21 Variation of! i during ECH

22 Variation of! ie during ECH

23 Comparison between DIII-D & TFTR Results DIII-D TFTR plasma type QH mode ERS mode Heating ECH + ctr NBI NBI dq/dr normal negative T e /T i 6 kev/13 kev 7 kev/20 kev n e0 (cm -3 ) < 4x10 13 fell during ech < 7x10 13 rose during NBI k! 7 cm -1 9 cm -1 k! " i 6 5 Electron- convection at "<" res, # e /D e ~30, D e ~0 Transport conduction at ">" res # e /# i ~12

24 What Causes Density Pumpout during ECH? Since! e doubled in plasma core during ECH, ETG is probably unstable Since k" i ~6, ions cannot respond to the observed small scale turbulence ETG alone is not expected to cause pumpout, other larger scale turbulence are probably present that raise # i above 10 4 cm 2 /s during ECH Need detailed analysis for various microinstabilities during ECH - only small changes in! i and! ie were observed

25 Further Analysis Calculate the linear growth rate for ETG, ITG and TEM After determine (!,k) in the plasma frame, try to identify the mode - good data on Er do exist Do gyrokinetic simulation for ETG to find out the k " spectrum in our experiment Understand the ion channel transport during ECH. ETG is not expected to affect ion transport; search for evidence of ITG & TEM when T e /T i increases during ECH

26 Summary We try to investigate ITG/TEM fluctuations during ECH density pumpout. Find 1cm -1 fluctuation amplitude & frequency bandwidth are reduced during ECH - need detailed analysis together with Er data Find good correlation btn signal at 7cm -1 and pumpout. TRANSP analysis indicates that convection plays a dominant role in core transport. Since k! i ~6, these small scale turbulence alone cannot cause pumpout - need growth rate calculations for ITG/TEM Is it possible that the 7cm-1 signal come from ETG streamers? Still a subject of debate among us - need more analysis and more experiments Further simulations with experimental parameters are needed. These data can provide critical information to resolve the dispute on ETG effects and electron transport.