Study of Enhanced D α H-modes Using the Alcator C-Mod Reflectometer

Transcription

1 Study of Enhanced D α H-modes Using the Reflectometer Y. Lin 1, J.H. Irby, E.S. Marmar, R. Nazikian, M. Greenwald, A.E. Hubbard, J. Hughes, I.H. Hutchinson, B. LaBombard, A. Mazurenko, E. Nelson-Melby, J. Rice, J. Snipes, S. Wolfe MIT, Plasma Science and Fusion Center, Cambridge, MA 02139, USA Plasma Physics Laboratory, Princeton, NJ 08543, USA 1 ylin@psfc.mit.edu

2 Abstract Reflectometry in the tokamak plays an important role in identifying the quasi-coherent fluctuations, which affect the edge particle transport in Enhanced D α H-modes. The O-mode amplitude modulated reflectometer currently has 5 channels with critical densities n e = m 3. The quasi-coherent fluctuations have been observed by the reflectometer, phase contrast imaging (PCI) system and Langmuir probes. The most likely location of these fluctuations can be determined by comparing the fluctuation signals of different reflectometry channels. The location is compared with other local plasma parameters. The frequency evolution of these fluctuations have also been studied. The observed fluctuations frequency variation is correlated with changes of plasma rotation velocity inferred from plasma stored energy. Possible mechanisms responsible for the quasi-coherent fluctuations will be discussed.

3 Reflectometer in O-mode: Five channels with frequencies at f 0 = 50, 60, 75, 88, and 110 GHz, corresponding to cutoff densities n e =0.31, 0.45, 0.69, 0.96 and m 3. Amplitude modulated: Upper and lower side-bands in each channel: f U,L = f 0 ± f. Group delays τ = dφ/df (φ U φ L )/2 f for density profile calculation. Group delay τ fluctuations in all five channels. Fluctuations of both the upper-side band and lowerside band in the 88 GHz channel. Higher sensitivity to fluctuations than other channels 2. Reflectometry fluctuations: Major contribution from density fluctuations near the cutoff layer. Small contribution from fluctuations far away from to the cutoff layer. 2 Y. Lin, J. Irby, P. Stek, R. Nazikian et al, Upgrade of reflectometry profile and fluctuation measurement in C -Mod, Rev. Sci. Instrum. 70 (1), (1999).

4 Reflectometer in PCI, probes and edge Thomson scattering volume are also shown.

5 Reflectometer Accessibility m 20 Electron Density (10-3 ) GHz Present 88 GHz 5 Channels 75 GHz 60 GHz 50 GHz Major Radius R 1. Not subject to any fluctuations inside the critical layers. 2. Usually more sensitive to fluctuations near the cutoff layers than to those far away from the cutoff layers.

6 Quasi-coherent mode and EDA H-modes In EDA H-modes, a quasi-coherent mode exists in the H-mode pedestal region. Has a large poloidal wave-number k θ 3 6cm 1.k θ =k for reflectometer. Correlates to the D α enhancement. Affects plasma transport. Pinpoint the location of the mode: Reflectometer: Both ICRF heated and Ohmic EDA H-modes. Langmuir probes: Low power Ohmic EDA H-modes only. PCI: Line-integrated fluctuations. Limited mode location information.

7 QC mode is correlated with EDA

8 Mode Location reflectometer observation The outermost point: determined by the lowest reflectometry frequency channel. For instance, if the mode appears in the 88 GHz channel, but not in the 75 GHz channel = the outermost point must be in-between the cutoff layers of 75 and 88 GHz waves, i.e., <n e < m 3. The innermost point: assumed to be no further than the density pedestal top. The mode usually seen in 75, 88 and 110 GHz channels. High density EDA H-modes: Seen by PCI, but not by any reflectometer channel. Low density EDA H-modes: Also seen in the 60 GHz channel.

9 Mode observation in high density shots

10 Mode observation in low density shots

11 Mode location inferred from reflectometer The reflectometry cutoff positions can be determined by comparing with edge Thomson n e profiles. The mode exact location is not correlated with a specific pedestal feature though it is in the pedestal region. The location is possibly correlated with certain q surfaces. For shots with I p = 800 ka and B t0 5.4 T, q 6 7 as mapping density profiles from edge Thomson scattering with q profiles from EFIT. Other plasma parameters, such as P, I p and certain plasma shape, may all be necessities to trigger the mode. This issue is still under study.

12 Mode location is not correlated to specific pedestal feature

13 Mode location is possibly correlated with certain q surfaces

14 Quasi-coherent mode level Reflectometry signal amplitude/phase fluctuations: NOT always proportional density fluctuations. Real geometry and plasma density profile needed to be considered. A 2-D full wave realistic geometry simulation to estimate the QC mode level 3 : Density profile: n e0 (z)/n c =0.8 [ z tanh where lengths are in cm, and ( )] z n c = cm 3. Plasma edge at z =0. The modelled QC mode: [ ( ) ] z ñ e (z)/n e0 = η exp cos(k y + θ) 0.35 where, k =4.5cm 1. The total density is n e = n e0 +ñ e. 3 Y. Lin, J. Irby, R. Nazikian, E.S. Marmar, A. Mazurenko, Two dimensional full wave simulation of reflectometry in, Rev. Sci. Instrum., January, To be published.

15 Radius of plasma curvature from EFIT is also included.

16 Simulation results and experimental data

17 Plasma curvature effect Observed reflectometry signal level for the mode that predicted by slab geometry models. Slab geometry models: ) E rms exp ( k2 w2 σ φ 2 where E rms is the root-mean-square E field of the reflected wave, σ φ is the equivalent 1-D phase modulation by density fluctuations, and w is the 1/e radius of incident beam power. With strong curvature, w ρ/2 k 0 w 2, [ ( ) ] E rms ρ 2 k 2 exp w2 2k 0 w 2 2 σ φ where, ρ =2ρ c ρ w /(ρ c +2ρ w ) is the effective radius of curvature at the cutoff layer. ρ c and ρ w are the curvature radii of plasma and wave-front respectively. 2-D full wave simulation: Supported the plasma curvature effect 4. 4 Y. Lin, R. Nazikian, J. Irby, E.S. Marmar, Plasma curvature effects in reflectometry fluctuation measurements. To be published in Plasma Phys. Control. Fusion.

18 Greater response to high k fluctuations with higher plasma curvature

19 Mode frequency behavior observation The mode frequency, as observed in the laboratory frame, usually starts at 250 khz and decreases to a steady frequency in the range of khz during the EDA H-mode period. The wavenumber of the mode is nearly constant in an EDA period as measured by PCI. f lab = f plasma + k θ v θ /2π The frequency change is correlated with plasma stored energy change, which is correlated to the toroidal rotation velocity. The frequency drop is also correlated with the reduction of pedestal density gradient.

20 Mode frequency behavior in EDA

21 Frequency is correlated with W p /I p

22 Indications of mode frequency behavior Radial force balance equation: E r = P i n i Ze + v φb θ v θ B φ The observed frequency change is probably due to changes in E r and P i, which result in the change of plasma rotation velocity. Direct E r (or rotation velocity) measurement in the pedestal region will be available from diagnostic neutral beam. The correlation between the frequency and W p /I p indicates a correlation in the plasma core rotation and plasma rotation in the pedestal region.

23 Future Work Study the location of the quasi-coherent mode in detail. Determine the favorite parameters triggering the mode. Determine the causes of the observed mode frequency change. Two higher frequency channels (132 GHz and 140 GHz) will be added so that the reflectometer is able to study internal transport barrier and turbulence correlation length.