Comparisons of Edge/SOL Turbulence in L- and H-mode Plasmas of Alcator C-Mod

Transcription

1 Comparisons of Edge/SOL Turbulence in L- and H-mode Plasmas of Alcator C-Mod J.L. Terry a, S.J. Zweben b, O. Grulke c, B. LaBombard a, M.J. Greenwald a, T. Munsat b, B. Veto a a Plasma Science and Fusion Center - MIT b Princeton Plasma Physics Lab. c MPI for Plasma Physics, EURATOM Association and Ernst-Moritz-Arndt University PSI (1)

2 Motivation, "Blobs", and Purpose - edge/sol turbulence with many similar characteristics (nondiffusive, intermittency, etc.) has been seen on many tokamaks. - the intermittent turbulence has a distribution of "events" (I sat, V fl, emissivity) skewed to larger amplitude events - named "blobs". - this "blobby" turbulent transport is a significant fraction of G R. - here we report on the edge/sol turbulence in H and L-mode using "gas-puff-imaging" and examining: "blob" frequency, "blob" magnitude, "blob" spatial size, and R,Z velocity fields - since E D a ~n e a T e b, we assume that the emission characteristics represent the particle densities - previous L/H-mode comparisons using probe measurements have been done on C-Mod by LaBombard, et al., and on DIII-D by Boedo, et al., and Rudakov, et al. PSI (2)

3 300 frame/250 khz movies of Edge/SOL Turbulence - using D a emission from "Gas-Puff-Imaging" (GPI) ELM-free H-mode L-mode note: E Da ~ n o n e a T e b with 0.5<a<0.8, 0.3<b<1.4 PSI (3)

4 Turbulence structures are extended along & aligned with the field. When viewed to field, these filament structures look like blobs! Side view - 4ms frame showing "filaments" in Da recycling light vertical Schematic of outboard GPI filament B gas cloud camera view toroidal 10 cm (<- down) Vertical (up ->) frame 52 2 cm nozzles (4) (<- in) Radial (out ->) A special movie camera and discrete radial array view D a emission fluctuations in a plane perp. to B There is toroidal localization since views are crossed with a gas puff "beam" Princeton Scientific Inst. Ultra-fast Camera takes 300 consecutive frames at 250 khz (1.2 ms sample) outer wall a 13-view outboard radial array & a 6-view inboard radial array sample at 1MHz for 131 ms PSI (5)

5 Turbulence is ballooning-like - not "induced" by gas puff - blobs are still present on outboard side with no gas puff - there are no blobs at inboard side ballooning-like drive for blobs Da GPI emission (arb.) Outboard r=8 mm Inboard r=7 mm time (s) Comparing inboard/outboard emission statistics normalized RMS fluctuation level of D a emission outboard side: between 0.15 and 0.55 increasing with r inboard side: between 0.05 and 0.10 constant with r skewness of D a emission (>0 means more larger amplitude events) outboard side: between 0.3 and 3 increasing with r inboard side: ~0, i.e gaussian, constant with r & PSI (5)

6 intensity freq. of events with amp. > 1.2xmean (khz) Frequency of "blobs" is similar in L and H-mode Magnitude of "blobs" decreases in ELMfree H-mode ELMfree H-Mode r=0.6 cm H-to-L transition L-Mode time (s) Frequency 1.2 x mean L-mode ELMfree H-mode r (cm) Magnitude relative to L-mode Magnitude=Integral of shaded portions above L-mode EDA H-mode R major (cm) PSI (7)

7 generate time-averaged image Analysis of Movie Images - for "blob" spatial size-scale - for velocity fields - for time-averaged emission subtract it from each individual frame, removing the "DC" component (<- down) Vertical Direction (up ->) time-averaged image 2 cm (<- in) Radial Direction (out ->) frame 52 with time-averaged image subtracted (<- in) Radial Direction (out ->) PSI (7)

8 Turbulence Spatial Size-Scale Comparison - poloidal auto-correlation length Poloidal correlation length (cm) L pol L-mode EDA ELMfree L pol ~ ~L pol ~ >L pol r (cm) L-mode EDA ELMfree PSI (8)

9 Generating Time-delay X-correlation Velocity Fields from Movies - cross-correlate time-history for each pixel with those of all other pixels - do this for time "lags" on other pixels from -16 to +12 ms - find maximum in X-correlation in space for each lag time - movement of maximum in space & time yields local velocity lag:-16 ms lag:-12 ms lag:-8 ms lag:-4 ms all X-corr. maxima are >0.55 +/-1.25 cm lag: 0 ms lag: 4 ms lag: 8 ms lag: 12 ms [<V R >,<V Z >] for this point= [107,-393] m/s +/-1.25 cm PSI (9)

10 Velocity Fields are Qualitatively Similar - fluctuation motion is poloidal inside - fluctuation motion is poloidal and radial in SOL - velocity magnitudes V R,V Z up to 1000 m/s, with no obvious dependence on confinement mode - radial acceleration in SOL -1.5 ELMfree =[V R =1000,V Z =0] m/s -1.5 L-mode Z (cm) shot t= s R R major (cm) major (cm) =[V R =0,V Z =-1000] m/s shot t= s PSI (10)

11 Shot-to-shot Variation in Velocity Fields -1.5 EDA =[V R =1000,V Z =0] m/s -1.5 L-mode Z (cm) R major (cm) shot t= s =[V R =0,V Z =-1000] m/s R major (cm) shot t= s PSI (11)

12 Summary - using "gas-puff-imaging" we have investigated the edge/sol turbulence in ELMfree, EDA H-mode, and L-mode plasmas with the goal of measuring any differences among these confinement modes. - the absence in the inboard SOL of the intermittent, "blobby" turbulence that is found on the outboard side is strong evidence that the observed turbulence is ballooning-like. - the 300 frame/250 khz movies of the gas puff emission provide a unique tool for studying the edge turbulence, with a demonstrated capability for measuring velocity fields with unprecedented spatial resolution and help to visualize the turbulence there. PSI (12)

13 Summary regarding effects of L- and H-mode upon edge/sol turbulence - the nature of the intermittant ("blob") turbulence is qualitatively similar in L- and H-mode. - the frequency of intermediate amplitude "blob" events is similar in L- and H-mode. - however, the magnitude (amplitude) of the "blobs" in the far-sol is smaller in ELMfree H-mode compared to EDA and L-mode. - the spatial size-scale of the turbulent structures is somewhat smaller in ELMfree H-mode compared to L-mode. - the time-averaged fluctuation velocity fields show almost purely poloidal motion inside the, with a mixture of radially outward and poloidal motion outside the. - the magnitudes of the poloidal and radial velocities of the turbulent structures range from 0 to ~1000 m/s in the SOL. PSI (13)