Helium ELMy H-modes in Alcator C-Mod in Support of ITER Helium Operating Phases

Transcription

1 Helium ELMy H-modes in Alcator C-Mod in Support of ITER Helium Operating Phases C. E. Kessel 1, S. M. Wolfe 2, M. L. Reinke 3, M. A. Chilenski 2, J. W. Hughes 2, Y. Lin 2, S. Wukitch 2 and the C-Mod Team 2 1 Princeton Plasma Physics Laboratory 2 Plasma Science and Fusion Center/MIT 3 Oak Ridge National Laboratory (ORISE) APS-DPP, Savannah, GA, November 16-20, 2015

2 Helium/Hydrogen discharges are the first phases of ITER operations He/H discharges will be used to prepare for DT operations on ITER IOS-2.1 joint experiment: characterize He ITER-like discharges and compare with D q 95 ~ 3-4 Access to H-mode Access to high performance ELMy regimes (Type I) W transport H-mode characteristics: P ped, H 98, ELM behavior, ELM control Radiative divertor operation H-L transition

3 Alcator C-Mod Expts: ELMy H-modes We did not match ITER plasma parameters for these discharges (q 95, shape, b N, n/n Gr, H 98,.) Ip = 0.9 MA, B T = 5.4 T n = x /m 3, n Gr flattop ~ 5.9x10 20 /m 3 P ICRF = MW (1 st harmonic H-minority) ELMs were accessed, at varying density, with limited power variation q95 = = 1.55 d U = 0.2, d L = Comparing to similar Deuterium ELMy H-modes from and Plasma shape used to access ELMy H-modes

4 ELMy He Discharge on C-mod H-mode established with ~ 2.3 MW of ICRF ELMs begin ~ 20 ms later ELMs evident on ICRF waveform (coupling), normally not present for EDA Neutron rate is lower than deuterium discharge when Helium is present Strong He puff and resulting disruption in rampdown was used to keep He fraction higher H-mode H-mode

5 ELMy D discharge on C-Mod,

6 ELMy D discharge on C-Mod,

7 Good ELMy regimes and Infrequent ELM regimes

8 H-mode H-mode L-mode L-mode H-mode

9 Lower densities led to good ELMy regimes, while higher density lead to large infrequent ELMs and H-L- H transitions Good ELMy H-mode regime H-L-H regime L-mode H-mode Dt = 20 ms Dt = 200 ms Note time frames

10 The He discharge at high and low density The best ELMy regimes were accessed for H-mode n line ave < 2.0x10 20 /m 3 Large infrequent ELMs with H-L-H behavior were obtained for H-mode n line ave > 2.6x10 20 /m 3 Pedestal n and T, and collisionality may be a good variable for separating these regimes For n line ave ~ x10 20 /m 3 clusters of ELMy phases occur between no-elm phases appear to be ELM-free for He, need to examine for any fluctuations In deuterium this transition has been described as transitioning to EDA H- modes from ELMy H-modes, with ELMs + EDA between

11 Quantifying the Deuterium and Helium Levels

12 He fraction in the C-Mod discharges, neutron rate is significantly lower for He

13 Fitting the Te(R) and ne(r) to calculate the neutron rate and infer D density Use 20 zones in minor radius Bosch-Hale reactivity formulation Assumed flux geometry based on EFIT n e (R-R o ) = n DD [ (1-f n,edge ) {1-(r/a) 2 ) an } + f n,edge ] f n,edge = n ped /n o T i (R) = T e (R), taken from ECE/TS data Deriving T i from HIREX n DD determined to match neutron rate measurements (fast He3 data) Te(R-R o ) = T o [ (1-f T,edge ) {1-(r/a) 2 ) at } + f T,edge ] f T,edge = T ped /T o

14 Preliminary analysis of neutron data show that He discharges significantly reduced n D line ave / n line ave D D

15 L-H Threshold in He and D Discharges

16 L-H threshold power Net or loss power to enter the H- mode indicates the He discharges are near/or in the low density regime (rising P thr ), limited available power likely made the higher density cases poor ELMy discharges No mass dependence applied P loss * P net 0 P loss P net B A The D discharges indicate the low density regime for L-H transition is at lower densities than reached, and the power to enter the H-modes was lower than He P loss + P net X P loss # P net X The D discharges show that all cases entered the H-mode at the same density, with a wide spread in power to enter the H- mode n line ave, /m 3 x10 20 See J. W. Hughes, poster on D threshold sensitivity

17 Mass dependence in energy confinement scaling IPB98(y,2) and L-H threshold formulations derived for deuterium plasmas, mass dependence inferred from hydrogen experiments..no He experiments used t 98(y,2) = I p 0.93 B 0.15 n L 0.41 R e 0.58 P (M i /2) 0.19 derived for deuterium plasmas, mass dependence inferred from hydrogen having ~ 2x threshold of D, and tritium experiments P L-H,thr = n L 0.72 B 0.84 S 0.96 (2/M i ) There is a Z dependence that affects the ion density at fixed electron density, and mass effects should be separate from this since the source of these mass factors is not from He expts, evaluations have not accounted for mass, using t 98(y,2)D and P L-H,thr D

18 ELM Frequency for He and D Discharges

19 Comparing ELM Frequency for He and D Discharges Deuterium ELMy discharges from and The power variations are small, so density provides the more significant variation As the density increases the ELM frequency decreases, both for D and He has high radiated power, and may be in a high freq ELM regime due to P net /P thr proximity?? Examining the power (loss and net) against the threshold to characterize possible regimes (Type III, Type I, etc.).better to run pedestal stability

20 Plasma Performance for He and D Discharges

21 He discharges tend to underperform the best D discharges in terms of energy confinement and stored energy, in flattop ELMy H-modes n Gr = 5.92 x /m 3 n line ave, /m 3 x10 20

22 Powers relative to the threshold power in sustained ELMy H-mode phases Scatter makes deriving trends difficult, but there does appear to be a somewhat higher P net required to maintain an H-mode in He than D, which persists over n line ave range P thrd = n L 0.72 B T S 0.94 Lines to guide eye Helium Deuterium

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24 The flattop H-modes indicate that higher P net relative to P thrd is required for He There is variability in the D discharges in terms of achieved stored energy, or energy confinement, but similar P net /P thr D He D D

25 The Power terms are used to characterize the discharges in different run days, both He to D and D to D He D

26 Global W Impurity Transport for He and D Discharges

27 Tungsten LBO provided global particle confinement times in He discharges t E ~ ms t E ~ ms t E ~ ms t E ~ ms t W* / t E ~ 4 t W* ~ 129 ms t W* ~ 127 ms t W* ~ 146 ms t W* ~ 123 ms ELMs provided effective W particle removal Infrequent ELM regimes show long holdup of W M. Reinke H/L Trans. Infrequent ELMs ELM clusters H/L Trans. 1.0 t Z [ms] = Normalized W Brightness [AU] ELMy cases Time [sec] Normalized W Brightness [AU] ELMy Time [sec]

28 Pedestal Comparison of He and D Discharges

29 The electron pedestal pressures appear similar between He and the D discharges Both the density and temperature of the pedestal appear to be similar for the He and D discharges Exceptions are the high density He cases which have large infrequent ELMs, and ELM crashes caught by the TS ELM crashes T ped is presently determined only to 25 ev increments Based on TS data for electrons Infrequent large ELMs

30 Electron pedestal collisionalities ranged from ~ for the majority of He and D discharges The He and and higher performing D discharges reach similar electron pressures in similar collisionality regimes T ped is presently determined only to 25 ev increments Based on TS data for electrons - ped

31 Examining MHD modes in H-mode phases for He to compare to D No mode in early H-mode (ELM-free?) High frequency mode appearing between long no-elm phase High frequency mode between ELMs that occur every ~ 5 ms Closer examination shows a dropping frequency just before an ELM (S. G. Baek and A. Diallo)

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33 High frequency mode over long ELM-less period, is this the QCM? L-mode

34 Observations and Future Work Overall, the He discharges behaved similarly to D discharges - Similar ELMy regimes were accessed - Similar T ped, n ped, ELM freq, etc. - Loss of ELMy regime with rising density, progression toward infrequent large ELMs is similar - There may be differences in the transition away from ELMy at increasing density.need to verify There is variability for the ELMy D discharges evidenced by and The underlying differences between these are still unclear, although ELM frequency is much different

35 Will add more discharges to the comparison between He and D ELMy discharges - Add (other He ELMy data) - Add companion D - Add D, D, D (others at 0.9 MA and T) Tungsten global particle confinement times from W LBO identify t W* / t E ~ 4, and that sustained ELMy phases avoid W holdup associated with infrequent ELM regime - W holdup was seen for the infrequent ELMs Examine the presence of or lack of modes (eg. QCM) between ELMs in the He and D discharges, both frequent ELMy and infrequent ELMs