C. E. Kessel1, S. M. Wolfe2, M. L. Reinke3, M. A. Chilenski2, J. W. Hughes2, Y. Lin2, S. Wukitch2 and the C-Mod Team2

1Princeton Plasma Physics Laboratory 2Plasma Science and Fusion Center/MIT 3Oak Ridge National Laboratory (ORISE)

APS-DPP, Savannah, GA, November 16-20, 2015

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

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

q95 ~ 3-4

Access to H-modeAccess to high performance ELMy regimes (Type I)W transport H-mode characteristics: Pped, H98, ELM behavior, ELM controlRadiative divertor operationH-L transition

Alcator C-Mod Expts: ELMy H-modes

We did not match ITER plasma parameters for these discharges (q95, shape, bN, n/nGr, H98,….)

Ip = 0.9 MA, BT = 5.4 T

n = 1.6-3.0 x 1020 /m3, nGrflattop ~ 5.9x1020 /m3

PICRF = 1.75 – 4.0 MW (1st harmonic H-minority)

ELMs were accessed, at varying density, with limited power variation

q95 = 3.65-3.9

= 1.55dU = 0.2, dL = 0.7-0.72

Comparing to similar Deuterium ELMy H-modes from 1090914 and 1120815 Plasma shape used to access ELMy

H-modes

ELMy He Discharge on C-mod 1150716

H-mode H-modeH-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

ELMy D discharge on C-Mod, 1090914

ELMy D discharge on C-Mod, 1120815

Good ELMy regimes and Infrequent ELM regimes

H-m

od

e

H-m

od

e

H-m

od

e

L-mo

de

L-mo

de

Lower densities led to good ELMy regimes, while higher density lead to large infrequent ELMs and H-L-

H transitions

Note time frames

Dt = 20 ms Dt = 200 ms

Good ELMy H-mode regime H-L-H… regime

H-mode

L-mode

The He discharge at high and low density

The best ELMy regimes were accessed for H-mode nline ave < 2.0x1020 /m3

Large infrequent ELMs with H-L-H behavior were obtained for H-mode nline ave > 2.6x1020 /m3

Pedestal n and T, and collisionality may be a good variable for separating these regimes

For nline ave ~ 2.0-2.5x1020 /m3 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

Quantifying the Deuterium and Helium Levels

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

Fitting the Te(R) and ne(R) to calculate the neutron rate and infer D density

Te(R-Ro) = To [ (1-fT,edge) {1-(r/a)2)aT} + fT,edge]ne(R-Ro) = nDD[ (1-fn,edge) {1-(r/a)2)an} + fn,edge]

Use 20 zones in minor radiusBosch-Hale reactivity formulationAssumed flux geometry based on EFIT

Ti(R) = Te(R) , taken from ECE/TS data Deriving Ti from HIREXnDD determined to match neutron rate measurements (fast He3 data)

fn,edge = nped/no fT,edge = Tped/To

Preliminary analysis of neutron data show that He discharges significantly reduced nD

line ave / nline ave

D

D

L-H Threshold in He and D Discharges

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 Pthr), limited available power likely made the higher density cases poor ELMydischarges

The 1090914 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

The 1120815 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

No mass dependence applied

Ploss *Pnet 0

Ploss +Pnet X

Ploss BPnet A

Ploss #Pnet X

nline ave, /m3 x1020

See J. W. Hughes, poster on D threshold sensitivity

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

t98(y,2) =0.0562 Ip0.93 B0.15 nL

0.41 R1.970.78 e0.58 P-0.69 (Mi/2)0.19

derived for deuterium plasmas, mass dependence inferred from hydrogen having ~ 2x threshold of D, and tritium experiments

PL-H,thr = 0.0488 nL0.72 B0.84 S0.96 (2/Mi)

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 t98(y,2)

D and PL-H,thrD

ELM Frequency for He and D Discharges

Comparing ELM Frequency for He and D Discharges

Deuterium ELMy discharges from 1120815 and 1090914

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

1120815 has high radiated power, and may be in a high freq ELM regime due to Pnet/Pthr proximity??

Examining the power (loss and net) against the threshold to characterize possible regimes (Type III, Type I, etc.)….better to run pedestal stability

Plasma Performance for He and D Discharges

He discharges tend to underperform the best D discharges in terms of energy confinement and

stored energy, in flattop ELMy H-modes

nGr = 5.92 x 1020 /m3nline ave, /m3 x1020

Powers relative to the threshold power in sustained ELMy H-mode phases

DeuteriumHelium

PthrD = 0.0488 nL

0.72 BT0.803 S0.94

Scatter makes deriving trends difficult, but there does appear to be a somewhat higher Pnet required to maintain an H-mode in He than D, which persists over nline ave range

Lines to guide eye

The flattop H-modes indicate that higher Pnet

relative to PthrD is required for He

There is variability in the D discharges in terms of achieved stored energy, or energy confinement, but similar Pnet/Pthr

D

1150716 He

1120815 D

1090914 D

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

D to D

He D

Global W Impurity Transport for He and D Discharges

Tungsten LBO provided global particle confinement times in He discharges

1.05 1.10 1.15 1.20 1.25Time [sec]

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d W

Brig

htn

ess [A

U]

129

1150716

025

127

11507

16026

146

11

507160

27

123

115071

6030

tZ [ms] =

1.05 1.10 1.15 1.20 1.25Time [sec]

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d W

Brig

htn

ess [A

U]

1150716

025

H/L Trans.

11507

16028

H/L Trans.

11

507160

29

ELMs provided effective W particle removal

Infrequent ELM regimes show long holdup of W

M. Reinke

tE ~ 33-31 mstE ~ 36-29 mstE ~ 31-43 mstE ~ 27-34 ms

tW* ~ 129 ms

tW* ~ 127 ms

tW* ~ 146 ms

tW* ~ 123 ms

ELMy cases

ELMy

Infrequent ELMs

ELM clusters

tW* / tE ~ 4

Pedestal Comparison of He and D Discharges

The electron pedestal pressures appear similar between He and the D discharges

Based on TS data for electrons

Infrequent large ELMs

ELM crashes

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

Tped is presently determined only to 25 eVincrements

Electron pedestal collisionalities ranged from ~ 0.2-1.0 for the majority of He and D discharges

- ped

The He and and higher performing D discharges reach similar electron pressures in similar collisionality regimes

Based on TS data for electrons

Tped is presently determined only to 25 eVincrements

Examining MHD modes in H-mode phases for He to compare to D

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)

No mode in early H-mode (ELM-free?)High frequency mode appearing between long no-ELM phase

1150716033

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

L-mode

Observations and Future Work

Overall, the He discharges behaved similarly to D discharges

- Similar ELMy regimes were accessed- Similar Tped, nped, 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 1120815 and 1090914

- The underlying differences between these are still unclear, although ELM frequency is much different

Will add more discharges to the comparison between He and D ELMy discharges

- Add 1101027 (other He ELMy data)- Add companion 1101026 D- Add 1101214 D, 1110325 D, 1101117 D (others at 0.9 MA

and 5.3-5.4 T)

Tungsten global particle confinement times from W LBO identify tW

* / tE ~ 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