Recent Progress in ICRF Research on Alcator C-Mod

Yijun Lin (林毅君 ), Steve Wukitch and the Alcator C-Mod Team

MIT Plasma Science and Fusion Center

Presented at the 6th US-China Magnetic Fusion Collaboration Workshop, University of California, San Diego, July 10-12, 2012

The Alcator C-Mod Tokamak

• R = 0.68 m• a = 0.22 m• Bt < 8.1 T• Ip < 2 MA• ne < 5×1020 m-3

• Te < 9 keV• ICRF: 8 MW source• LHCD: 4 MW source

Compact high-performance divertor tokamak research to establish the plasma physics and engineering necessary

for attractive fusion reactors.

Alcator C-Mod has a World-Leading Program Critical to the Success of ITER and the Development of Fusion Energy

• Pioneered the vertical-plate divertor and solid high-Z plasma facing components– Both adopted for ITER: currently only viable solution for high power

density, low retention– Now taking this to the next level with high temperature tungsten: best hope

for a fusion reactor (Bruce Lipschultz’s talk this afternoon)• RF heating, current drive, and plasma control

– Unique LHCD studies at fields, densities, frequency planned for ITER. Only proposed solution for steady-state tokamak reactor

– Making rapid progress on the challenge of ICRF-induced impurities: • Discovered and developing potential solution to the ELM

problem: I-mode• Alcator C-Mod is a major educator of the next generation of

plasma physicists and fusion scientists in the US.

ICRF related research on C-Mod

• New field-aligned 4-strap antenna and performance evaluation

• ICRF mode conversion flow drive• Neoclassical tearing modes (NTMs) in high ICRF

heated I-mode plasmas• RF and edge interaction: density profiles and sheath

potential• Impurity seeding enhanced antenna performance• Fast particle lost-ion detection and fast ion transport• ICRF and LH interaction• ICRF simulation

Standard ICRF antennas cannot eliminate E field along magnetic field lines (E||)

Antenna straps: vertical, conform to plasma shapeFaraday screen rods: field-aligned (or horizontal)

Antenna structure is symmetric horizontally along the torus, but it is not symmetric along the field lines E|| from different straps cannot be fully canceled RF sheath and impurities.

Two 2-strap antennas Old 4-strap antenna (replaced)

E|| in front of ICRF antennas generates RF sheath which leads to impurities

• Impurities has been a long-standing problem in ICRF application in tokamaks.

• In a good absorption regime, E|| in front of an antenna RF sheath acceleration of ions local sputtering impurities transport core impurities

• And to reduce ICRF related impurities, we need to find a way to minimize E||

• Innovative approach: Field-aligned antenna

After boronization, molybdenum radiation increases shot by shot due to the boron layer erosion by ICRF power.

Field-aligned 4-strap Antenna

Field-aligned antenna to reduce E||

• Straps, Faraday screens and the entire structure are rotated by 10o and made to match the plasma shape (5.4 T and 1 MA);

• Straps are shaped to maintain equal RF flux;

• Antenna model calculation shows E|| field is < 20% of a standard antenna at the same power.

Assembly on a bench Assembly in vessel

Antenna strap

The new field-aligned antenna generates less impurities in the core than standard antennas

• In L-mode, impurity contamination appears to be lower as seen in the radiated power and Molybdenum.

• Roughly, radiated power is ~0.4-0.6 MW lower per 2.5 MW injected.

• FA-antenna voltage handling is ~40 kV and power up to 2.5 MW.

The new field-aligned antenna generates less impurities in the core than standard antennas

• Reduced core Mo levels is also observed in H-mode.

• Total radiated power is also lower at the same power level.

Mo source on the antenna is also reduced

• Two spectroscopic view: one viewing the stand antennas (D/E view, black curves), and the other one viewing the FA-J antenna (red curves);

• Result:• When FA-J antenna is on, the

Mo source on the FA antenna is much smaller than the Mo source on D/E antennas when D/E antennas are powered.

• When FA-J antenna is on, the Mo source on D/E antenna is much smaller than the Mo source on the FA-J antenna when D/E antennas are powered.

Field-aligned 4-strap antenna has been a success

• Field aligned ICRF antenna does reduce impurity contamination– In L-mode, the radiated power fraction is lower 30%. In

H-mode, the radiated power fraction is reduced by 20-30%.

– Core Mo concentration is significantly reduced.– Mo source at the FA antenna is significantly reduced

compared to when the standard antennas are operated.More detailed results will be presented by Steve Wukitch at the coming IAEA Fusion Energy Conference and the APS-DPP annual meeting.

ICRF mode conversion heating can drive significant plasma rotation

Mode Conversion:• ΔV ~ 90 km/s at 3

MW 50 MHz rf • Co-current direction.

Minority Heating:• ΔV~ 35 km/s at 3

MW 80 MHz rf • Co-current direction.• Intrinsic plasma

rotation

Developing RF flow drive as a unique external control tool for reactor-level plasmas

ICRF mode conversion drives strong toroidal flows

– Scaling with current and RF power

– Scales inversely with density & frequency

– Hypothesis: power going to the He3 minority ions asymmetrically

In H-mode plasmas, the effect is weak due to high density. We have studied flow drive in I-mode plasmas.

I-mode plasmas: good thermal confinement but no density pedestal

• I-mode is an improved confinement mode with H-mode-like thermal confinement, but L-mode-like particle confinement. Typical I-mode has confinement factor HITER98,y2 ~ 1.0.

• Typical I-mode plasmas have a much lower density than H-mode, thus are potentially favorable for MC flow drive (based on the density dependence observed in L-mode).

Intrinsic rotation in I-mode is as large as in H-mode

• Te, but not p, near the edge may be the key drive for the intrinsic rotation. [J. Rice et al, PRL (2011)]

• If ICRF MCFD can be superimposed on the intrinsic rotation, we could produce plasmas with very large total rotation.

MCFD in I-mode seems to help plasma performance, but incremental rotation is small

L-mode IL-mode I

50 MHz MC 50 MHz + 80 MHz

Reversed field and lower-null plasma, i.e., BB unfavorable to H-mode transition. Bt0 ~ 5.1 T, Ip = 1 MA, [3He] ~ 10%.

50 MHz ICRF power via MCFD to drive significant rotation, ΔV0 > 100 km/s; And then adding 80 MHz via D(H) minority heating to trigger I-mode.

I-mode confinement HITER98,y2 ~ 1.4 and Te0 > 7 keV.

Similarly good confinement plasmas also obtained by using 80 MHz MH to trigger I-mode, and then adding 50 MHz for MCFD.

But the rotation in I-mode is not much higher than that in L-mode.What is limiting the rotation? Answer: Neo-classical tearing modes (NTMs)

NTMs appear in high , low collisionality plasmas and often triggered by large sawtooth crashes

“Seed” island necessary for growth

Saturation width proportional toβθ - hence limits plasma

NTM “Phase diagram”• A seed island created by large sawtooth crash;

• Plasma pressure profile is flattened within the island – bootstrap current is turned off;

• The corresponding induced δB has the same direction as the initial perturbation and enhances it.

dW/dt

W

NTM effects on plasma• Limiting plasma performance• Braking plasma rotation

NTMs structure

NTM =(i/)/e*

Characterizing the NTMs

• The NTM onset condition scales vs. the collisionality and βN, similar to ASDEX-upgrade and DIII-D.

• Saturated width W ~ 1 cm ~ 4×(ion banana width), and this causes about 5% loss of stored energy.

• Effect on rotation braking is W4.

To avoid or control NTMs on Alcator C-Mod• Limit sawtooth crash magnitude• Use LHCD to modify current profile

Summary

• New field-aligned ICRF antenna on Alcator C-Mod has been a success.

• More experiments on mode conversion flow drive in I-mode plasmas lead to observation of NTMs on Alcator C-Mod.

Collaboration work with ASIPP on ICRF in 2010-2012:– Participated EAST campaigns in 2010 and 2012;– Characterized the present EAST antennas:

• Studied ICRF heating in many different scenarios• Obtained ICRF-only (~1.7 MW) H-mode.

Plan for future collaboration on ICRF: – A field-aligned long-pulse ICRF antenna on EAST;– Flow drive experiments on EAST to control rotation profile.