Alcator C-Mod Research Highlights

Compact high-performance divertor tokamak research to establish the plasma

physics and engineering necessary for attractive

fusion reactors.fusion reactors.

55th Annual Meeting, APS-DPP, Denver, CO, November 11, 2013, CO4-01E. S. Marmar for the Alcator Group

Runaway Suppression May Not Require the Very High Density Implied by Collisional Theory

• Data from ITPA joint experiments and mining of previously existing data consistently show that the electric field required (Ecrit) to generate significant relativistic runaway e− populations is 5 to 10 times higher than expected from Connor, Hastie*

• Could be good news for runaway mitigation during disruptions in future facilities

V/m

)

Threshold for Runaway Production on Multiple Facilities

ric F

ield

(Vda

l Ele

ctr

Toro

i

0 0 2 0 4 0 6 0 8

• R. Granetz, BI2-03, Mon. AM*J.W. Connor, R.J.Hastie, Nucl. Fus. 15(1975)415.

Line-Average Density (1020m-3)0 0.2 0.4 0.6 0.8

Core transport, Gyrokinetic modeling

• Previous gyrokinetic modeling has shown sensitivity of ion transport to main ion dilution by impurities

• Series of nitrogen seeding experiments

• Under-prediction of electron heat flux increases with IP

• Series of nitrogen seeding experiments shows χi decrease, little/no change in χe

/s)

/s)

Unseeded

χ e(m

2 /

χ i(m

2 /Nitrogen Seeded

r/a r/a0.5 0.6 0.7 0.8 0.5 0.6 0.7 0.8

Nitrogen SeededUnseeded

• P. Ennever, CO4-07, this session• N. Howard, CO4-06, this session

Density (1020m-3)r/a r/a

• Correlates with absence of long-λ sub-dominant electron modes

Gyrokinetic Simulations of TEM Turbulence: Control of ITB Density Profile by ICRH

● Absolute fluctuation levels match experiment within

ncertainties

Gyrokinetic Simulations of TEM Turbulence in ITB match measured fluctuation level and transport within uncertainties

uncertainties

● Nonlinear upshift heightens TEM Sensitivity to Tey e

● Provides mechanism for density and impurity control

ith ICRHwith ICRH

Nonlinear upshift of TEM critical density gradient: strong sensitivity to Te

• D. Ernst, TP8-50, C. Fiore, TP8-35, Thurs. am

Additional gyrokinetic studies:C. Sung, CO4-08, this session, K. Liao, TP8-39

Current Profile Control with Lower Hybrid Waves Leads to Dramatic Changes in Core Plasma Rotation

• Unifies results from 4 facilities

s)

• Change in j(r) modifies core plasma rotation

cy (k

Rad

/s

ty (k

m/s

)

n fre

quen

c

ion

velo

cit

in ro

tatio

n

ge in

rota

ti

Cha

nge

i

Cha

ng

• J. Rice, CO4-09, this session

Magnetic Shear Scale Length, LS (m)

Lower Hybrid RF Edge Interactions Lead to Significant Confinement Increase of High Density H-modes

• ~25% increase in normalized confinement, with <15% increment in power

• LH waves absorbed in edge/SOL

• With LH, dramatic reduction in ped. d SOL d i fl i (GPI) power

• ~15% increase in absolute τEand SOL density fluctuations (GPI)

mal

ized

τE

ge in

Nor

mC

hang

• J. Terry, CO4-13, this session

I-mode Pedestal Peeling-Ballooning Stable;Pedestal βp scales with Density

• Poloidal beta of pedestal scales linearly with density

• Leads to linear increase of plasma pressure with input powernt

• ELITE modeling shows that I-mode pedestal is stable to peeling-ballooning

pressure with input power

esta

l Cur

ren

(kJ)

I modeNor

m. P

ede

d E

nerg

y (

I-modeN

Norm. Pressure Gradient (α) Sto

red

Power x IP ( MW-MA)• Favorable compared to H-mode, which

has ~constant pedestal pressure ⇒

• J. Walk, UI2-03, Thurs. PM

degradation of τE with power

Fluctuations and Energy Transport Reduced in I-mode Core

I-modeL-mode

● L-mode■ I-mode

Ĩ e/T e

(%)

T

Noise level

• Te fluctuations (CECE) reduced by at least 40% in core

• Energy transport reduced by similar amount across the core confinement region

ρ

Whit CO4 03 thi i

region

• White, CO4-03, this session

Additional I-mode studies: A. Hubbard, TP8-34, Thurs. am

Unique Mirror Langmuir Probe Reveals Unprecedented Details of Pedestal/Edge Modes

• Direct measurements of lag between density and potential fluctuations

• MLP electronics adjusts I-V in real-time to match that of an actual probe

• Yields Isat, Te, Vf, from single probe with μs real-time responseμs real time response

• EDA H-mode QCM very narrow (~3 mm) at LCFS

QCM i l t d ift ith• QCM is an electron drift-wave with significant interchange drive and electromagnetic contributions

• B. LaBombard, YI2-06, Friday, noon

Additional H-mode pedestal studies:T. Golfinopoulos, YI2-05, Fri. am, A. Diallo , CO4-04, this session; E. Davis, PP8-17, Wed. pm

Accelerator-based In-situ Materials Studies (AIMS):Between-shot measurements of surface changes

Isot

ope

hang

e in

I

1 M V D+ l t actio

nal C

h

• 1 MeV D+ accelerator

– Use toroidal and vertical B fields to steer beam

R lti l ti d tFr

aMeasurement Sequence

– Resulting nuclear reactions used to probe changes in surface conditions • In‐situ monitoring of evolution in 

deuterium and boron surface concentrations at multiple locations during wall conditioning experiments

• Zach Hartwig, NI3-03, Wed. AM

wall conditioning experiments

With Continued Operation, C-Mod Ready to Provide Critical Results for ITER, FNSF and DEMO

Study/Exploit UniqueField-Aligned ICRF Antenna

Develop Advanced Off-Midplane LH Launcher

Deploy Actively Heated (900 0K) Tungsten DivertorExploit Unique

Capabilities for In-situ Measurementssitu Measurements

of Plasma Wall Interactions

Statistic:11 of 99 invited talks at this APS-DPP are by current or former MIT PSFC graduate students

Extreme Materials Challenge: Remove Heat and Particles with Close to Zero Erosion

• X‐Point Divertor: put the radiation where it belongs

• Concept of facility to test revolutionary approach: Alcator DX

• LaBombard, CO4-02, this session

Alcator C-Mod Presentations at 2013 APS-DPP Meeting

• Review– Martin Greenwald: Twenty Years of Research on the Alcator C-Mod Tokamak,

Monday, 8:00 am• Invited• Invited

– Robert Granetz: ITPA Joint Experiment to Measure Threshold E-fields and Densities for Runaway Electron Onset and Suppression, Monday, 10:30 am

– Zach Hartwig: The in-situ diagnosis of plasma-wall interactions on magnetic fusion devices with accelerators Wednesday 10:30 amfusion devices with accelerators, Wednesday, 10:30 am

– Jungpyo Lee: Ion intrinsic rotation in a tokamak caused by momentum transport due to diamagnetic flow effects, Wednesday, 4:00 pm

– John Walk: ELM Suppression and Pedestal Structure in I-Mode Plasmas, Thursday 3:00 pmThursday, 3:00 pm

– Ted Golfinopoulos: External Excitation of a Drift-Alfvén Wave Response in the Alcator C-Mod Edge Plasma and its Relationship to the Quasi-Coherent Mode, Friday, 11:30 am

– Brian LaBombard: New insights on boundary plasma turbulence and the Quasi-Brian LaBombard: New insights on boundary plasma turbulence and the QuasiCoherent Mode in Alcator C-Mod using a Mirror Langmuir Probe, Friday, 12 noon

• Oral Contributed (Monday, PM)• Poster Contributed (Thursday, AM)• ITER Oral Contributed (Wednesday, PM)ITER Oral Contributed (Wednesday, PM)

Research supported by USDOE

The mind is not a vessel to be filled, but a FIRE to be kindled. — Plutarch