Lower Hybrid RF: Results, Goals and Plans

J.R. Wilson Alcator C-Mod Program Advisory Meeting

January 27, 2010

ITER Needs and the RENEW Report Provide a Context for LH Research on C-Mod •  ITER Needs: 

–  Hea-ng and Current Drive Strategy •  Baseline  (LH no) •  Upgrade (possibly) 

–  Integrated Scenarios •  Hybrid scenario (Can C‐Mod get something like this?) •  AT (Demonstrate that LHCD is necessary) 

•  ITER Physics Issues –  Elm Control –  H‐mode Pedestal –  Rota-on –  C‐Mod experiments have seen intriguing phenomena and will con-nue exploring this physics 

LH Research Contributes to Progress on Several RENEW Thrusts

•  Thrust 4 – Qualify opera-onal scenarios and the suppor-ng physics basis for ITER 

•  Thrust 5 – Expand the limits for controlling and sustaining fusion plasmas –  Ac-ve control 

•  Current profile control in steady state –  Stabiliza-on 

•  Can LHCD affect instabili-es in a controlable fashion 

•  Thrust 6 – Develop predic-ve models for fusion plasmas, supported by theory and challenged with experimental measurement 

Physics Issues being studied

•  Density Limit – LHCD efficiency falls as density is increased (~1/ne) un-l the density limit is reached –  Both C‐Mod and FT‐U observe a lower density limit than expected from theory and earlier experiments 

–  Joint ITPA Experiment ‐IOS‐5.3: Assessment of lower hybrid current drive at high density for extrapola@on to ITER advanced scenarios  

•  Rota-on – C‐Mod observes Counter Ip plasma rota-on during LHCD 

•  Pedestal Modifica-on – Modifica-on of H‐mode pedestal observed during LHCD –  Possible effect on ELM’s??  

Hard X-rays disappear at high density

  HXR emission drops much faster with density than 1/ne theoretical prediction   Precipitous drop in emission above 1.0x1020 m-3   Higher toroidal field and plasma current help at high density   Changing antenna phasing (n||) has little effect

1/ne For AT operation need to operate in H-mode regime

Accessibility limits do not explain fall-off

Parametric decay does not explain fall-off

Computational modeling does not predict drop in HXR at high density

  GENRAY*: Follows ray trajectories in plasma

  CQL3D*: Calculates damping and current drive

  Predicts HXR emission similar to 1/ne

CQL3D

*codes courtesy of CompX

Large currents measured in the Scrape-Off-Layer during LHCD at high density

  Parallel electric currents in the SOL flowing between inner and outer divertors

  Currents increase during LH at high ne, at similar density to loss of HXR, suggesting SOL absorption

  Current driven in SOL is larger for higher n||

  Direction of SOL current does not change with n||

Bt Ip

Jsol

LSN

What can be causing fall-off in emission at high density ??

Computational models did not allow for propagation and absorption outside of the last closed flux surface

A Scrape-off Layer (SOL) model has been added to the GENRAY code that allows for propagation and collisional absorption in the SOL

Collisional damping calculated by replacing me with me(1 +iν/ω) in the wave equations where ν is the electron-ion collision frequency (νie ~ Te-3/2 - becomes strong in cold SOL plasma)

For L-mode plasmas agreement between experiment and modeling improved

Addition of collisional damping in the SOL improves agreement with experiment

•  Collisional damping in SOL calculated by GENRAY 

•  SOL parameters –  Tmin = 5 eV –  nmin = 1e11 –  λT = 5 mm –  λn = ~2 cm 

•  Collisional damping increases at high density as rays propagate farther in the SOL 

2D SOL w/ collisions

If model is correct can we find conditions where SOL absorption is less??

H-mode plasmas have significantly different SOL plasmas but high edge density

Newly discovered I-mode has hot core plasma without edge density barrier

Do I-mode plasmas look better as LHCD targets?? (High core temperature should increase single pass damping)

Can this hypothesis of collisional damping be tested be comparing LHCD for different plasmas??

Increasing minimum edge temperature increases x-ray emission, decreases SOL absorption and increases CD

Temin = 50 eV ILH = 157 kA PSOL/Pcore = 0.07

Temin = 20 eV

Temin = 5 eV ILH = 17 kA PSOL/Pcore = 0.7

Temin = 10 eV ILH = 86 kA PSOL/Pcore = 0.23

~1/ne

Application of LHCD causes changes to core, pedestal and SOL in H-mode above ne limit •  Density decreases in core 

and top of pedestal •  Density increases at 

pedestal foot and in SOL •  Temperature increases in 

core plasma 

•  Increase in HXR emission •  Decrease in Vloop, mostly 

due to changes in ne, Te 

! !"#$ !"! !"!$ !"% !"%$ !"& !"&$ !"' !"'$ !"$#

!###

%###

()*

+

+

! !"#$ !"! !"!$ !"% !"%$ !"& !"&$ !"' !"'$ !"$!

%

&,+!#

%#

(-!"*

+

+

! !"#$ !"! !"!$ !"% !"%$ !"& !"&$ !"' !"'$ !"$!

%

&

(./0*

+

+

! !"#$ !"! !"!$ !"% !"%$ !"& !"&$ !"' !"'$ !"$#

#"$

!

12-/+(3*

(4"5"*

+

+

678

69:;<

=/#

=/>4?@

1AB-3B=+1/#

C:C+1/#

%!+DBEB-/F/G

#$%&'()*+,&-!./

In the Presence of LHCD a counter Ip plasma rotation is observed

•  Rota-on appears inside loca-on of CD •  Magnitude of rota-on velocity change is propor-onal to apparent current driven 

•  In H‐mode with edge CD rota-on change is seen to propagate inward 

•  [1.]   A. Ince‐Cushman, et.al., Phys. Rev. Leh., 102, 035002 (2009) 

•  [2.]  J. E. Rice, et. al., Nucl. Fusion 49, 025004 (2009) 

LH driven rotation localized to Core

R(cm)

Rate of injected wave momentum sufficient to explain plasma momentum buildup

Mechanism for rotation remains to be determined

•  Inward pinch of trapped electrons? •  Deviation of energetic passing particles from

flux surface? •  Other?

New Diagnostics Being added to aid understanding of LH Physics

•  PCI •  Reflectometers for density measurements at

launcher location •  Reflectrometry to directly detect 4.6 GHz

component of density fluctuation

Direct detection of the LH wave by a reflectometor

Promising result was already obtained on the prototype instrument (2008 APS Dominguez)

  Mo-va-on -  To study the LH wave propaga-on close to LCFS (spectrum broadening, 

PDI, “density limit” etc ) 

-  Complementary to PCI (measurement is close to LCFS) 

-  To understand the reflectometer (measurement of “known” density perturba-on) 

Full wave simulation using COMSOL   Phase response to Gaussian shaped density perturba-ons shows that the localized 

(dx ~ cm) reflectometer signal response.  

  The either upshio or downshio is allows depending on the propaga-on direc-on of the reflectometer wave due to the fact that LH wave is the backward wave.  

dR (m)

density

phase response

cutoff

Maximum response happens not “at” but “in front of” the cutoff layer

scattering upshift

downshift

transient simulation shows the difference of time of flight of three wave components

PCI setup in Alcator C‐Mod for LH waves detec-on 

Inside the cell 

R = 60‐79 cm 

M

M

OPM

M

M

OPM

LASERM

M

Phase plate 

Detector

Modulator

plasma 

M: mirror OPM: off‐axis parabolic mirror 

CO2 60W CW 

32ch HgCdTe photoconduc-ve 

extra phase π/2 

New Launcher to be installed this spring

Reflectometer waveguides

In FY2010 will Assess the power handling limits of the new antenna

Using ten klystrons direct fed guides can fall between hard and weak limits – split guides will be just below weak limit

“Hard” limit

“weak” limit

In FY11 with more sources can push beyond “strong” limit

Goal is to have 4 MW of klystron Power and 2 Launchers by end of FY2012

•  16 klystrons available in 2011 •  2nd launcher in 2012 

–  Need experience with present launcher – Mesh with ICRF and Divertor upgrades 

LH Research Program •  2010 

–  Characterize new launcher performance •  Coupling, Power Handling, Effect of reflec-ons 

–  Study Density limit •  Use new reflectometer to to measure SOL wave fields •  Explore different configura-ons to modify SOL characteris-cs 

–  Con-nue pedestal modifica-on studies •  Apply to ELMing discharge 

•  2011 –  Design 2nd launcher – begin fabrica-on 

•  Increased source power available to test power limit –  Con-nue studies on density limit and rota-on 

•  Add PCI diagnos-c for core wave detec-on •  Combine with ICRF for rota-on profile sculp-ng 

•  2012 –  Complete fabrica-on of second launcher 

•  Full system capability by end of FY 12 –  Determine most promising LH scenario for AT physics studies 

Progress on Advanced Scenarios can be made with Existing System

•  U-lize current profile diagnos-cs to test ability to manipulate current profile and effect of MHD stability on profile modifica-on. 

•  Con-nue studies on LHCD during ramp‐up (ITER like scenarios) 

•  Explore I‐mode with its large core electron temperatures which yield strong absorp-on. 

•  Goal is to establish best candidate advanced scenarios in -me for full capability (FY12)