P' M' Ryan, D'B' Batchelor, G'L' Bell, T'S' Bigelow, M'D' Carter, J' B' Caughman, E'F' Jaeger, D'A'

1
EBW Research for NSTX and QPS

• P. M. Ryan, D.B. Batchelor, G.L. Bell, T.S.
  Bigelow, M.D. Carter, J. B. Caughman, E.F.
  Jaeger, D.A. Rasmussen, J.B Wilgen
• ORNL
• G. Taylor, P.C. Efthimion
• PPPL
• R. Harvey
• CompX
• US/Japan/Europe RF Technology Workshop
• Amsterdam, October 4-5, 2004

2
Motivation for electron Bernstein wave emission
heating

• High b plasmas with wpe gtgt Wce cannot use ECH, or
  ECE Te(R,t) diagnostics
• Electron Bernstein waves (EBWs) can propagate in
  these overdense plasmas
• EBWs have high optical thickness at EC
  resonances t 1000 for NSTX, QPS, (and TJ-II)
• Potential for local heating, current drive and
  Te(R,t) diagnostic
• EBW emission can yield information about
  viability of EBW heating and current drive

3
Incoming waves undergo a mode conversion to EBW
at the Upper Hybrid Resonance
Perpendicular launch (X-B)
Oblique launch (O-X-B)
Laqua et al. on W7-AS stellarator
Gary Taylor
With a locally steepened density profile at the
Upper Hybrid Resonance, the X-mode can tunnel
through to the EBW mode Heating is the reciprocal
of the emission
4
Plasmas in NSTX have wpe/wce 3-10 on Axis and
wpe/wce gt1 Beyond Last Closed Flux Surface

• Cannot use ECRH or ECCD

5
EBWs Can Generate Critical Off-Axis Current
Drive in NSTX at High b

• 100 kA of off-axis CD needed
• to sustain b 40 in NSTX
• Cannot use ECCD in NSTX
• since wpe/wce 3-10
• Modeling indicates that EBWCD can
• provide needed current
• EBWCD may also assist startup
• and stabilize NTM's
• 4 MW, 28 GHz EBWCD system is being planned for
  NSTX

NSTX, b 40
0
r/a
0
1
Charles Kessel (PPPL) Tokamak Simulation Code
6
Require 4 MW EBW System to Support NSTX Research
Plan

• Ohkawa EBWCD can use the large off-axis electron
  trapping in NSTX to achieve high CD efficiency
• Modeling predicts EBWCD efficiencies 40-50
  kA/MW in projected high b NSTX plasmas
• 100 kA of EBW off-axis CD --gt 4 MW of RF
  source power
• System designed to operate for RF pulse lengths
  of at least 2 s

7
Choice of RF Source Frequency Constrained by Bt,
EBW Coupling and Accessibility

• EBW damping is on
• Doppler-downshifted
• EC harmonics
• Launch at 14 21 GHz
• looks OK for plasma access
• Launch at 28 GHz may
• damp on both 3 fce 4 fce
• - But small changes in
• Bt can help

28 GHz
21 GHz
14 GHz
NSTX Bt 3.5 kG b 42
8
Modeling Indicates that Oblique, "O-X-B", Launch
is Resilient to Changes in Edge Density Gradient

• OPTIPOL surveys EBW coupling - uses impedance
  matrix from GLOSI

• Optimum n// 0.55
• toroidal angle 34o
• from normal to B
• gt 75 coupling for
• O-X-B antenna with
• 5 degree beam
• spread

Frequency 14 GHz
EBW Coupling ()
80 60 40 20
OPTIPOL/GLOSI
9
Maximum EBW Coupling Efficiency Obtained for
Near-Circularly Polarized Launch
OPTIPOL/GLOSI
Frequency 14 GHz

• Optimum polarization insensitive to edge field
  pitch variations of up
• to 15 degrees but may need ellipticity
  control for startup

10
Obliquely Viewing Quad-Ridged Antenna Installed
on NSTX as a Te(R) Diagnostic for EBW Coupling
Studies

• Two frequency tunable 8-18 GHz EBW radiometers
  
• simultaneously measure orthogonal
  polarizations
• Focusing lens improves collimation presently
  optimized for
• measuring 16-18 GHz EBW emission
• Antenna views along 35 degree B field pitch,
  suitable for NSTX plasmas with Ip 0.8-1.2 MA
  and Bt(0) 4 kG

11
Ray Tracing Calculations Show 16.5 GHz EBW
Emission is Generated Locally at r/a 0.4
EBW Emission Frequency 16.5 GHz
GENRAY t 325 ms
Shot 113544
Bt(0) 4 kG

• GENRAY ray tracing uses EFIT equilibrium and
  Te(R) ne(R) from Thomson scattering

• Antenna acceptance angle much larger than
  predicted 90 EBW conversion region

CompX
12
EBW Emission Analysis Indicates Near-Circular
Polarization EBW Trad/Te 70 Consistent with
Theory

• Emission fluctuations
• due to fluctuation in Ln
• at EBW conversion
• layer
• Fluctuations should be
• smaller at 28 GHz
• smaller antenna
• acceptance angle
• smaller Ln
• fluctuation

Freq. 16.5 GHz
13
Obliquely Viewing 20-40 GHz EBW Radiometer to
Measure 28 GHz EBW Mode Conversion on NSTX Next
Year

• Larger vacuum window higher frequency should
  allow much
• better collimation
• current 16-18 GHz antenna has 12 degree
  acceptance angle,
• 20-40 GHz antenna should achieve less
  than 5 degrees
• Detailed 28 GHz coupling study using
  OPTIPOL/AORSA1D and
• realistic EBW launcher model planned for FY05
• compare to 28 GHz emission measurements
• 1 MW, 60 GHz and 100 kW, 28 GHz EBW
  experiments on
• MAST will also test oblique O-mode
  conversion theory

14
1 MW Proof-of-Principle EBW System Tests
Viability of Heating Current Drive in NSTX

• 750 kW EBW power delivered to plasma
• allowing for transmission loss and EBW conversion
• drive 30-40 kA
• Final 4 MW system will add three more gyrotrons,
  transmission lines launchers
• provides 3 MW of EBW power in the plasma
  generates gt 100 kA

15
QPS
16
Oblique launch (O-X-EBW) of 28 GHz waves looks
feasible in QPS
Mark Carter
17
Density gap between 53 GHz X-mode cut-off (
1.6 -1.9 x1019 m-3) and EBW leaped by 28 GHz

• Hyperbolic density profiles with gradient
  scale-lengths 1- 3 cm show good transmission
  for 28 GHz EBW
• Upper hybrid layer moves rapidly to edge as
  density increases to allow access to the EBW
• gt80 transmission for 30-40 degree launch (from
  normal)
• Polarization control during shot may be needed
• Polarization 60 (quasi-O) for 2 to 4 cm scale
  lengths
• Steeper gradients make larger window for 80
  transmission and drive polarization closer to
  circular (45)

18
28 GHz EBW transmission window opens up when ne
1.5 x 1019
19
Existing 28-GHz and 56 GHz ECH/EBW waveguide
hardware can be used on QPS (and possibly TJ-II?)
20
QPS EBW conclusions

• 28 GHz EBW possible in QPS for densities gt2x1019
  m-3 if gradient scale-lengths can be reduced to
  2-3 cm
• 53 GHz EBW X-mode or O-X-B
• Density gap ( 1.6 -1.9 x1019 m-3)
• 28 GHz EBW is a possible alternative or addition
  to ohmic heating to fill the density gap between
  53 GHz X-mode cut-off and 53 GHz EBW launch
• Issues to investigate (on TJ-II?)
• 53 GHz low field coupling
• EBW coupling efficiency
• Collisional edge damping