Rf heating and current drive experiments on mst
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RF Heating and Current Drive Experiments on MST. Jay Anderson for the MST team. Summary. Two rf experimental approaches are underway, complementary strengths Lower hybrid: established physics, technically challenging antenna

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Rf heating and current drive experiments on mst

RF Heating and Current Drive Experiments on MST

Jay Anderson for the MST team

14th IEA RFP Workshop


Summary

Summary

  • Two rf experimental approaches are underway, complementary strengths

    • Lower hybrid: established physics, technically challenging antenna

    • Electron Bernstein wave: simple antenna, complicated wave coupling in RFP edge plasma

  • Modest power (~100kW) shows rf-plasma interaction

    • Power levels too low for significant current drive, observing and understanding any effect is encouraging.

    • EBW: localized SXR increase

    • LHCD: localized HXR emission

  • Power upgrades under development for each experiment

14th IEA RFP Workshop


Outline

Outline

  • Motivation

  • EBW

    • Coupling between edge EM and EBW waves occurs

      • Blackbody-level emission measured: EBE

      • Reflected power ratio, wave electric fields from grill antenna understood

    • SXR enhanced during select launch conditions

    • Field error induced by port hole has substantial effect

    • Upgrade to MW level source power underway

  • LHCD

    • Generation of large HXR flux with injection of ~100kW

      • Toroidal localization, asymmetries understood

      • Particle trapping and guiding center drifts important

    • Upgrade to 400+kW system underway

14th IEA RFP Workshop


Motivation

Motivation

14th IEA RFP Workshop


Ebw heating and current drive

EBW Heating and Current Drive

EBW is efficiently damped at

cyclotron resonance; coupling

power to the EBW is key issue

Genray/CQL3D case, zero diffusion

This would be an interesting

experiment in the RFP

14th IEA RFP Workshop


Rfp geometry is challenging for rf heating cd

RFP geometry is challenging for RF heating/ CD

Goal: heat and drive current at cyclotron resonance

Overdense: wp >> wc

EM waves not accessible to ECR

There is no high field side.

Mode conversion at UHR, in antenna near-field, is critical

Edge density fluctuations hinder coupling, particularly O-mode

Field error caused by hole in conducting shell (MST) has deleterious effect on coupling,

Limits maximum size of antenna.

14th IEA RFP Workshop


Coupling to ebw in mst x mode launch

Coupling to EBW in MST: X-mode launch

EBW is an electrostatic wave carried by gyromotion of electrons.

Blackbody levels of EBE demonstrate coupling

R/F from waveguide grill understood in terms of local density

lvac ~ 8 cm

l B ~ 1mm

Simulation

data

14th IEA RFP Workshop


Correct interguide phase grill antenna is critical for coupling to ebw

Correct interguide phase (grill antenna) is critical for coupling to EBW

  • Interguide phasing critical parameter in optimization of coupling.

  • BN Antenna cover improves coupling

    • Affects local electron density gradient

    • Blocks plasma from entering antenna (source of arcing at high power)

No cover, PPCD

BN cover

simulation

14th IEA RFP Workshop


Bn cover steepens local density gradient

BN cover steepens local density gradient

Effect of field error: field lines protrude into antenna in port

No cover, PPCD

BN cover

Antenna cover acts as

limiter due to field error.

14th IEA RFP Workshop


Measurement of wave e in plasma

Measurement of wave E in plasma

  • Crossed dipole RF probe measures Er, Ef within plasma (few cm)

    • Probe position scanned for fixed ne

    • Probe position fixed for evolving ne

Cold plasma dispersion:

For x-mode launch

At upper hybrid resonance

Electric field becomes longitudinal

14th IEA RFP Workshop


Wave e field within plasma consistent with ebw

Wave E field within plasma consistent with EBW

Discharge reaches state where |B|, ne(edge), and antenna phase (scanned) are optimal.

Vacuum: Er/ Ef~ 0: TEM

Probe at Xuh: Er/ Ef> 1

Recall cold plasma dispersion:

14th IEA RFP Workshop


Localized sxr measured with ebw injection

Localized SXR measured with EBW injection

Genray predicted

ray trajectory

RFX SXR camera,

Measuring 4-7 keV

PPCD discharge

PPCD + rf

14th IEA RFP Workshop


Sxr enhancement requires good confinement

SXR enhancement requires good confinement

Net Power in

  • 4 arm antenna, ~130 kW forward power.

  • PPCD discharge.

  • SXR, outboard edge.

  • Signal <0 during confinement loss; real effect of rf pickup

  • m=0 indicator of PPCD quality

  • Qualitatively in agreement with CQL3D: Diffusion reduces emission.

  • Boron injected into plasma during rf; emission enhanced

14th IEA RFP Workshop


Ebw experiment upgrading to mw level

EBW experiment upgrading to MW level

  • Move from 3.6 GHz to 5.5 GHz system (tube availability)

    • Target discharge higher Ip

    • Shorter wavelength, smaller antenna, smaller porthole

  • Goal: Demonstrate feasibility of MW level EBW experiment

    • Optimize launch through 11cm port

    • Test power capability in 5cm port

    • Test OXB scheme; very simple with cylindrical antenna.

1 MW generated in bench test,

20 April 2010

14th IEA RFP Workshop


Ebw experiment upgrading to mw level1

EBW experiment upgrading to MW level

  • Move from 3.6 GHz to 5.5 GHz system (tube availability)

  • Prototypes being tested:

    • 1/4 l quartz vacuum window

    • Circular choke joint

    • Cylindrical molybdenum antenna

14th IEA RFP Workshop


Lower hybrid current drive

Lower Hybrid Current Drive

  • Fokker-Planck modeling predicts efficient current drive

    • 0.5 A/W at 250 MHz

  • Experiments ongoing at 800 MHz

    • Efficiency still quite high: ~0.3 A/W

    • Physical size of antenna more tenable

    • Make use of existing klystrons

14th IEA RFP Workshop


Meticulously designed antenna successful to klystron power limit

Meticulously designed antenna successful to klystron power limit

  • 800 MHz launcher

    • Interdigital line antenna.

    • Power (up to 220kW) fed in one port, then along structure

    • co-, counter- CD by choice of port

  • Clear RF/ plasma interaction:

    • Hard x-rays generated

14th IEA RFP Workshop


Large hxr flux generated during lhcd

Large HXR Flux Generated During LHCD

Viewing chords look across MST toward antenna.

Large flux up to 40keV, intensity follows electric field strength.

14th IEA RFP Workshop


Strong near field e accelerates electrons

Strong near-field E accelerates electrons

  • Test particle computation: e- initially 40eV Maxwellian

  • Single pass through antenna electric field (COMSOL) shows

  • acceleration to ~50 keV, mostly perpendicular

  • Particle trapping.

  • Directionality in parallel velocity, consistent with proposed wave

14th IEA RFP Workshop


Launch direction toroidal hxr asymmetries

Launch Direction, Toroidal HXR Asymmetries

Stronger flux to lower toroidal angle

- consistent with drift of trapped particle orbit

Higher flux for Co- launch than Counter-

- 2nd pass through antenna more likely.

14th IEA RFP Workshop


Lh summary

LH Summary

  • Successful antenna designed for strict space constraints in MST

    • Small port holes for coaxial power feeds

  • Strong HXR flux in antenna near field is understood: Acceleration of plasma electrons via Lorentz force

    • COMSOL modeling of antenna E field

    • Test particle calculation shows electrons are primarily heated in perpendicular direction

      • Explains existence of localized high energy x rays.

      • Explains co-, counter- magnitude difference and toroidal asymmetry

    • Also shows directional current drive qualitatively consistent with Fokker-Planck modeling: asymmetry in parallel speed near 0.2c

  • Complete power accounting is required:

    • Measured HXR flux does not consume full radiated power

    • Near term plans are to double input power: 2 tubes.

14th IEA RFP Workshop


Summary1

Summary

  • Two rf current drive schemes are being tested on MST

    • EBW: Simple antenna, coupling verified.

      • Building MW level experiment, rf source tested short pulse.

    • Lower Hybrid: Complex antenna, successful to 200+ kW

      • HXR generation explained by large perpendicular E in antenna near-field

      • Computed near-field effect also shows parallel directionality

        • Yet to be measured

      • Next step: Double power with 2nd tube, 2nd antenna.

  • Broader impact than just MST/ RFP:

    • EBW, LH waves are of general interest in high b plasmas

    • Ongoing modeling: Fokker-Planck and ray tracing validation in unique parameter space (RFP)

14th IEA RFP Workshop


Second pass of inboard going trapped e

Second pass of inboard-going trapped e-

Test particle initial distribution: inboard-travelling trapped

electrons from first pass calculation.

Co- current direction now has higher density of 30-50 keV e-

14th IEA RFP Workshop


Ebw current drive efficiency in mst tbd

EBW current drive efficiency in MST: TBD

Fisch-Boozer and Ohkawa effects both factors in MST

Ohkawa

Fisch-Boozer

EBW resonance

14th IEA RFP Workshop


Four waveguide grill heating experiments

Four waveguide grill, heating experiments

Optimum phasing of 4-guide

antenna qualitatively similar

to that of 2-guide grill

Sustained good coupling at > 100kW

14th IEA RFP Workshop


Ebw hardware upgrades power supply

EBW Hardware Upgrades: Power Supply

  • Require -80kV at 40A for 10-20ms to run klystron tube

  • 0.3F at 1200V capacitor bank

  • 3 phase IGBT inverter 1200V at 5000A

  • Resonant transformers

  • Voltage doubling rectifier

  • Harmonic filtering for low ripple

14th IEA RFP Workshop


Empirical power handling waveguide grills

Empirical power handling: Waveguide grills

This may give insight to:

How much power can we

get through the antenna?

Pericoli et al Nuc Fusion 2005

14th IEA RFP Workshop


Empirical power handling waveguide grills1

X

Empirical power handling: Waveguide grills

This may give insight to:

How much power can we

get through the antenna?

X

X EBW 3.6 GHz achieved

X EBW 3.6 GHz proposed

X 5.5 GHz: 1 MW, 4.5” port

X 5.5 GHz: 1 MW, 2” port

EBW on MST is different than LH grills on tokamaks:

X-mode launch: E perp. to B0 may enable higher power density.

X

X

Pericoli et al Nuc Fusion 2005

14th IEA RFP Workshop


X mode to ebw conversion

X-mode to EBW Conversion

  • Fast X-mode launched from RFP edge

  • Cold plasma approximation valid for Fast X-mode region

  • X-mode wave crosses R cutoff layer and begins to evanescently decay

  • Steep edge density gradient leads to closely spaced R, UH, and L layers leading to efficient coupling

  • Slow X-mode propagates between UH and L layers

  • Electric field becomes predominantly parallel to k near UH layer

  • Slow X-mode reflected off of L

  • Interference between UH and L minimizes reflected wave traveling past UH

  • Mode conversion to EBW between UH and L layers

  • EBW propagates past L layer into plasma

Cold plasma approximation

For x-mode launch

At upper hybrid resonance

Electric field becomes predominantly longitudinal

14th IEA RFP Workshop


Rf heating and current drive experiments on mst

14th IEA RFP Workshop


Raw sxr vs input power level

Raw SXR vs input power level

14th IEA RFP Workshop


X mode launch coupling to ebw

X-Mode launch coupling to EBW

  • No high field side in RFP; fast X-mode launch.

  • Evanescent layer encountered at R cutoff

  • Width of layer sensitive to edge density profile, typical value ~2cm

14th IEA RFP Workshop


Oxb conversion in mst

OXB Conversion in MST

  • Most other machines use OXB conversion scheme for heating and current drive (most others at higher field)

  • OXB efficiency on MST is less than XB efficiency

14th IEA RFP Workshop


Ebe verifies mode conversion

EBE verifies mode conversion

Conversion efficiency  ~TEBE/T

  • X mode >  O mode

14th IEA RFP Workshop


Coupling to ebw in mst

Coupling to EBW in MST

Launched EM wave couples to Bernstein mode at

upper hybrid resonance

In near field of antenna

Cutoff ( L )

Cutoff ( R )

Upper hybrid resonance

~ 2 cm

lvac ~ 8 cm

l B ~ 1mm

Reflection occurs from

each cutoff;

Distance between layers determined

by ne and B profiles.

Interference of reflected waves leads

to optimized transmission

14th IEA RFP Workshop


Coupling improvements available at 5 5 ghz

Coupling Improvements available at 5.5 GHz

S-band antenna in 4.5” port

C-band antenna (~2” OD) in 4.5” port

Insertion to steeper Ln possible;

Partial field error mitigation

Antenna cover acts as

limiter due to field error.

Field error reduction by

use of smaller port:

C-band antenna in 2” port

14th IEA RFP Workshop


Ebw high voltage supply transformer

EBW high voltage supply transformer

  • Resonant secondary configuration

    • Parallel LC resonator

    • Large leakage inductance

  • 20 turn primary, 160 turn secondary (8:1)

  • 50:1 voltage multiplication due to resonance

  • Microcrystalline iron core, low hysteresis loss at high frequency

  • 20kHz operation for low output ripple

  • 3 phase Y configuration, center tap connected to rectifier positive terminal

  • Oil filled secondary

14th IEA RFP Workshop


Ebw hardware upgrades waveguide and launcher

EBW Hardware Upgrades: Waveguide and Launcher

  • Previous copper rectangular waveguide arced in vacuum with 3.6GHz at 150kW

  • Now injecting 5.5GHz at 1MW

  • Rectangular to circular transition

  • Circular fused silica RF window and choke joint transition

  • Cylindrical molybdenum waveguide

    • Cylindrical waveguides have lower electric fields reducing arcing risk

    • Molybdenum has high electron affinity and good plasma damage resistance.

    • Possible use without boron nitride limiter

    • Capable of using smaller port on MST

14th IEA RFP Workshop


Lower hybrid current drive experiment

Lower Hybrid Current Drive Experiment

  • 800 MHz launcher

    • In MST vacuum vessel.

    • Power fed through antenna (more in than out)

    • 80+ kW at present

  • Antenna loading depends on edge plasma conditions

    • Localized puffing used for density control

  • Clear RF/ plasma interaction observed:

    • Hard x-rays generated

  • Upgrade to 320+ kW in progress


Particle trapping toroidal drift explain asymmetries

Particle trapping, toroidal drift explain asymmetries

Delta phi 6-15 cm

Antenna aperture ~3cm

Delta_phi short way = 0-5cm

14th IEA RFP Workshop


Inference c band coupling via emission

Inference: C-band coupling via emission

Still needs to be measured in

launch mode; different geometry

Conversion efficiency  ~Tebe/T

  • @ 5.5 GHz >  @ 4. GHz

  • X mode >  O mode

14th IEA RFP Workshop


Next step 300 kw antenna

Next Step: ~300 kW Antenna

  • Larger coax feed through; expect 320 kW power handling capability

  • RF source development under way (need to run outside design parameters for pulsed experiment)


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