EBW Experiments on MAST

V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

MAST

EBW Experiments on MAST

V. Shevchenko1, G. Cunningham1, A. Gurchenko2, E. Gusakov2, A. Saveliev2, A. Surkov2, F. Volpe1

1EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon,Oxon, OX14 3DB, UK2Ioffe Institute, Politechnikheskaya 26, 194021, St. Petersburg, Russia


MAST

Long-term goals

CTF or ST Power Plant will not allow the use of a central solenoid

NBCD is considered as a main source to sustain steady state current in ST plasma

EBW can provide a critical off-axis current drive to sustain ST plasma in equilibrium

EBW H&CD can also assist plasma start-up

Midplane topology of cut-offs and resonances in MAST (H-mode)

EC harmonics are usually obscured by cut-offs in ST

IntroductionIntroduction


MAST

Btotal

ne

PlasmaLH O-mode

RH O-mode

Antenna Configuration for O-X-B ConversionAntenna Configuration for O-X-B Conversion

• O-X-B conversion is possible when ce < RF < pe

• Angular width of mode conversion cone depends on ne

• Launch plane is determined by Btot and ne at the plasma edge

• Angle between ne and optimal direction depends only on | Btot | in the layer where RF = pe

• Btot at the plasma edge is the most crucial parameter for the optimal launch configuration

sin2=N2||,opt=Y/(Y+1), Y=ce/

a) b)

O-X-B mode (N|| < 0) conversion window calculated for the 60 GHz launcher located 22.5 cm below the midplane in MAST: a) high density ELM-free H-mode, b) high density L-mode plasma. Contours indicate 10% steps in

conversion efficiency, i.e. 0.5 means 50% mode conversion efficiency etc.


MASTEBW Modelling in MAST

Midplane topology of cut-offs and resonances in sawtoothing H-mode plasma, shot #7798 in MAST.

like O/X

Power deposition radius against frequency within the range of fundamental EC resonance. 40 cm above midplane launch, N|| < 0.

0.2 0.4 0.6 0.8 1.0 1.20

20

40

60

80# 7798, 0.24 sec

Fre

qu

ency

, GH

z

Major Radius, m

fpe

fUHR

n fce

1ce


MASTEBW Emission in MAST

EBW emission spectrogram measured in sawtoothing H-mode plasma, shot #7798. Red areas correspond to higher emission intensity. ECRF power was injected at 0.21 - 0.24 s, ITF = 91 kA.

EBW emission spectrogram in ELM-free H-mode plasma at optimised TF (ITF = 83 kA), shot #11156. ECRF power (0.5 MW) was injected at 0.22 - 0.29 s.

0.0

0.5

1.0

1.1

1.2

0.0 0.1 0.2 0.3 0.4

80

90

shot #7798

Plasma CurrentI p, M

A

Plasma Size

RL

CF

S, m

Toroidal Field

I TF, k

A

Time, s

20

30

40

Fre

qu

en

cy,

GH

z

From Ruby TS and EFIT

Fre

qu

en

cy,

GH

z


MASTEBW Steerable Launcher in MASTEBW Steerable Launcher in MAST

• Final polarisation can be chosen from linear to circular

• Resultant beam divergence is less than +/-2.5o (w = 25 mm)

• Poloidal steering range of +/-13o, toroidal +/-24o, accuracy of 0.5o

21 mirrors7 beams200 kW each60 GHz


MAST

• O-X-B mode conversion window is broad (about ± 5° at 50% efficiency). Relaxed launch configuration.

• EBW absorption is expected to be very peripheral, r/a ~ 0.8. Non-linear effects can be observed.

Plasma Targets for EBWHPlasma Targets for EBWH

ELM-free H-mode Sawtoothing H-mode Ohmic H-mode

•Mode conversion window is moderate (about ± 3° at 50% efficiency). Relatively stringent to launch parameters.

• EBW absorption is more central, can reach r/a ~ 0.6. Heating effects can be observed.

•Mode conversion window is narrow (about ± 1.5° at 50% efficiency). Very stringent to launch parameters

• EBW absorption can reach transiently r/a ~ 0.4. Heating effects must be detectable.

EBW deposition


MASTLower Hybrid Probe Head

Can be moved to a specified distance from the plasma in the midplane Allows axial rotation ± 45o

Loop (20x40 mm2) & L shape antennas 76-545MHz spectrum analyser with 10 MHz resolution

50 mm


MASTParametric Effects in ELM-free H-mode

100 200 300 400 500 6000

1

2

3

4

MAST #11420

LHE

sig

nal,

mW

Frequency, MHz

255 ms 285 ms 290 ms

• A strong emission enhancement around 134 MHz has been observed during RF injection.

• This emission was identified as LH emission originating in the X-B mode conversion layer near UHR at 60 GHz

• Parametric Decay (subject to ~80 kW RF power threshold at UHR) indicates the mode conversion is no less than 50%


MAST

34

3145121131

6133131323

22102

L

BTTf

eVGHzTcm

W

cm

WP eeffUHR

Parametric Decay Threshold

where Teff =Te + 4Ti

ρ is the radius of the heating beam

L is the inhomogeneity scale: L-1 = grad(ne)/ne + 2(ωce/ωpe)2grad(B)/B

For typical MAST parameters: f = 60 GHz, Ti ≈ Te = 140eV, B =0.38 T, L = 3 cm

PUHR*/(πρ2) 260 W/cm2 PUHR* ≈ 80 kW for ρ ≈ 10 cm


MASTSawtoothing H-mode Ray-tracing Modelling

UHR

EBW power deposition profile

UHR

Poloidal projection of EBW ray-tracing results


MASTEBWH in Sawtoothing H-mode PlasmaEBWH in Sawtoothing H-mode Plasma

Average heating result. Shots #9262, #9263, #9267.

EBW emission has been used to optimise magnetic field and launch angles EBE has a maximum when O-mode cut-off is about 2/3 of the gap between 5ωce and 6ωce

mode coupling is strongly modulated by sawteeth and ELMs heating effect is better seen after averaging over a few shots no parametric decay has been observed in this scenario

EBW radiative temperature measured from 2ωce

during ELM-free intervals at optimal magnetic field.

a)

60 kJ (~3 MW)

7 kJ (~0.25 MW)

250 kW

Plasma Energy, kJ

RF Power, kW

Time, s

60

70

80

50

40400

300

200

100

0

0.20 0.25 0.30 0.35

b)

Before RFinjection

During RFinjection


MASTEBE in High Density Ohmic H-modeEBE in High Density Ohmic H-mode

Resonance topology for high density Ohmic H-mode EBW emission (60.5 GHz) in high density Ohmic H-mode

during plasma compression

Outer plasma radius, m

#10638

#10639

ECE

ECE

EBE

4ce

3ce2ce

EBE4ce

3ce

As the plasma boundary moves into the higher magnetic field during compression, EBE comes first from 4ωce, then from 3ωce and finally from 2ωce


MAST

-7 -6 -5 -40.0

0.2

0.4

0.6

0.8

1.0

f=60.5GHz, = -7deg

BXO

-EBW

Sig

nal (

a.u.

)Poloidal angle (deg)

-11 -10 -9 -8 -7 -6 -5

0.0

0.2

0.4

0.6

0.8

1.0f=60.5GHz, = -5.6deg

BXO

-EBW

Sig

nal (

a.u.

)

Toroidal angle (deg)

Angular window for O-X-B mode conversion measured at 60.5 GHz

Launcher Aiming in Ohmic H-modeLauncher Aiming in Ohmic H-mode

EBE has been used to optimise the launch configuration for EBWH at 3ωce


MAST

SXR signal in the RF heated shot (red) and reference shot (black)

SXR Signal During EBWH

SXR signal from the RF heated shot with subtracted reference signal

Plasma outer radius

D signal

60 GHz EBE signal

SXR differential signal

RF power

SXR signal was doubled during RF pulse


MASTPlasma Energy Increase During EBWH

Plasma energy (EFIT) during RF injection. Shots: #10704, #10706, #10707, #10709.

Plasma Energy

RF Power

Plasma Density

D signal


MAST

Electron temperature profiles measured by Thomson scattering in RF heated (red) and ohmic (blue) plasmas at 0.20 s.

Electron Temperature Increase During EBWH

Electron temperature profiles measured by Thomson scattering in RF heated (red) and ohmic (blue) plasmas at 0.18 s.

Electron Temperature increased by 10-15% due to RF injection


MASTConclusions

Proof-of-principle studies of the O-X-B scheme have been conducted on MAST at 60 GHz. Antenna aiming was performed using EBW emission measurements and mode coupling modelling:

• In ELM-free H-mode only peripheral absorption is possible but the mode coupling efficiency was proven to be high. Lower Hybrid emission (subject to ~80 kW RF power threshold at UHR) indicates the mode conversion is no less than 50%.

• Some evidence of EBW heating was observed in the sawtoothing H-mode target plasma. Total plasma energy shows 10% increase during RF pulse.

• In high density Ohmic H-mode the mode conversion window is very narrow. However, after careful antenna alignment using EBW plasma emission at 60 GHz 10% electron temperature increase has been measured during RF power injection.

EBW heating has therefore clearly been observed via the O-X-B mode conversion process


MASTFurther Plans

Conduct EBW assisted plasma start-up experiments with the recently installed 28 GHz, 200 kW start-up system.

As seen in experiments EBW emission is strongly anisotropic. Angular co-ordinates of maximum EBE intensity are predominantly determined by the magnetic field pitch angle at the plasma edge. The magnetic field pitch angle experiences strong variations during the plasma shot due to L-H transitions, sawteeth and other plasma activities. To study the dynamics of the magnetic pitch angle we are planning to:

• Upgrade the FSR with a remotely controlled spinning mirror• Make a real time angular scan over the range, expected due to pitch angle variations.

Real time measurements of EBW emission angular dependence should give us a direct estimate of the magnetic pitch angle potential q-profile diagnostic.

It would allow more accurate predictions of launch parameters for EBWH experiments and to clarify the edge plasma physics during L-H transition, ELM-free H-mode etc.


MAST28 GHz EBW Assisted Plasma Start-up

0 10 20 30 40 50 60 70 80 90 100 110-50

-40

-30

-20

-10

0

10

20

30

40

50

Z, c

m

R, cm

O-mode beam

Vessel wall

Central rod

X-mode beam

EBW-mode beam

UHR ECR

BT IP

Grooved area X-mode beam

Beam pattern

Modelling results. EBW driven current (trapping effects included) in the range of plasma temperatures and densities. Input power 150 kW, 28 GHz.

Ray-tracing modelling (poloidal view) of the EBW CD plasma initiation. The RF beam pattern, as measured at low power, is well within the grooved area of the graphite mirror-polariser.


MAST28 GHz Antenna Mock-up Assembly28 GHz Antenna Mock-up Assembly

In-vessel mirrors of the 28 GHz launcher Mock-up assembly of the launcher (upside-down)


MAST

EBE from

plas

ma

to ra

diom

eter

Tilted spinning mirror for angular scan of EBW emission (red ellipses).Inclination of the contours of BXO conversion efficiency (colour ellipses) Inclination of field lines at cutoff location q-profile

Spinning Mirror Principle Spinning Mirror Principle


MAST

Manual replacement of mirrors of different tilt

4.5o

1.1Kg

1.5o

0.9Kg3o

1Kg

Replacement Mirrors


MASTSpinning Mirror Set-up on MAST Spinning Mirror Set-up on MAST


MASTSummary

• A dedicated complex of high power RF heating systems, EBW diagnostics, and modeling tools has been developed at Culham in order assess the potential of EBW assisted plasma start-up, EBW heating and CD in MAST.

• EBE measurements provide a powerful tool for EBW heating optimisation

• Proof-of-principle (60 GHz) EBWH experiments confirm modelling predictions

• Real time angular EBE measurement is a potential q-profile diagnostic

• 28 GHz EBW plasma start-up/assist system has been installed on MAST

• Low frequency (~18 GHz) 1 MW EBW system is considered for MAST

EBW Experiments on MAST

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