The 2 nd A3 Workshop on ST January 6 th ~8 th , 2014

The 2nd A3 Workshop on STJanuary 6th~8th, 2014

Hyunyeong Lee, Jong Gab Jo, D. H. Choi, Y. H. An,S. H. Kim, K. J. Chung and Y. S. Hwang

NUPLEX, Dept. of Nuclear, Seoul National University, San 56-1, Shilim-dong, Gwanak-gu, Seoul 151-742, Korea

[email protected]

Study on the effect of ECH/EBW mode conversion on the wall reflection in linear device

Center for Advance Research in Fusion Reactor Engineering

Department of Nuclear Engi-neering Seoul National Univer-sity

2/20LHY_A3_JAN07

Study on the effect of ECH/EBW mode conversion on the wall reflection in linear device on January 07th 2014, at Tsinghua University

Contents

Experiment Result- Feasibility of EBW mode conversion- Density profile along the magnetic field- HFSX, LFSX and LFSO injection along the wall

position

Summary

Introduction- Motivation and Objectives- Electron Bernstein Wave(EBW) mode conversion

Experiment Setup- Experiment setup, Diagnostics

Pre-ionization experiment in VEST- XB mode conversion in pre-ionization- Comparison between O&X mode injection

Simulation Result(COMSOL)- Comparison between the CMA diagram- The simulation based on diagnosis of the exper-

iment

3/20LHY_A3_JAN07


IntroductionMotivation & Objectives

Electron Cyclotron Heating(ECH) is widely used for various purposes in fusion device that the pre-ionization, local heating and current drive to control the pedestal and the instabilities such as sawtooth, NTM etc.

Especially non-inductive current drive and startup using ECH/CD is essential for Spherical Torus (ST) which has difficulty in start-up due to lack of space for the center stack.

In ST devices which have characteristics of high beta operation and conse-quent low cutoff density, alternative method using the Electron Bernstein Wave (EBW) which is no cutoff density has been studied.

In most previous works, EBW mode conversion for the wall covered with the chamber has not been studied in detail. The investigation of this EBW mode conversion is essential for the fusion plasma research including ST.

4/20LHY_A3_JAN07


Electron Bernstein Wave(EBW)- Electrostatic wave that has no high density cutoff- Electromagnetic wave(ECH) is converted to the electrostatic wave(EBW) only in-

side the plasma. Conventional ECH is not suitable to ST due to low cutoff density

- EBW : Attractive for heating and CD in ST

EBW mode conversion coefficient : [1]

Budden parameter :

Schematic of the EBW assisted plasma current start-up in MAST[2] Maximum mode conversion efficiency C versus Ln at NSTX[1]

4 (1 )C e e 21

22

2

2 1)/(11

)/(2

BneBne

nece

LLLLcL

[1] Heinrich Peter Laqua, PPCF, 49, R1-R42, 2007[2] V.F. Shevchenko et.al., Nuclear Fusion, 50, 022004, 2010

IntroductionElectron Bernstein Wave Mode Conversion

5/20LHY_A3_JAN07


-6 -4 -2 0 2 4 6700

750

800

850

900

950

Mag

netic

Fiel

d (G

)

Distance (cm)

Radial FieldQuartz

Quartz

HFS LFS875G

`

WR284

PumpSystem

3 StubTuner

2.45 GHzMagnetron

Hydrogen Injection

Quartz Tube

Stainless Steel Wall

MW Loss Protection

Bent Magnetic Field(BT)

Bent Magnetic Field(BT)

LFS Injection

HFS Injection

Experiment SetupMagnetic Field and Waveguide System

Objective• Confirm the effect of ECH & EBW

mode conversion Small cylindrical device with axial magnetic

field bent similar to tokamak Possible to change the direction of mi-

crowave(LFS&HFS and O&X mode) Easily changeable to the magnetic field and

MW power Antenna : open waveguide(WR284)

TB

Magnetron

TB

6/20LHY_A3_JAN07


Experiment Setup Diagnostics – Langmuir Probe

Diagnostics – Radial Langmuir probe• Planar type tip(r=1.5mm)• Measured in Z=0cm from the center• Te (linear fit slope of the log scale V-I curve)• ne (the ion saturation current & Te )

Comparison between Interferometer & Probe• To confirm the density measurement• Assuming that parabolic density profile due to the line

integrating density using interferometer• The result shows the similar tendency of the density• In this result the diagnostics was utilized by this probe. 300 400 500 600 700 800 900

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

X(L) cutoff

n e [10

17m

-3]

Microwave Power [W]

Interferometer probe

O cutoff

2 234 3 41 AxialProbe

Ra-dialProbe

Interfer-ometer

7/20LHY_A3_JAN07


Experiment Setup

The Wall with X/O Polarized Slit

Objective• Confirm the effect of EBW mode conversion on the wall

reflection with polarized slit The wall is possible to change the position along the injection

and opposite side. X/O slit : The wall with slits that reflects the X/O mode wave

• Formation of polarized wave with specific direction to ra-diate the electric field of wave perpendicular/parallel on the magnetic field

• The other wave with different direction will pass through the slit

Center

AxialProbe

Ra-dialProbe

2.45GHz

Hydrogen Injection

PumpSystem

Bt

E1┴Bt

perpendicular

E1

O slit wall

∥ parallel

X slit wall

8/20LHY_A3_JAN07


In case of HFS injection, the O/X(L) cutoff layer exists in CMA diagram.

O mode cutoff density : ~7.5×1016 #/m3

X(L) mode cutoff density : ~1.5×1017 #/m3

The result of COMSOL simulation also shows the cutoff layer.

Simulation Result CMA Diagram – the cutoff from HFS injection

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

HFS

LFS

: Cutoff(reflection)

0 1E16 5E16 3E17

HFSX injection HFSO injection

0 1E16 5E16 3E17Background

Plasma

9/20LHY_A3_JAN07


Also in case of LFS injection, the O/X(R) cutoff layer ex-ists in CMA diagram.

O mode cutoff density : 7.5×1016 #/m3

X(L) mode cutoff density : ~1.5×1017 #/m3

X(R) mode cutoff density : ~2.5×1016 #/m3

The result of COMSOL simulation also shows the cutoff density.

Simulation Result CMA Diagram – the cutoff from LFS injection

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

HFS

LFS


0 1E16 5E16 3E17

LFSX injection LFSO injection

0 1E16 5E16 3E17Background

Plasma

10/20LHY_A3_JAN07


In HFS case, the injected microwave does not exceed the cutoff density as same as COMSOL result.

In LFSX case, over dense plasma generates which overcomes the cutoff layer.

In LFSO case, the tendency of the density is similar but weak tendency of the LFSX injection.

The feasibility of EBW mode conversion(XB/OXB) has been shown.

Experiment Result The Feasibility of EBW Mode Conversion

300 400 500 600 700 800 9000

1x1017

2x1017

3x1017

4x1017

5x1017

6x1017

7x1017

O cutoff

HFSO HFSX LFSO LFSX

Dens

ity (#

/m3)

Power (W)

X(L) cutoff

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

HFS

LFS


: XB conversion

11/20LHY_A3_JAN07


700WLFSX injection

-4 -3 -2 -1 0 1 2 3 40

2

4

6

8

10ECR in 800G ECR in 875G

Quartz WallLFS

X(L) cut-offO cut-off

700G 800G 875G

n e [10

17m

-3]

Distance [cm]Quartz WallHFS

The density profile is measured for moving the probe shot by shot.

Due to the limit of the probe size, it was impossible to measure near the quartz wall, so the presumption of the density profile is shown in the dashed line.

Higher plasma density with the low magnetic field not in the ECR, but near the UHR(dashed line)

The density is higher with the lower magnetic field

Broader and higher density in the UHR(steep density gradient due to the narrow chamber)Feasibility of XB conversion

Experiment Result The Density Profile along the Magnetic Field

-4 -3 -2 -1 0 1 2 3 4

5.0x1016

1.0x1017

1.5x1017

2.0x1017

2.5x1017

3.0x1017

3.5x1017

4.0x1017

UHR @ 875GUHR @ 800GUHR @ 700G

X(L) cutoff

O cutoff

ECR @ 800G

Dens

ity (#

/m3)

Distance (cm)

700G 800G 875G

ECR @ 875G

UHRLayer

12/20LHY_A3_JAN07


The result of COMSOL simulation is conducted with the experiment result(density).

In case of HFS injection, the density from the experi-ment does not exceed the X(L) cutoff density.

The microwave from LFSO meet the cutoff layer, but LFSX injection overcomes the cutoff layer.

It might be occurred due to the steep density gradient near the UHR.The feasibility of EBW mode conversion

Simulation Result The Feasibility of EBW Mode Conversion

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

HFS

LFS


: XB conversion

HFSO/X injection LFSO/X injection

13/20LHY_A3_JAN07


Experiment Result HFSX Injection

In case of HFSX injection, the density does not exceed the X(L) cutoff layer.

In all cases of injection side wall, opposite side wall and both wall, the density has the similar ten-dency – single pass absorption.

300 400 500 600 700 800 9000.0

5.0x1016

1.0x1017

1.5x1017

2.0x1017

2.5x1017

Dens

ity (#

/m3)

Power (W)

None Opposite Injection Both

X(L) cutoff

O cutoff

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

HFSX

HFSXInjection Opposite

ECR

14/20LHY_A3_JAN07


Experiment Result LFSX Injection – XB Mode Conversion

In case of LFSX injection XB mode conversion might be occurred directly.

- Steep density gradient near UHR.

The wall positioned in the opposite side of the wave injection affects to the multiple reflection – multi pass absorption.

300 400 500 600 700 800 900

2x1017

3x1017

4x1017

5x1017

6x1017

7x1017

Dens

ity (#

/m3)

Power (W)

None Opposite Injection Both

X(L) cutoff

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

LFS

LFSXOpposite Injection

ECR

15/20LHY_A3_JAN07


Experiment Result LFSX Injection with Polarized Wall

In case of LFSX injection XB mode conversion and multi pass power ab-

sorption due to the wall reflection.

Oslit wall has the similar tendency with the none wall case – no effect of the reflected O wave

Xslit wall – similar to the full wall(XB conversion)

300 400 500 600 700 800 900

2.00E+017

3.00E+017

4.00E+017

5.00E+017

6.00E+017

7.00E+017

Dens

ity (#

/m3)

Power (W)

None Oslit Xslit Full

X(L) cutoff

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

LFS

LFSXXslit Oslit

ECR

16/20LHY_A3_JAN07


Experiment Result LFSO Injection – OXB Mode Conversion

Opposite side & both wall- The feasibility of OXB mode conversion- OX conversion : O cutoff & wall reflection- Multi pass absorption due to the opposite wall

Injection side & None wall- No overcomes the X(L) cutoff density

Further research is required to understand the wave movement after MW meets the O cutoff layer.

300 400 500 600 700 800 9000.0

5.0x1016

1.0x1017

1.5x1017

2.0x1017

2.5x1017

3.0x1017

3.5x1017

Dens

ity (#

/m3)

Power (W)

FullNone NoneFull Full None

X(L) cutoff

O cutoff

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

LFS

LFSOOpposite Injection

ECR

17/20LHY_A3_JAN07


Experiment Result LFSO Injection with Polarized Slit

1

1

Resonance

R(X) cutoff

O cut off

X(L) cut off

UHR

LFS

LFSOXslit Oslit

ECR

300 400 500 600 700 800 9005.0x1016

1.0x1017

1.5x1017

2.0x1017

2.5x1017

3.0x1017

Dens

ity (#

/m3)

Power (W)

None Xslit Oslit Full

X(L) cutoff

O cutoff

In case of LFSO injection Xslit & Full wall

- The feasibility of OXB mode conversion- the similar density tendency

Oslit & None wall- No existence of the reflected X wave- XB conversion does not occurs.

18/20LHY_A3_JAN07


30 35 40 45 50 55 60 65 70 75 80 850.0

0.2

0.4

0.6

0.8

1.0

1.2

n e [1

017m

-3]

R [cm]

X-mode_2kW X-mode_3kW X-mode_4kW X-mode_6kW

Electron Cyclotron Resonance

Chamber Port

Based on the linear device experiment, the pre-ionization experiment in VEST is con-ducted with only TF field.

Initial breakdown occurs in ECR with low ECH power, and then UHR moves outward increasing ECH power that means the electron density buildup.

The density profile in VEST shows that the electron density peak exists near the UHR which has the steep density gradient and it means the feasibility of XB mode conver-sion.

R cutoff

UHR

Pre-ionization experiment in VESTXB Mode Conversion in Pre-ionization

LFSX In-jection

19/20LHY_A3_JAN07


30 35 40 45 50 55 60 65 70 75 80 850.0

0.2

0.4

0.6

0.8

1.0

1.2n e [

1017

m-3

]

R [cm]

X-mode_6kW O-mode_6kW


Cham

ber Port

30 35 40 45 50 55 60 65 70 75 80 853

6

9

12

15

18

21

T e [eV

]

R [cm]

X-mode_6kW O-mode_6kW


Cham

ber Port

The density and temperature of LFS O mode injection is similar to LFS X. Reflected MW diagnostics shows OX and no XO mode conversion.

300 320 340 360 380 400 420 440 460 480 5000

100

200

300

400

500

600O-mode Injection

Ref

lect

ed P

ower

[W]

Time [ms]

Upper_X Main_X Upper_O

300 320 340 360 380 400 420 440 460 480 5000

100

200

300

400

500

600

Ref

lect

ed P

ower

[W]

Time [ms]

Upper_X Main_O Upper_O

X-mode Injection

OnlyX-mode

O-mode

X-mode

Pre-ionization experiment in VESTComparison Between O & X Mode Injection

20/20LHY_A3_JAN07


Summary To confirm the effect of ECH and EBW mode conversion

on the wall reflection, the experiment is prepared.

With the simulation named as COMSOL based on the experiment result, the feasibility of EBW mode conversion near the steep density gradient is found.

In LFS injection, the feasibility of XB & OXB mode con-version is confirmed and higher plasma density with the low magnetic field not in the ECR, but near the UHR as same as the simulation result.

In case of LFSX injection, XB mode conversion is oc-curred directly due to the steep density gradient and multi pass power absorption with opposite side wall and in case of LFSO, OXB mode conversion may occur in the UHR and O cutoff layer.

The pre-ionization experiment in VEST is conducted and the result also shows the feasibility of XB and OXB mode conversion near the UHR.

Back up

22/20LHY_A3_JAN07


300 400 500 600 700 800 9000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

n e [1017

m-3]

Microwave Power [W]

O_Slit Wall No Wall X_Slit

LFSX injection

No_Wall O_Slit X_Slit0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Pow

er (a

.u.)

Wall

Leakage MW power monitoring

Preliminary ECH Experiment Result(3)

X_Slit/O_Slit : The wall with X/O mode MW radiation when reflec-tion is occurred in the wall

Density : Wall > X_Slit > O_Slit > No_Wall Over dense plasma and X mode polarized wave effect shows the

evidence of the EBW mode conversion(XB) The result of monitoring the leakage MW

No_wall > O_Slit > X_slit

23/20LHY_A3_JAN07


Future Work 7.9GHz MW source installation

• Inboard startup using trapped particle configuration – the peak density of the pre-ionization plasma near the inboard side.

• Study for the ECH harmonic efficiency

EBW mode conversion experiment• Closed flux surface formation in trapped

particle configuration – the possibility of the steep density gradient to make EBW mode conversion

• In conventional startup, the steep density gradient will be made using local limiter in front of the waveguide launcher

Non-inductive current drive experiment• In steady state plasma, the possibility of

the ECCD/EBCD will be investigated.0 0.2 0.4 0.6 0.8 1

-1.5

-1

-0.5

0

0.5

1

1.5Coil Geometry

R (m)

Z (m

)

Various EC Resonance Layer in VEST

O-modecutoff

7.5GHz FHM 2.45GHz FHM7.5GHz THM

2.45GHzUHR

7.5GHz SHM

24/20LHY_A3_JAN07


1e16 1e16 LFSX 5e165e16LFSX injectionLFSO injection Experiment Setup LFSX injection

25/20LHY_A3_JAN07


Documents

The 2 nd A3 Workshop on ST January 6 th ~8 th , 2014