Progress of RF system development for LHFW Heating and Current Drive on VEST

2016 Korea-Japan Workshop

16th. Dec. 2016

S. H. Kima, B. K. Junga, S. H. Jeonga, H. W. Leeb, B. J. Leeb, J. G. Joc, H. Y. Leec, and Y. S. Hwangc

aKorea Atomic Energy Research InstitutebKwangwoon University

cSeoul National University

2

Contents

Introduction to LHFW (Lower Hybrid Fast Wave)

- LHFW on CMA / Dispersion relation / Propagation Boundary

- Advantages & Disadvantages

Status of RF system development for LHFW experiment on VEST

- Klystron

- RF components

- Antenna

- Safety & Diagnostics

Summary

3

Introduction to LHFW(0)

The continuous current drive is one of the key issues toward steady-state

operation of tokamak fusion reactors.

Though the current drive by using bootstrap is the main strategy in tokamak,

the external current drive is still required to control pressure profile and fill the

gap between the bootstrap and target current.

Most efficient current drive ever is the LHCD which is a scheme by using slow

wave in lower hybrid resonance frequency range.

There, however, is density limit and it is hard to propagate into the central

region in high temperature plasmas due to highly damping.

Therefore, alternatives are required.

LHFW could be an alternative.

4

Introduction to LHFW(1)Lower Hybrid Fast Wave(LHFW) on CMA diagram

<H2 plasmas>

LHFW is a fast wave

between lower hybrid

resonance frequency

and electron cyclotron

resonance frequency.

2 lh ceω ω ω< <<

5

Dispersion Relation1/ 22

||

1/ 22 2|| ||

2||

( )

( )( )( )

⊥

− − ≅ − −− −

r

P N SSlow Wave

SN

N R N LFast Wave

N S

Introduction to LHFW(2)Dispersion & Propagation Boundary

Propagation Boundary

The perpendicular wave number of LHFW is less than

that of slow wave in this frequency range(LHSW).

1/ 2 3 2,

1/ 2 3 2

, 2 2||

2

exp( )

exp( )

1

r SW

i r FWce

pe

N LHSW

N N LHFWN

π η η

π η ηωω

⊥

⊥ ⊥

− −≅ −

The imaginary wave number of LHFW becomes

higher as the density increases.

6

Advantage of LHFW(1)Parallel electric field Ez to B

<Polarization of FW and SW on CMA>

It has favorable polarization for current drive compared to FWs in other frequency range.

|| ||

22

0 ||2||0|| /

12

ps sLD z

v k

FP v v dv Evk

ω

ω ωε ω πω

∞

⊥ ⊥

=

∂≅ − ∂

∫

7

It has deep penetration property in high density.

<Angle of Group Velocity Vg to magnetic field of LHFW and LHSW>

Advantage of LHFW(2)More perpendicular Vg to B

B

Vg

8

Launching condition

Accessibility condition.

( )

202

20||2 1

e

launche

ce

m LHSWen

m N LHFWe

ε ω

ε ωω

≅ −

2 20||2

14

econfluence ce

mn Neε ω≅

Disadvantage of LHFWCoupling Issue

< Propagation Boundary >

* High density is required to launch

LHFW compared to LHSW.

☞ Coupling Problem

* The same accessibility condition as LHSW

should be satisfied.

* The confluence density condition above is a more generalized accessibility condition.

2||~ ce Nω

ω

LHFW launching

LHSW launching

9

RAY tracing simulation on VEST (GENRAY)

Comparison between LHFW and LHSW launching (0.2T/ 500 MHz / 3x1018 #/m3/ 3 keV/ N||=4)

LHFW can propagate into more central region. In addition, the current drive is comparable to

LHSW.

LHFWlaunching

LHSWlaunching

100 kA / 10kW125 kA / 10kW

Normalized radius

10

RF System for LHFW experiment on VEST

Schematic of LHFW RF System

- Frequency : UHF

- Power : 10 kW(Klystron)

- Antenna : Comb-line antenna

Klystron (UHF)

- Old klystron system for UHF broadcasting has been refurbished.

11

Klystron (1)

VarianFrequency 470~700 MHzOutput 37.5 kWDrive Power 600 mWGain 48 dBBeam voltage 19.5 kVBeam current 5.4 AElectrode voltage 19.5 kVHeater voltage 7 VHeater current 17 ABody current 50 mAMagnet Voltage 145 VMagnet Current 32 AMagnet Cooling Water(2.0 gal/min)Cooling(Collector) Water 1.5 gal/minCooling(Body) Water 2.0 gal/min

Cooling(Gun/Heater) Forced Air 50 ft3/min

Harris transmitter frame(SNU)

Varian Tube (SNU/KAPRA)

NEC Beam P/S(KAPRA) Harris UHF transmitter Specification

12

Klystron (2)

Front control panel

Controlpanel

High VTR

IVRTR

RectifierDiode

Rectifiercapacitor

RectifierInductor

ModulatingAnode

Surgeprotector

Front view of Inside

Refurbished klystron beam power supply : NEC (15kV-6A)

Rear view of Inside

13

Klystron (3)

Test of beam power supply : NEC (15kV-6A)

DC beam P/S (15kV-6A)

Air cooled resistor assembly : 3.58 kOhm

Test of the vacuum degree

14

Circuit for Vacuum test

- Normal conditionGas current(Digital volt meter) < 0.5uA(5V)

- Measurement : 0.1772uA (1.772 V )- Filament : 7V -17Aa

Filament V, I

Gas I

Klystron (4)

Measurement of beam guiding magnet

15

Klystron (5)

Magnet (Harris)- Resistance : 2.5 Ohm- Water cooling : 7.6 lpm

DC P/S (In-housing)- Output : 80V-32A - Input : 220V (3ph)

Integrated of klystron RF system

16

Klystron (6)

DC P/S20kV-6A

Dummy Load(10kW CW)

Magnet

H/V, Filament Connector

Collector

T/L3-1/8 EIA

Cooling WaterInlet

Test of electron beam extraction

17

Klystron (7)

Test of the klystron performance (frequency response & power)

18

Klystron (8)

- Beam V, I : 15 kV, 2.5 A / Mod. Anode : 13 kV- Bandwidth variation will be tried with cavity tuning.

- Beam V, I : 15 kV, 2.5 A / Mod. Anode : 13 kV- Frequency : 651 MHz- 10 kW power generation is achieved.

19

RF components – DC breaker/VFT

Power transmission components( DC breaker / Vacuum Feed-through)

Vacuum Feedthru

Model Model

VSWR<1.1

VSWR<1.1DC breaker under H.V. test

DC breaker (Model/VSWR/H.V.test)

Research Objective Consideration Points

Frequency UHF Antenna Length ~ 418 mm

Bandwidth < 50 MHz Antenna width < 200 mm

VSWR < 3 E-field ratio(Ey/Ez) 4~6

Peak n∥ 3 ~ 5 Peak E-field < 30 kV/cm

<Antenna Design>

<VEST>

LHFW Antenna(1) : comb-line

20

Antenna Structure

LHFW Antenna(2)

Faraday shield

418 mm

174 mm

Port 1 : source

Port 2 : dummy load

Comb-line current strap

128.5 mm

31.5 mm

Simulation Domain for Design(COMSOL)

Antenna in Vacuum

Plasmas or Vacuum

Perfect Matched LayerPeriodic B.C. Periodic B.C.

21

S-parameters (500 MHz) S11 = -25.6 dB S21 = -0.12 dB

S-parameters & field distribution (Vacuum load)

LHFW Antenna(3)

22

S-parameters (500 MHz) S11 = -28.1 dB S21 = -4.0 dB

S-parameters & field distribution (Plasma load)

LHFW Antenna(4)

23

470 475 480 485 490 495 500 505 510 515 520-20

-15

-10

-5

0

S11 S21

Frequency [MHz]

|S-p

aram

eter

s| [d

B]

Antenna Parallel wave number N|| ~ 3-5 S parameters : S11 <-10 dB, S21 ~ -5 dB

480~510 MHz

S21 ~ -5 dB

S11 < -10 dB

480~510 MHz

N|| : 3.5~4.7

LHFW Antenna(5)

24

470 475 480 485 490 495 500 505 510 515 5203.0

3.5

4.0

4.5

5.0

5.5

Frequency [MHz]

n ||

(Plasma load)

25

RF diagnostic (1)

Measurement : Forward/Reflected power, Voltage at antenna/TL

26

RF diagnostic (2)

Schematic of peak/phase detector & safety circuit

27

RF diagnostic (3)

Fabricated peak/phase detector & safety circuit

<Safety box>

Peak Detectors

Gain Phase Detector

Power Splitters

Comparators

RF switch

<Test signal of safety box>

TTL signal generated for RF

switchingReference

for switching

Test input

<Peak Phase Detector>

<Gain-Phase on Smith> <Peak measured>

Gain : 10, Phase : 117°

28

Summary

The steady-state operation is one of the key issues of tokamak fusion reactor.

A new concept of current drive by using a fast wave in lower hybrid frequency range(LHFW)

is suggested and the proof of principle experiment is planned on VEST.

For the experiment,

Klystron RF power system are prepared through collaboration with SNU and KAPRA .

It is refurbished and integrated, and confirmed that 10 kW power can be stably generated.

For LHFW launcher, compact comb-line antenna has been designed with COMSOL

simulation code by KWU. As a result, it is shown that it can couple 10 kW power to plasmas

with N|| 3~5.

RF diagnostic and safety device are also successfully fabricated and tested.

All RF system will be assembled after fabrication of LHFW antenna and integration

test will be carried out sooner or later.

29

Continuous Current Drive (1)

The continuous current drive is one of the key issues toward steady-state

operation of tokamak fusion reactor.

M. Kikuchi

30

Continuous Current Drive (2)

Though the current drive by using self-generated bootstrap is the main strategy,

the external current drive is still necessary to fill the gap between the

bootstrap and target current, and to control the current and pressure profile.

Nuclear Fusion 30(1990) 343

1/ 2

~bT dnj

B drθ

ε−

1/ 2~bp

I cI

ε β

Fusion Eng. Des. 38(1997) 27

31

Continuous Current Drive (3)

The most efficient external current drive scheme is the LHCD where slow wave in

lower hybrid resonance frequency range(LHSW) is used.

It, however, has inherent density limit and the high efficiency, paradoxically, hinders it

from being used for reactor grade high temperature plasmas.

Therefore, alternatives are necessary.

Tokamaks (2004) J. Wesson

SchemeCurrent drive efficiency η :

A/m2/WMachine

LHCD ~ 0.34 (3.6 MA) JT-60U

ICRF ~ 0.04 (180 kA) DIIID

ECCD ~ 0.03 (150 kA) TCV

NB ~ 0.05 (340 kA) TFTR

32

Propagation and Absorption of LHFW on VEST

Real and Imaginary perpendicular wave number (0.2T/ 500 MHz /3x1018#/m3/ N||=4)

1/ 22||

1/ 22 2|| ||

2||

( )

( )( )( )

r

P N SLHSW

SN

N R N LLHFW

N S

⊥

− − ≅ − −− −

1/ 2 3 2,

1/ 2 3 2

, 2 2||

2

exp( )

exp( )

1

r SW

i r FWce

pe

N LHSW

N N LHFWN

π η η

π η ηωω

⊥

⊥ ⊥

− −≅ −

33

RAY tracing simulation (GENRAY) Comparison between LHFW and LHSW launching

(0.2T/ 500 MHz / 3x1018 #/m3/ 3 keV/ N||=4)

LHFW can propagate into central region. In addition, the current drive is comparable to

LHSW.

(0.2T/ 500 MHz / 3x1018 #/m3/ 3 keV)

34

Coupling efficiency (1D full(cold)+WKB boundary at RHS)

Comparison between LHFW and LHSW launching

for LHFW launching for LHSW launching

Parameters Values

B0 0.2 TFrequency 500 MHzN∥ 4.0

n0 6x1015 ~ 5x1017 #/m3

nb 4x1015 #/m3

d(X2-X1) 3 cm

X1 2 cm