design of standing wave vft and mtc for kstar helicon...

Korea-Japan Workshop, Seoul, march 19-20 2019

Design of standing wave VFT and MTC for KSTAR helicon current drive

systemKwangho Janga, Sonjong, Wanga, Kenji Saitob, Hyunho Wia, Jeehyun Kima,

Hyungyong Leea and Jonggu Kwaka

aNational Fusion Research Institute, 169-148, Gwahak-ro, Yuseong-gu, Daejeon, KoreabNational Institute for Fusion Science, National Institutes of Natural Sciences, 322-6

Oroshi-cho, Toki, Gifu 509-5292, JapanE-mail : [email protected]

발표자

프레젠테이션 노트

Good afternoon. My name is Kwangho Jang for NFRI I graduated last February and joined the KSTAR current drive research team in last May. I’m delighted to have this chance to speak in front of you. Today, my topic of presentation is “Design of standing wave VFT and MTC for KSTAR helicon current drive system.”

• Introduction

• Problems during previous helicon current drive system experiment

• Design of high power Vacuum Feedthrough (VFT)

• Design of Multipactor Test Chamber (MTC)

• Summary

Korea-Japan Workshop on Physics And Technology of Heating and Current Drive /152

Contents

발표자


Contents is shown as follows. First of all, I want to start by introduction and problems during previous helicon current drive system experiment Secondly, I’m going to talk about design of high power vacuum feedthrough and multipactor test chamber Finally, I’ll will finish the presentation with a summary.

• Steady-state tokamak scenarios require sufficient off-axis current drive.

• High frequency, (ωci << ω ≤ ωLH) helicon wave current drive (HCD) is being tested for efficient off-axis current drive in high beta plasmas.

• After confirming the coupling characteristics at low power, we carried out the experiment with power about 100kW, but we found a effect similar to multipactor in Vacuum feedthrough (VFT).

• We need a high power VFT study considering multipactor effect for high power injection into plasma.

• Additionally, we designed the Multipactor Test Chamber (MTC) for analysis of multipactor effect with high power


Introduction

발표자


Steady-state tokamak scenarios require sufficient off-axis current drive. High frequency, helicon wave current drive is being tested for efficient off-axis current drive in high beta plasmas. After confirming the coupling characteristics at low power, we carried out the experiment with power about 100kW, but we found a effect similar to multipactor in Vacuum feedthrough (VFT). We need a high power VFT study considering multipactor effect for high power injection into plasma. Additionally, we designed the Multipactor Test Chamber for analysis of multipactor effect with high power

Korea-Japan Workshop on Physics And Technology of Heating and Current Drive

Problems during previous helicon current drive system experiment (1/2)

2017 campaign

VFT

Alumina

• In the 2017 campaign, we found traces of alumina damage in the VFT used in HCD system experiments.

• As a results of component analysis, most of them were identified as copper particles.

• Perhaps this is due to multipactoring or arcing.

발표자


In the next slide, I will present the problems during previous Helicon Current Drive system experiment The KSTAR helicon current drive system has confirmed the low power coupling of the mock up type Traveling wave antenna (TWA). The first problem was found in the 2017 campaign. in the 2017 campaign, we found traces of alumina damage in the VFT used in helicon current drive experiments. As a results of component analysis, most of them were identified as copper particles. Perhaps this is due to multipactoring or arcing.


Problems during previous helicon current drive system experiment (2/2)

Conductive surface(in Teflon)

2018 campaign

VFT

• In the 2018 campaign, we tried to use TiNcoated alumina to reduce the multipactoreffect, but we replaced it with Teflon because manufacturing problems.

• KSTAR Shot 3555 showed the phenomenon that the transmitted power gradually disappears.

• As a result, the expected evidence for a multipactor was shown in the figure.

발표자


In the 2018 campaign, we tried to use TiN coated alumina to reduce the multipactor effect, but we replaced it with Teflon because manufacturing problems. This KSTAR Shot showed the phenomenon that the transmitted power gradually disappears. As a result, the expected evidence for a multipactor was shown in the figure. The VFT inner conductor appears to have been stripped of metal and Teflon coated with a conductive material.


What is multipactor?

• In its simplest form, multipactor discharge occurs when electrons move back and forth between two electrodes in synchronism with an RF field

• In this figure, if the secondary emission coefficient of the electrodes is greater than unity, then the number of electrons involved in the process builds up with time.

• This phenomenon is a multipactor and the emission coefficient is called Secondary Electron Yield (SEY).

• During multipacting, large amounts of energy can be discharged in a small volume with the result that electrodes or RF windows can be damaged or destroyed[1].

[1] A.S. Gilmour, Jr., “Klystrons, traveling wave tubes, magnetrons, crossed-field amplifiers, and gyrotrons”, Artech house, 2011

발표자


So what is a multipactor? In its simplest form, multipactor discharge occurs when electrons move back and forth between two electrodes in synchronism with an RF field In this figure, if the secondary emission coefficient of the electrodes is greater than unity, then the number of electrons involved in the process builds up with time. This phenomenon is a multipactor and the emission coefficient is called Secondary Electron Yield (SEY). During multipacting, large amounts of energy can be discharged in a small volume with the result that electrodes or RF windows can be damaged or destroyed[1].


Solution for reducing multipactor effect

• Remedy is being introduced• Geometry optimization• TiN coating• Intensive conditioning

f (MHz)

Vop

(kV)

Geometry optimization [3,4]

TiN coating [5]

Intensive Conditioning [6]

Multipactoring probability in 6-1/8” coax [2]

VFT

MTC

[2] S.J. Wang, et al., 20th IOS-TG Meeting, (2018)[3] M.Neubauer, et al., IPAC’10, (2010)[4] E. Kako, HF2014, (2014)[5] V.Baglin, et al., EPAC 2000, (2000[6] Jefferson Laboratory, 30th HPCW, (2002)

발표자


This multipactoring should be noted in the HCD frequency band. in this graph, it shows higher probability of occurrence toward red color. The frequency band and power of helicon current drive system are prone to multipactoring. So, there are three ways to solve this problem. The remedy is Geometry optimization, TiN coating and Intensive conditioning. Geometry optimization is to reduce the electric field by structure and design technic Previously used geometry optimizations include double alumina type and choke type and so on. Secondly, the graph shows that TiN has a lower SEY value than alumina. So there is a method to coat alumina to reduce SEY. Finally, one of the most common methods in accelerators is to overcome the problem using conditioning. We will apply geometry and TiN coating to VFT, and we will analyze TiN coating and conditioning in MTC.


Design of high power Vacuum Feedthrough (VFT) (1/2)

• Design main point• Low value of absolute electric field at alumina

• Low value of Z-direction (RF propagation) electric field (EZ) at alumina

• The outer and inner diameter of the alumina were made the same as the 50 ohm coaxial line for

reduction EZ.

• There two impedance transformer match the VFT and minimize the electric field in the alumina. at

this structure, the electric field is like a standing wave.

Ez=1.4kV/cm

(Pin = 1MW)

Very low Ez

Outer conductor

Inner conductor

General Coaxial VFT

Alumina

Standing wave VFT

Ez

V am

plitu

de

Minimum E-field point

Z

ZT = Impedance transformer

(ɛr=9.9)ZT ZT

발표자


This slide is VFT design part. there are two main considerations in the VFT design. The absolute value of the electric field in alumina should be small. And the z direction value of the electric field in alumina should be small. The first design part is a modification of the alumina joint. The joint of general VFT has a structure with a high Ez value. This value is 1.4kV/cm with 1MW input power. So, the size of the inner conductor and outer conductor of the alumina were changed for to reduce EZ value. So the impedance of alumina was changed from 50 ohm to 15.8ohm. There two impedance transformer match the VFT and minimize the electric field in the alumina. at this structure, the electric field is like a standing wave.


Design of high power Vacuum Feedthrough (VFT) (2/2)

-400 -200 0 200 4000

2

4

6

8

10 Eabs Ez

Distance (mm)

Eabs

(kV/

cm)

0.0

0.5

1.0

1.5

2.0

2.5

Ez (V

/cm

)400 450 500 550

-50

-40

-30

-20

-10

0

S-pa

ram

eter

s (d

B)Frequency (MHz)

S11 S21

RF direction

VacuumAir

• The VFT was designed to have the lowest electric field in alumina by impedance transformer. In this

structure, Eabs is 1.6 kV/cm and nearly half compared to conventional VFT. EZ is negligible in alumina.

• The center frequency of the VFT is designed to be 476MHz, which is the operating frequency of the MW

klystron.

Alumina

Z

Y

Simulation conditionPin = 1MWFrequency = 476MHzCeramic = Alumina (Kyocera A476B)

~λ/2 ~λ/2

Eabs : absolute E-fieldEZ : Z-direction (propagation) E-field

I.T. : Impedance transformer

I.T.I.T.

발표자


The designed VFT has the following design conditions. Input power is 1MW, operating Frequency is 476MHz and ceramic is Kyocera A476B alumina. The VFT structure is as follows. This direction is RF propagation direction. This is alumina and two impedance transformer. The VFT was designed to have the lowest electric field in alumina by impedance transformer. In this structure, Eabs value is 1.6 kV/cm and nearly half compared to conventional VFT. EZ value is negligible in alumina.


Mechanical simulation for fabrication of ceramic junction

Equivalent stress

Thermal stress

Deformation

Modeling

Air area

Vacuum area

Alumina (A479B)

Thin Kovar

RF Pin = 1MW(HFSS loss data import)

• We analyzed the deformation of each

component in MW power transmission to

develop high power VFT.

• The mechanical design was carried out

to minimize the stress of parts due to

thermal expansion.

• Thermal loss was estimated by using

volume loss and surface loss when 1MW

power was transmitted using HFSS.

• Alumina joint was finally designed

considering yield strength.

• In addition, one vacuum side of alumina

will be coated with TiN to reduce

multipactor effect.

Equivalent stressKovar = 193MPa (Y.S. = 270MPa)Alumina = 107MPa (Y.S.= 300MPa )Y.S. : Yield Strength

발표자


Next slide is mechanical simulation results. We analyzed the deformation of each component in MW power transmission to develop high power VFT. The mechanical design was carried out to minimize the stress of parts due to thermal expansion. Thermal loss was estimated by using volume loss and surface loss when 1MW power was transmitted using HFSS. Alumina joints were finally designed considering yield strength. Kovar stress is 193MPa, alumina stress is 107MPa. Both parts were lower than the yield strength. In addition, one vacuum side of alumina will be coated with TiN to reduce multipactor effect.


Design of the Multi-pactor test chamber (MTC) setup

• Purpose of experiment• Identify multipactor effect at high power.

• Analysis to overcome multipactor effect.

• Analysis of electric field characteristics at 1 MW using 50kW power source.

VCO SSPA

Klystron(~50kW,490MHz)

Impedance transformer

Impedance transformerDUT

Vacuum pump Measurement

Circulator

Dummy load Dummy load

RF propagation

T.L. (6-1/8”)

T.L. (6-1/8”)

Multipactor Test Chamber (MTC)

발표자


The next slide is multipactor test chamber. The main purposes are as follows. Identify multipactor effect at high power. Analysis to overcome multipactor effect. Analysis of electric field characteristics at 1 MW using 50kW power source. The MTC experiment configuration is as follows. This configuration is arranged in the order of high power source, circulator, MTC, and dummy load. A vacuum pump is used for the MTC to check the multipactor effect in the vacuum condition. Next, I will explain the structure of MTC


Structure of Multipactor test chamber (MTC)

Quartz

V am

plitu

te

Max 15kV/cm from 50 kW

Sample holder

Z-axis

RF propagation

Directional coupler

Light sensor

Vacuum pump Pressure gauge

• Design of Multipactor Test Chamber (MTC)• A standing wave is used to generate a high electric field using low power injection.

• The impedance transformer is designed to generate a high electric field on the sample as opposed

to the VFT concept.

발표자


The specific structure of MTC is shown in the following figure. The basic concept of MTC is similar to VFT. However, it different from using the maximum electric field of the standing wave. As shown in the figure, the two impedance transformers create VFT matching condition and standing wave. The goal of this structure is to obtain a maximum electric field of 15kV/cm in the sample with 50kW input power. This electric field value is the maximum electric field of designed 1 MW antenna.


Simulation results of MTC

Quartz

Sample~15kV/cm


RF Transmission direction(Pin=50kW)


Quartz

Vacuum pumping

Pressure gauge

~λ/2

• The impedance transformer focused the electric field at the center of the MTC, and it used a

sample holder to generate a stronger electric field.

• A maximum electric field of 15 kV/cm was generated in the MTC sample position.

• Generally easily available Teflon and Quartz are often used for vacuum sealing at low power VFT.

Melting point Alumina : ~2100 C°Quartz : ~1600 C°Teflon : ~327 C°

발표자


The next slide is the designed MTC using simulation. The impedance transformer focused the electric field at the center of the MTC, and it used a sample holder to generate a stronger electric field. A maximum electric field of 15 kV/cm was generated in the MTC sample position. At that time, the input power is 50kW. Generally easily available Teflon and Quartz are often used for vacuum sealing at low power VFT. Among them, we designed using quartz.


Simulation results of RF design

14/15

-100 -50 0 50 100

0

5

10

15

E-fie

ld (k

V/cm

)

Distance (mm)

S11

450 460 470 480 490 500 510 520 530-60

-50

-40

-30

-20

-10

0

S-pa

ram

eter

s (d

B)

Frequency (MHz)

S11 S21

Sample position

• Quartz was chosen because it has a low Secondary

Electron Yield (SEY) value among the available

components at low power.

• Simulation results show an electric field strength of about

15kV/cm in the sample.

• This value is the maximum field strength of the 1MW

antenna structure.

Melting point Alumina : ~2100 C°Quartz : ~1600 C°Teflon : ~327 C°

발표자


Quartz was chosen because it has a low Secondary Electron Yield (SEY) value among the available components at low power. And it can withstand temperatures higher than Teflon. As shown in the figure, Simulation results show an electric field strength of about 15kV/cm in the sample. This value is the maximum field strength of the 1MW antenna structure. The designed MTC will analyze the multipactor and study how to improve it using coating and conditioning.

• During the HCD experiment, multipactor effect were found in VFT

• So, we designed the high power VFT and MTC to solve this problem.

• The main point of both designs is to use the E-field maximum and minimum of the standing

wave.

• VFT is designed to minimize Eabs and Ez in order to reduce the multipactor.

• Eabs value has been reduced to nearly half value compared with the previous VFT. Ez value is

negligible value.

• Currently, the alumina joint is on going fabrication. In additionally, TiN coatings will be applied in

alumina surface.

• The MTC is designed to test and analyze the multipactor effect that can occur in 1MW systems.

And it is currently under construction.


Summary

발표자


Finally a summary During the HCD experiment, multipactor effect were found in VFT So, we designed the high power VFT and MTC to solve this problem. The main point of both designs is to use the E-field maximum and minimum of the standing wave. VFT is designed to minimize Eabs and Ez in order to reduce the multipactor. Eabs value has been reduced to nearly half value compared with the previous VFT. Ez value is negligible value. Currently, the alumina joint is ongoing fabrication. In additionally, TiN coatings will be applied in alumina surface. The MTC is designed to test and analyze the multipactor effect that can occur in 1MW systems. At present, the structure is designed to obtain an electric field of 15kV/cm at 50kW input power and It is currently under construction.

16

Thank you for your attention

발표자


Thank you for your attention!

design of standing wave vft and mtc for kstar helicon...

Documents