Center for Advance Research in Fusion Reactor Engineering

Center for Advance Research in Fusion Reactor Engineering

Design Features and Commissioning of Versatile Experiment Spherical Torus (VEST) at Seoul National University

The 8th General Scientific Assembly of Asia Plasma and Fusion Association (APFA 2011)

November 1-4, 2011, Guilin, China

K. J. Chung, Y. H. An, B. K. Jung, H. Y. Lee, C. Sung, Y.S. Na and Y. S. Hwang

Department of Nuclear Engineering Seoul National

University

2/25 Versatile Experiment Spherical Torus (VEST) at SNU

Outline

Background & Motivation Introduction to VEST Device Design Features

Key Design Points with Operational Scenario Research Plans

Commissioning Vacuum Chamber and Center Stack TF Coil System PF Coil System ECH Pre-ionization System Diagnostics Plan Port Assignment

Summary


Background & Motivation

Spherical Torus (ST)

Advantages

• High performance : High β, High plasma current• Compactness

Weakness

Difficulty in start-up & sustainmentdue to the lack of space for solenoid

Innovative start-up method is critical issue for ST!By developing new start-up and non-inductive current drive methods,

ST can be a high performance fusion research device.

R

a

Spherical Torus (ST) : Low aspect ratio (A<2) fusion device

Small aspect ratio(Spherical Torus)Small aspect ratio(Spherical Torus)

Large aspect ratio(Conventional Tokamak)

Large aspect ratio(Conventional Tokamak)


Introduction

VEST Status

23 June 2011VEST successfully installed at SNU

23 June 2011VEST successfully installed at SNU

Man Power, but NO Power yet


Introduction

VEST(Versatile Experiment Spherical Torus)

Initial Phase Future

Chamber Radius [m]0.8 : Main Chamber0.6 : Upper & Lower Chambers

Chamber Height [m] 2.4

Toroidal B Field [T] 0.1 0.3

Major Radius [m] 0.4 0.4

Minor Radius [m] 0.3 0.3

Aspect Ratio >1.3 >1.3

Plasma Current [kA] 30 100

Safety factor, qa 7.4 6.7

Specifications

Objectives

Basic research on a compact, high- ST (Spherical Torus)

with elongated chamber in partial solenoid

configuration

Study on innovative partial solenoid start-up, divertor, etc

* Elongation : 3.3 assumed


Key Design Point

Solenoid-Free Start-up

Existing injection scenario

Compression-Merging method : Use in-vessel PF coil’s swing

(START, MAST)

Drawbacks of in-vessel PF coil Impurity

Engineering problems

Plasma injection and merging is one approach for solenoid-free start-up

Double Null Merging (DNM) : Use Outer PF coil swing (UTST)

Limitation : Hard to get equilibrium


Key Design Point Double Null Merging Start-up with Partial Solenoids

Partial Solenoid

Partial Solenoid

Plasma Merging

Relatively smaller space for central solenoid in ST.Hard to supply sufficient magnetic flux.

Relatively smaller space for central solenoid in ST.Hard to supply sufficient magnetic flux.

Solenoid Start-up

Partial Solenoid Operation

• Inherits the merits of solenoid start-up• Possible to maintain low aspect ratio

Effective Start-up method in Spherical Torus

• The most effective start-up method• High shaping and equilibrium ability• Hard to keep low aspect ratio• Difficult to apply to spherical torus

Breakdown by Partial Solenoid



Operational Scenario

Loop Voltage Calculation with Circuit Model

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-500

0

500

1000

1500

2000

2500

3000

Time [ms]

Cur

rent

[A]

Eddy current

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-4

-2

0

2

4

6

8

10

12

14

Time [ms]

Loop

vol

tage

[V]

Loop voltage( R=0.46, Z=0.98)

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) Most severe at thick cover

and wall close to PF2Most severe at thick cover

and wall close to PF2

Eddy current decay most slowlyin thick cover wall

Eddy current decay most slowlyin thick cover wall

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-500

0

500

1000

1500

2000

2500

3000

Time [ms]

Cur

rent

[A]

Eddy current

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-4

-2

0

2

4

6

8

10

12

14

Time [ms]

Loop

vol

tage

[V]

Loop voltage( R=0.46, Z=0.98)

Without eddy currentWith eddy current

Eddy current @ Chamber wall

Loop voltage deceasedby eddy current

38% Reduced

eddy currents are accounted in this model


Operational Scenario

Field Null Formation and Lloyd Condition

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-1000

-800

-600

-400

-200

0

200

400

600

800

1000

Time [ms]

Cur

rent

[A]

PF2 & 3 / PF 5 / PF 8 ( R=0.46, Z=0.98)

PF 2&3PF 5PF 8

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-140

-120

-100

-80

-60

-40

-20

0

20

40

Time [ms]

Lloy

d C

ondi

tion

Lloyd Condition (R=0.46, Z=0.98)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-1000

-800

-600

-400

-200

0

200

400

600

800

1000

Time [ms]

Cur

rent

[A]

PF2 & 3 / PF 5 / PF 8 ( R=0.46, Z=0.98)

PF 2&3PF 5PF 8

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30-140

-120

-100

-80

-60

-40

-20

0

20

40

Time [ms]

Lloy

d C

ondi

tion

Lloyd Condition (R=0.46, Z=0.98)

Lloyd condition Max. at ~15 ms> 100 V/m for 0.7 ms

Lloyd condition Max. at ~15 ms> 100 V/m for 0.7 ms

Field Null

PF2, 3, 5, 8 Current WaveformPF2, 3, 5, 8 Current Waveform

Lloyd condition:

100 /B

E V mB

Field null formation with circuit model

* Induced eddy current at the chamber wall considered.

Lloyd condition


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)

Operational Scenario Start-up Scenario utilizing Pressure Driven Current

Trapped Particle Configuration

Trapped Particle Configuration

Pressure Driven Current by ECH HeatingPressure Driven Current by ECH Heating

• Trapped Particle Configuration at midplane.

• Pressure driven current by ECH heating

Opposite direction to the main plasma current induced by partial solenoid.

But, it can be used as virtual coil for further optimization of null formation.



X

∙

Pressure driven current by ECH

2.45GHz3kW

2.45GHz3kW∙

2.45GHz6kW


Research Topics

Sequential Tokamak Injection for Ramp-up

Solenoid 0A +40kAt

+40kAt

PF #1 -0.044kAt PF #1 -0.044kA

PF #2 -0.449kA

PF #3 -1.833kA

PF #4 -6.546kA

Solenoid 0A

PF #1 -0.044kAt

PF #4 -6.55kAt

PF #2 -0.45kAt

PF #3 -1.83kAt

0A

0A

PF #2 -0.449kAt

PF #3 -1.833kAt

PF #4 -6.546kAt

Plasma injection

Small Plasma

Small Plasma

Solenoid charging

Solenoid Swing Down

Ip ~ 12kA Ip ~ 12kA Ip >12kA

Study on the feasibility of a sequential tokamak plasma injection methodfor maintaining plasma currents through partial solenoid operations.Study on the feasibility of a sequential tokamak plasma injection methodfor maintaining plasma currents through partial solenoid operations.


Research Topics

EC/EBW H&CD

2.45GHz System under Preparation EBW heating and current drive will be studied2.45GHz System under Preparation EBW heating and current drive will be studied

• Use of ECRH is limited in ST device due to cutoff density.

• However, Electron Bernstein Wave (EBW) heating through mode conversion is possible since EBW has no cutoff density.

[V.F. Shevchenko, et. al., Nucl. Fusion 50 (2010) 022004]Schematic of the EBW assisted plasma current start-up in MAST

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)

O-modecutoff

5GHz ECR7.5GHz ECR

2.45GHz FHM5GHz SHM

7.5GHz THM

HFS Launching

LFS Launching

2.45GHzUHR

Various EC Resonance Layer in VEST

Various EC Resonance Layer in VEST


Research Topics

Innovative Divertor Concepts• Control of heat flux and neutron flux is one of important issue in fusion research

• ST with high heat density and compactness is appropriate to heat flux study.

• VEST with enough space for divertor and sufficient number of PF coils is suitable for divertor research

Innovative divertor concepts such as super-X and snow-flake divertors will be investigated.Innovative divertor concepts such as super-X and snow-flake divertors will be investigated.

Enough Space for Divertor

Enough Space for Divertor

8 pairs of PF coils

8 pairs of PF coils

Single Null (Left) and Snowflake (Right) Configuration in TCV[F Piras et al,Plasma Phys. Control. Fusion 52 (2010) 124010 ]


0.8 m

0.6 m

0.6 m

1.2 m

~ 1.1 m

~ 2.7 m

Vacuum Chamber and Center Stack

Overall Dimensions of VEST

Overall dimension of VESTOverall dimension of VEST

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)

15mm17mm

3.4mm

2.8mm

5mm

6mm

15mm

6mm

13mm15mm

15mm13mm

Thickness of vacuum chamber wallThickness of vacuum chamber wall

PF1

PF2PF3 PF4

PF5

PF6

PF7PF8

PF9

PF10



Design and Fabrication of Vacuum Chamber

Lower ChamberLower Chamber

Upper ChamberUpper Chamber

Rectangular PortRectangular Port

12” Port12” Port

10” Port10” Port

Main ChamberMain Chamber

4” Port4” Port

6” Port6” Port

V/V : S/S 316LV/V : S/S 316L

Middle ChamberMiddle Chamber

Center Stack PartsCenter Stack Parts

Center StackCenter Stack

Center Stack and Bottom ChamberCenter Stack and Bottom Chamber



Design and Fabrication of Center Stack

Inner TF Coils

Inner TF Coils

Thin Solenoid (PF1)Thin Solenoid (PF1)

Partial Solenoid (PF2)Partial Solenoid (PF2)

Inner Pipe(PF1 Bobbin)

Inner Pipe(PF1 Bobbin)

Cu block is brazed

Cu block is brazed

Nipples for water cooling

Nipples for water cooling

G-10 BlockG-10 Block

Epoxy moldedEpoxy molded

Center StackChamber WallCenter Stack

Chamber Wall87mm

~268mm

A A

“Section A-A”

TF Coils (24ea)

PF1 Coil

87mm

2.4 m

165mm


TF Coil

Design Parameters of TF Coil

Structure of VEST TF coilStructure of VEST TF coil

Strand Specification for TF Coils

Parameters Design values

BTF [T] at R0 0.1

Coil length [m] 2.7

Wire size [mm2]Inner: 124 (12 x 12)

* Cooling channel : 6ΦOuter: 500 (50 x 10)

No. of turns [#] 24 (12 x 2 pairs)

ITF [kA] 8.33

Driving circuit Battery

R [mΩ] 15 (18.8 measured)

L [mH] 1.0 (0.93 measured)

Battery bank100 Ah battery x 200

ea

Internal Resistance of battery bank

~ 10 mΩ(200 ea Total)

V0 [V] 250(20 batteries in series)

Switching Magnetic Contactor


TF Coil

TF Coil Power Supply: Battery Banks

MC SwitchMC SwitchPneumaticSwitch

PneumaticSwitch

Fuse8.3kA for 4s

Fuse8.3kA for 4s

Emergency Safety Breaker

Emergency Safety Breaker

Battery BankBattery Bank

VEST TF CoilR = 15 mΩL = 1 mH

VEST TF CoilR = 15 mΩL = 1 mH

• Designed to able to supply up to 8.3 kA• Based on commercial deep-cycle battery• 5 banks with 40 batteries for each bank (Total 200 batteries are used to make 0.1 T)• Adv.: Cost effective and long flat-top• Disadv.: Always contain large energy

VEST TF Coil Power SupplyVEST TF Coil Power Supply


Commissioning

Battery-based TF Power Supply

TF coil current of 8.6 kA is achieved successfully by using 8 battery modules (Note: BT@R0 = 0.1 T for 8.3 kA TF coil current).

To minimize high current load on batteries during the turn-off, the sequential switch-off is adapted.

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

2

4

6

8

10

TF C

urr

ent [k

A]

Time [s]

2 Battery modules 4 Battery modules 6 Battery modules 8 Battery modules


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)

PF2

PF1PF3 PF4

PF5

PF6

PF7PF8

PF9

PF10

Partial solenoidfor plasma start-up

Long Solenoid for plasma sustaining

Null formation

Vertical stability & Control of small plasma

Equilibrium of small plasma

Vertical stability & Control of small plasma

Equilibrium of main plasma

Null formation

Null formation

PF Coil

Overall Features of Various PF Coils

Role of each PF coilRole of each PF coil

• Pancake design of PF 3~10

Pancake module : 6*2 strands4*2 strands for PF #3 & #4

• Strand Size - PF1 & PF2: 3.5mm*15mm - PF3 ~ 10 : 6.5mm*6.5mm (3.5ф hole)


PF Coil

Design Parameters for PF1 & PF2 Solenoid Coils

Parameters PF1 PF2

Initial Goal Large plasma of 30 kA Small plasma of 10 kA

Volt-sec [mV-s] ~ 55 ~ 56 (28 x 2)

Required A-T [MA] 5.2 0.21 x 2

Rin / Rout [m] 0.045 / 0.063 0.08 / 0.125

Coil length [m] 2.4 0.5 x 2

Wire size [mm2] 56.0 (3.5 x 16) 56.0 (3.5 x 16)

N [#] 632 (4 x 158) 250 (10 x 25) x 2

IPeak [kA] 7.3 0.84

Driving Circuit RLC double swing RLC double swing

R [mΩ] 68 104 (52 x 2)

L [mH] 1.6 7.4 (3.7 x 2)

C [mF] 200 / 1 / 500 10 / 0.2 / 50

V0 [kV] +1.0 / +1.0 / -2.0 +0.7 / +0.7 / -0.5

Max. achievable V-s [mV-s] @stress limit

130 545

IPeak [kA] @stress limit 27.3 14.0

BPeak [T] @stress limit 7.4 8.0

Max. sustaining time@thermal limit (90oC)

~ 50 ms ~ 180 ms

PF1 (Longsolenoid)

PF2 (Lower partial solenoid)

PF2 (Upper partial solenoid)

Stress limit:Tensile strength of Cu ~ 70 MPa


Cs1

S13

Rs1

C2C1

Cs1

S12

Rs1

Cs1

S11

Rs1

Rd1

Rd1

Rd1

Cs2

S22

Rs2

Cs2

S21

Rs2

Rd2

Rd2

C3

R_load

L_load

Cs3

S33

Rs3

Cs3

S32

Rs3

Cs3

S31

Rs3

Rd3

Rd3

Rd3

D2

D2

D1D3

Scheme: Double swing circuit Switching: Thyristor with ferrite isolation Control: Optical trigger

Thyristor Module

Gate trigger Module

Capacitor bank

HV Relay Module

SW1SW2

SW3

Power Supply for PF2 Coils

Commissioning

PF2 Coil Power Supply

-10 0 10 20 30 40 50-1000

-800

-600

-400

-200

0

200

400

600

800

1000

PF2

Coil

Curr

ent [A

]

Time [ms]

0

4

8

12

16

20

24

28

32

36

40

Mag

net

ic F

lux

[mV-s

]

C1(10 mF)

C2(0.2 mF)

C3(50 mF)

SW1

SW2

SW3

PF2 coil


Commissioning

ECH Pre-ionization for VEST

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)

2.45GHz3kW

2.45GHz3kW

2.45GHz6kW

MergedPlasma

Breakdown by Partial Solenoidwith ECH Pre-ionization

MagnetronMagnetron

Breakdown by Partial Solenoidwith ECH Pre-ionization

6” port

WR284Waveguide

Vacuum Window

ReducerWR340→WR284

WR340Waveguide

3kW, 2.45GHzMagnetron

ECH injection system for pre-ionizationECH injection system for pre-ionization

VEST Preionization

Upper & Lower Chamber•Two cost-effective homemade magnetron power supplies

(3kW, 2.45GHz)•Low field side launching

Main Chamber•Commercial microwave power supply (6kW, 2.45GHz )•Low field side launching

VEST Preionization

Upper & Lower Chamber•Two cost-effective homemade magnetron power supplies

(3kW, 2.45GHz)•Low field side launching

Main Chamber•Commercial microwave power supply (6kW, 2.45GHz )•Low field side launching


Preparation for Operation

Diagnostics Plan

Diagnostic Method Installation Status Remark

MagneticDiagnostics

Rogowski Coil Initial installationFabricated

Under fabrication3 out-vessel2 in-vessel

Pick-up Coil Initial installation Prototype Test Initially 16 pick-up coils

Magnetic Probe Array

Initial installation Under designVacuum field and merging

plasma measurements

Flux Loop Initial installation Under fabrication Initially 10 loops

ProbeElectrostatic

ProbeInitial installation Under design

OpticalDiagnostics

OES Initial installation Monochromator

Interferometry Initial installation

Fabricated(Under design of phase

comparator and supporting structure)

94 GHz

Fast CCD camera

Near Term Ordered 20 kHz

Soft X-ray array Near Term OrderedAXUV16ELGPhotodiode

Thomson Scattering

Long Term -Nd:YAG Laser 1.2J/pulse

10ns


Preparation for Operation

Port Assignment

MM 11 : Pumping Duct

UU1/MM1/LD1 : Halpha MonitoringUT1/ LB1 : ECH (Magnetron, 3kW)

UU3 : InterferometryMM3 : Laser In (TS,LIF)

UU8 : GDCMM8 : Fast CCDLD8 : Pressure Guage

UU9/LD9 : Vacuum BNCMM9 : Soft X-ray

MM10 : Vacuum BNC

UT7/LB7 : ECH (Power Supply)MM7 : Laser Out (TS,LIF)

UU5/LD5 : Fast CCDMM5 : Laser Detector (TS,LIF)

MM2 : NBI In

MM6 : NBI Dump

MM12 : 7.5 GHz Klystron 2.45GHz ECH Power, 6kW

UT4 : Piezoelectric valveMM4 : Detector for CXS

Yellow color in future plan.


Summary

Fist plasma is coming soon !

A new spherical torus named as VEST (Versatile Experiment Spherical Torus) has been built to be a low cost, compact, educational fusion research device at Seoul National University.

Operation scenarios with two partial solenoid coils are prepared by using a simple circuit model accounting eddy currents.

Various research plans such as double null merging start-up, sequential tokamak injection, innovative divertor concepts and non-inductive heating & current drive with EBW are considered.

Coil and heating powers with basic diagnostics are under preparation. Discharge cleaning and breakdown-test are ongoing.

Summary


Thank you for your attention !

VEST Installation Movie Clip


Thank you for your attention !

Collaboration is strongly Welcome !


Center for Advance Research in Fusion Reactor Engineering

(CARFRE)

<Group 3>Advance

Technologies of Fusion Energy

Conversion System

<Group 1>Fusion Reactor

Systems Integration and Plasma Control Technologies

<Group 2>Fusion Reactor

Edge Plasma Technologies

29

•Lead the development of key technologies crucial for continuous and stable operation of fusion

reactors

•Develop analysis tools and compile experimental database for fusion reactor design and systems

integration

• Foster well-trained fusion research personnel

• Promote international collaborations in fusion research

Organization: 10 Projects with 16 Principal Researchers from 5 Major Universities and 2 National InstitutesFunding: ~1M US$/yr for 6.5years(+ 3 years optional)

Organization: 10 Projects with 16 Principal Researchers from 5 Major Universities and 2 National InstitutesFunding: ~1M US$/yr for 6.5years(+ 3 years optional)


30

PFCs, BlanketHigh temp/

Low activation material

• Blanket analysis model

• Tritium behavior analysis

• Edge plasma models

• PFCs property tests

• 통합시스템 해석체계• 노심 플라즈마 모델

Group 3 Advance


Conversion System

Group 2 Fusion Reactor


• Analysis tool for the

integrated system• Core plasma

models


Systems Integration and Plasma Control

Technologies

Phase 1 (Sep 2008- Feb 2012): Key Technology Development with Research Infrastructure


31

PFCs, BlanketHigh temp/

Low activation material

• Blanket analysis model

• Tritium behavior analysis

• Edge plasma models

• PFCs property tests

• 통합시스템 해석체계• 노심 플라즈마 모델

Group 3 Advance


Conversion System



• Analysis tool for the

integrated system• Core plasma

models


Systems Integration and Plasma Control

Technologies

Phase 2 (Sep 2013- Feb 2015):Integration of Key Technologies for

Applications


Research Topics

Sequential Tokamak Plasma Injection

Suppose that 12kA main plasma initiated by merging two 6kA plasmas

0 1 2 3 4 5 6 7 8 9 10 11 120

50

100

150

200

250

300

350

400

450

500

Pla

sma

curr

ent [A

]

Coi

l's C

urre

nt [A

]

time [ms]

Partial solenoid Thin solenoid main plasma expected

10000

12000

14000

16000

18000

20000

22000

24000

26000

28000

30000

32000

34000

① : Solenoid charging-up phase (12kA main plasma sustaining) ② : Plasma current ramp-up by merging (ramp up from 12kA to 21kA)

③ : Solenoid charging-up phase (21kA main plasma sustaining) ④ : Plasma current ramp-up by merging (ramp up from 21kA to 30kA)

① ② ③ ④


List of Research Topics for VEST1. Fusion Engineering- Plasma startup and ramp-up- Innovative divertor concept- PFC surface coating and material test

2. Plasma Transport and Stability- Confinement scaling- Effect of SOL flow on plasma rotation- Alfven wave characteristics in spherical torus- Magnetic reconnection mechanism during merging phase- Bootstrap current in spherical torus- 3-D physics by internal coils like magnetic perturbations.- Asymmetric plasma characteristics in same magnetic flux surfaces- Transport & Stability studies during merging phase- Shaping effect on plasma including negative triangularity- Isotope effect as working gas

3. Heating and Current Drive- ECH&CD including mode conversion- Synergetic effect among heating schemes

4. Plasma-Wall Interactions- SOL physics- Dust

Documents

Center for Advance Research in Fusion Reactor Engineering