Experimental results and plans of VEST

Y.S. Hwang and VEST team

March 19, 2019

CARFRE and CATS

Dept. of Nuclear EngineeringSeoul National University

Versatile Experiment Spherical Torus (VEST)

toward Advanced Tokamak Study

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

Seoul National University, Seoul, Korea, March 19-20, 2019

1/25

Present Upgrade

Toroidal B Field 0.1 T 0.2T

Major Radius 0.44 m 0.4 m

Minor Radius 0.32 m 0.3 m

Aspect Ratio >1.37 >1.33

Plasma Current <120 kA <300 kA

H & CD

(ECH, NBI, LHFW)

ECH (2.45GHz, 30kW)

NBI (15keV, 600kW)

LHFW (500MHz, 10kW)

ECH (7.9GHz, 3kW)

NBI ( 20keV, 1.2MW)

LHFW (500MHz,200kW)

Specifications

VEST (Versatile Experiment Spherical Torus)

The first and only Spherical Torus (ST) device in Korea

Basic research on a compact, high- ST

Study on innovative start-up, non-inductive H&CD, high ,

disruption, innovative divertor concept, energetic particle, etc

2/25

Introduction : Versatile Experiment Spherical Torus (VEST)

Device and machine status

Start-up experiments

Robust start-up method with trapped particle configuration

Closed flux surface formation as a successful start-up criterion

Solenoid-free start-up from outboard with outside PF coils

Studies for Advanced Tokamak

Research directions for high-beta and high-bootstrap STs

Preparation for heating and current drive systems / diagnostic systems

MHD studies

Summary

Outline

3/25

VEST device and Machine status

VEST device and

Machine status

4/25

VEST device and Machine status

History of VEST Discharges

• #2946: First plasma (Jan. 2013)

• #10508: H2 GDC (Nov. 2014)

• #14945: He GDC, Boronization (Mar. 2016)

• #19351: Slower ramp-up and diverted plasma (May. 2018)

#2946 #10508

#14945

#19351

Time (ms)

Pla

sm

a c

urr

en

t (k

A)

R/a = 1.3

High beta and bootstrap current

Low aspect ratio and high Ip/ITF

5/25

Start-up Experiments

Start-up Experiments

6/25

Start-up experiments

Trapped Particle Configuration (TPC)

• Efficient and robust tokamak start-up demonstrated with wider operation window at VEST and KSTAR

Pressure, ECH power and low loop voltage

• TPC: Mirror like magnetic field configuration– Enhanced particle confinement– Inherently stable decay index structure for Bv

Y. An et al., Nucl. Fusion 57 016001 (2017)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Decay In

dex

Major Radius [m]

Trapped Particle

Conventional w/ stable Bv

Conventional

J.W. Lee et al., Nucl. Fusion 57, 126033 (2017)

7/25

400 401 402 403 404 405 406 407 408 409 410

0

1000

2000

3000

4000

5000

6000 B

0~500 G / ECH 6 kW

B0~500 G / ECH 4 kW

B0~500 G / ECH 2 kW

Time (ms)P

las

ma

Cu

rre

nt

(A)

-3.0x10-3

-2.0x10-3

-1.0x10-3

0.0

1.0x10-3

2.0x10-3

3.0x10-3

Ve

rtica

l Fie

ld (T

)

Start-up Experiment

Closed Flux Surface Formation for Successful Start-up in TPC

Criterion for successful start-up can

be set with closed flux formation, i.e.

poloidal field from plasma current

exceeds vacuum vertical field.

H.Y. Lee

𝐴 = 𝜋𝑟2𝐴 = 𝜋𝑟2

Bv (G) < Bp = 2*Ip(kA)/a(m)

Loop voltage,

Pressure and ECH power

There is another limit ofvertical field strength

for successful start-up !

8/25

Start-up Experiment

Solenoid-free Start-up from Outboard with Outer Poloidal Field Coils

Solenoid-free Startup using outer PF coils assisted by enhanced 2.45 GHz EBW pre-ionization

Startup criteria for Closed Flux Surface (CFS) formation

Successful outer PF Startup region of CFS location & size

Lowering plasma resistivity by optimizing EBW pre-ionization

𝑳 = µ𝒐𝑹𝒐 (ln𝟖𝑹𝒐

𝒂+𝒍𝒊

𝟐- 2)

𝒅𝑹𝒐

𝒅𝒕< 0

𝒅𝑳

𝒅𝒕< 0

𝑳𝒅𝑰𝒑

𝒅𝒕= 𝑽𝒍 - 𝑰𝒑 (

𝒅𝑳

𝒅𝒕+ 𝑹𝒑 ) > 0

R=0.60m

a=0.15m

R=0.40m

a=0.30m

To reach target resistivity, higher ECH power of ~45kW may be needed

to have sufficient density and temperature in the pre-ionization phase.

Required ECH power will be reduced by optimizing EBW pre-ionization

9/40

Start-up experiments in VEST

EBW Pre-ionization via Collisional Heating with Different Resonance Fields

In case of Bo ~ 500 G

• The density peak exists near ECR with low ECH

• Steep density gradient is produced near UHR : improvement on XB MC

• Over dense plasma : EBW collisional damping

In case of Bo ~ 1000 G

• Over dense plasma between ECR and UHR

10/25

Preparation for Advanced Tokamak Studies

Studies for Advanced Tokamak

11/25

Preparation for Advanced Tokamak Studies

Scopes of Advanced Tokamak Studies in VEST

Fusion reactor requires

high beta (or Q) and high bootstrap current

Simultaneously

Alpha heating dominant (high Q)

Centrally peaked pressure profile

Confinement and Stability?

High power neutral beam heating

High Bootstrap current fraction (high fb)

Hollow current density profile (low li)

Current density profile control

Bootstrap/EBW/LHFW

Profile diagnostics

12/25

Preparation for Advanced Tokamak Studies

Simulations for the VEST Advanced Tokamak Scenario C.Y. Lee/Y.S. Na

• The integrated modeling system constructed.

- ASTRA+TGLF and NEO for heat & particle transport: Valid even in low aspect ratio tokamak

• The steady state solution of beam discharges showing that 𝑇𝑒0~0.8 𝑘𝑒𝑉, 𝑇𝑖0~0.5 𝑘𝑒𝑉 can be achieved by considering beam heating & fueling simultaneously.

𝛽𝑁~7, 𝑓𝐵𝑆~0.5, 𝑓𝑁𝐼~0.75

13/25

Neutral Beam Injection System

with ~600kW at 15keV

from KAERI

Thomson Scattering

with Nd:YAG laser

from SNU/NFRI/JNU/SogangU

Preparation for Advanced Tokamak Studies

Diagnostics, Heating and Current Drive Systems in VEST

Low Hybrid Fast Wave H/CD

with ~10kW at 500MHz

from KAERI/KAPRA/KWU

ECH

Electron Cyclotron/

Bernstein Wave H/CD

with ~30kW at 2.45GHz

from SNU/KAPRA

Interferometry

with 94GHz, multi-channel

from SNU/UNIST

Magnetics and Spectroscopy

Interferometer

Heating and Current Drive Systems Profile Diagnostic Systems

14/25

High power central heating

High Power NBI System : NBI Commissioning on VEST

B.K. Jung/K.H. Lee

Neutral beam injection system (600 kW at 15keV)

1 MW NBI developed by KAERITested up to 200 kW / Target 600 kW (VEST)

15/25

High power central heating

High Power NBI System : NBI Commissioning on VEST

B.K. Jung/K.H. Lee

-2 0 2 4 6 8 10 12-10

0

10

20

-10

0

10

20

30

40

Vo

lta

ge

(k

V)

V_PE

V_SE

Time (ms)

Cu

rre

nt

(A)

I_PE

I_SENeutral Beam Center stack of VEST

Commissioning shots Up to 200kW (10kV 20A 10msec)

-2 0 2 4

0

10

20

30-10

0

10

20

30

40

50

Vo

lta

ge

(k

V)

V_PE_before

V_SE_before

V_PE_after

V_SE_after

Time (msec)

Cu

rre

nt

(A)

I_PE_before

I_SE_before

I_PE_after

I_SE_after

Conditionning effect

< Before NB injection > <NB injection >

16/25

In collaboration with KAERI and Kwangwoon Univ.Current density profile control

Feasibility study of LH Fast Wave (500MHz, 10kW)

S.H. Kim/J.G. Jo

Modified accessibility condition of LHFW by 𝒏∥-upshift via microwave scattering

𝒏∥-upshift via wave scattering

measured during propagation

in edge region

Modification of wave propagation

into more radially inward region Expanded coupling regime

in high density plasma

17/35Status and plan for VEST

EBW heating (6kW cw+10kW pulse at 402ms) on Ohmic plasmas

Current density profile control

EBW Heating with XB Mode Conversion

H.Y. Lee

400 401 402 403 404 405 406 407 408 409 410 411 4120

5

10

15

20

25

30

35

40

45

50

55

60

Pla

sm

a C

urr

en

t (k

A)

Time (ms)

off

on

• Boronization (impurity removal) and higher plasma

current (higher electron temperature)

• No change in the electron density profile with

additional MW – no additional collisional damping

• Electron temperature rises two times near 3rd

harmonics ECR resonance : the first resonance

position after EBW mode conversion

18/25

Preparation for Advanced Tokamak Studies

VEST Diagnostic Systems

Diagnostic Method Purpose Remarks

MagneticDiagnostics

Rogowski CoilPlasma current & eddy

currentin-vessel coils

Pick-up Coil & Flux Loop

Bz, Br &Loop voltage, flux

49 pick-up coils11 loops

Magnetic Probe Array

Bz, Br measurement inside plasma

Movable array

Probes Electrostatic Probe Radial profile of Te, ne

2 Triple ProbesMach probe

OpticalDiagnostics

Fast CCD camera Visible Image 20kHz

Hα monitoring Hα Hα filter+ Photodiode

Impurity monitoring O & C lines Spectrometer

Interferometry Line averaged ne 94GHz, multi-channel

Imaging Fabry-PérotInterferometer

edge Ti

Multi-channel with fiberarray

Charge Exchange Spectroscopy

Rotation and Ti CES/BES with DNB

Thomson Scattering Te, ne profile Nd:YAG laser

19/25

Profile diagnostics

Thomson Scattering System UpgradeIn collaboration with

New fast digitizer (CAEN V1742) 5 GS/s × 32 ch.

Additional polychromator 2 points measurement

Nd:YAG laser system (BeamTech Inc. SGR-20)

Laser energy: 0.85 J 2.0 J

Repetition rate: 10 Hz 1 kHz (10 pulses)

Optical loss: 23% 4%

Recent result of N2 Rayleigh scattering measurement

Y.G. Kim, D.Y. Kim, J.H. Kim

20/25

Profile diagnostics

Visible optical spectroscopy

Measurement quantities : 𝑻𝒊, 𝒗𝑻, 𝒏𝒁 Passive emission spectroscopy

CIII 464.74 nm and OII 464.90 nm Line-integrated spectra spectral inversion

Specification Detection range : 0.39 m< R <0.71 m Spatial resolution : 20-22 mm Temporal resolution : 2 ms for 9ch, 0.2 ms(for 1ch)

CES/BES combined system is in preparation

Y.S. Kim In collaboration with

⋯

Slit

⋯

7 channel

HDG105 mm

f/14

EMCCD

85 mmf/1.4

140 channel

Geometric centerLCFS

#20913 308.1 ms Ch9

Total fit

3-CIII

2-OII

21/25

MHD studies

Ohmic Discharges: Adjust ramp-up rate for MHD suppression

𝜓𝑁

𝒒

#18452, 0.305 s #19160, 0.305 s

• Hollow current density profile

– Fast ramp-up and high prefill gas pressure

• High current without MHD activity

– Slower Τ𝑑𝐼𝑃 𝑑𝑡: Peaked 𝐽𝜙 profile

– Low prefill gas pressure: Low resistivity

𝑱𝝓 (A/cm2)

Time (s)

OI / Hα

Δin

Τ𝒅𝑩𝒁 𝒅𝒕 (T/s)

𝑰𝑷 (MA)

𝒑𝟎 (10-5 Torr)

2/1

#18452 #19160J.H. Yang

22/25

MHD studies

Ohmic Discharges: MHD activity suppressed by prefill gas control

J.H. Yang

𝜓𝑁

𝒒

#18731, 0.306 s #19157, 0.306 s

• MHD depends on current density profile only?

• Higher prefill gas pressure:

– Lower 𝑇𝑒, less hollow 𝐽𝜙 profile but more TM

– Due to more neoclassical drive (higher 𝛽𝑁)

• Neoclassical feature of TM

𝑱𝝓 (A/cm2)

Time (s)

OI / Hα

𝑉𝜙 (V)

Τ𝑑𝐵𝑍 𝑑𝑡 (T/s)

𝐼𝑃 (MA)

𝑝0 (10-5 Torr)

2/1 + 3/2

#18731 #19157

23/25

MHD studies

Lower 𝒍𝒊 startup by tearing mode suppression J.H. Yang

𝒒𝟗𝟓

𝒍 𝒊

■ #19160, 0.308 s■ #18731, 0.306 s

□ Max. Τ𝛿𝐵 𝐵 < 1 %

□ Max. Τ𝛿𝐵 𝐵 > 1 %

(Stable)

(Unstable)Kink/double tearing limit

Startup at 𝒍𝒊 ~ 0.5 without MHD activity available

Suppress TM by e.g. lower 𝜷𝑵

Time (s)

𝛽𝑁

#19160 #18731

𝛿𝐵 (T/s) 𝐼𝑃 (kA)

Black squares:

Stable shots at unstable region

24/25

Summary

Ohmic operation with 𝑰𝑷 < 120 kA, 𝜿 < 2 and 𝒒𝟗𝟓 < 5 achieved

MHD suppression and diverted configuration

Efficient start-up with TPC (Trapped particle configuration) and EBW heating

TPC start-up with low 𝑉𝑙, low ECH power, and wider pressure window

Closed flux surface formation as a successful start-up criterion

Solenoid-free start-up utilizing efficient EBW pre-ionization at outboard under development

Preparation for the study of Advanced Tokamak with strong central heating

High power (>600 kW) NBI system commissioning

EBW/LHFW heating and current drive experiments

Various diagnostics are under preparation

MHD studies are performed

25/25

Thank you for your attention !

Tearing modes during tokamak plasma current ramp upPage 26

Profile diagnostics

External/Internal Magnetic Diagnostics

• Equilibrium reconstruction: EFIT [5] + transient open field current

– Reconstruction of low plasma current < 100 kA

• Mode identification: Fourier analysis + SVD [6]

– Poloidal & toroidal mode numbers

• Internal Magnetic Probes [7]

R (m)

Z (

m)

Poloidal plane view

Flux loops

Inte

gra

ted

Mirn

ov

coils

[5] Lao et al. NF 1985

[6] Kim et al. PPCF 1999

[7] Yang et al. RSI 2014

27/25

Profile diagnostics

Thomson Scattering System

• Measurement target

– ne: > 5×1018 m-3

– Te: 10-500 eV

– core plasma

• Laser: 0.65 J/pulse, 1064 nm

• Collection solid angle: ~50 msr

• Scattering length: ~5 mm

• Filter polychromator: 4 channels

Spectral Response

of the polychromator

In collaboration with

Schematic of the TS system

Y.G. Kim/D.Y. Kim

28/25

Profile diagnostics

94 GHz Multi-channel Interferometer

• Microwave system with heterodyne interferometer

• 3.2 GHz IF (avoid EC noise)– EC 2.45 GHz

• Single chord multi-chord

In collaboration with

J.I. Wang

29/25

Ramp-up Experiments

MHD mode number and island location from magnetics

R (m)

Tim

e (

s)

Τ𝑑𝐵𝑍 𝑑𝑡(T/s)

Shots #18452 – 18457

𝑅0

q=3q=2 q=2

q=3

Shot #18452

𝐼𝑃 (MA)

𝑚

𝑓 (kHz)

Τ𝑑𝐵𝑍 𝑑𝑡 (T/s)

Τ𝑑𝐵𝑍 𝑑𝑡 (T/s)

𝒏 = -1𝒏 = -2

2/13/1

3/2 4/2

Time (s) Time (s)

𝑞95

𝑞min.

• Island position:

Peak Τ𝑑𝐵𝑍 𝑑𝑡 location

• Agree with flux surface at

m/n = 3/1 and 2/1 (and 4/1)

• Central 𝑞 large uncertainty:

Surface m/n = 3/2

1.5

J.H. Yang

30/25

VEST device and Machine status

120kA Diverted Plasma (#19351) S.C. Kim

Coil Switching

O impurity drop

31/25

Start-up Experiment

Criterion for Successful Start-up in TPCH.Y. Lee

What is optimal vertical field strength for successful start-up?

PF1 current waveform

Different plasma resistivity

with different ECH powers