Experimental results and plans of VEST
Y.S. Hwang and VEST team
March 29, 2017
CARFRE and CATS
Dept. of Nuclear Engineering Seoul National University
Overview of Versatile Experiment Spherical Torus (VEST)
23rd IAEA Technical Meeting on Research Using Small Fusion Devices, Santiago, Chille
1/36
Versatile Experiment Spherical Torus (VEST)
Device and discharge status
Start-up experiments
Low loop voltage start-up using trapped particle configuration
EC/EBW heating for pre-ionization
DC helicity injection
Studies for Advanced Tokamak
Research directions for high-beta and high-bootstrap STs
Preparation of long-pulse ohmic discharges
Preparation for heating and current drive systems
Preparation of profile diagnostics
Long-term Research Plans
Summary
Outline
2/36
VEST device and Machine status
VEST device and
Machine status
3/36
Present Future
Toroidal B Field [T] 0.1 0.3
Major Radius [m] 0.45 0.4
Minor Radius [m] 0.33 0.3
Aspect Ratio >1.36 >1.33
Plasma Current [kA] ~100 kA 300
Elongation ~1.6 ~2
Safety factor, qa ~6 ~5
Specifications
VEST (Versatile Experiment Spherical Torus)
Objectives
Basic research on a compact, high- ST (Spherical Torus)
with elongated chamber in partial solenoid configuration
Study on innovative start-up, non-inductive H&CD, high and
innovative divertor concept, etc
4/36
History of VEST Discharges • #2946: First plasma (Jan. 2013) • #10508: Hydrogen glow discharge cleaning (Nov. 2014) Ip of ~70 kA with duration of ~10 ms • #14945: Boronization with He GDC (Mar. 2016) Maximum Ip of ~100 kA
• # ??: Long pulse with coil power supply upgrade (March. 2017)
5/36
VEST Discharge Status : 100kA with Boronization (#14945) Discharge Status (Shot #14945)
• EC-assisted OH discharge
– Plasma current 100 kA
– Hydrogen plasma with Boronization
– Tungsten & Graphite (inboard) limiter
• Equilibrium parameters
– R=0.45m/a=0.33m (A=1.36)
– Elongation: 1.6 – 2.0
– Edge safety factor: ~ 7
• Limited discharge length
– Fast ramp-up Require large Bv
in the beginning
– Wall eddy current Limited time
response of Bv
– Excessive vertical field near current peak early termination
6/36
Start-up Experiments
Start-up Experiments
7/36
Start-up experiments
Widely used Field Null Configuration (Breakdown/avalanche)
The field null configuration has been generally used even for the ECH-assisted start-up of tokamaks.
KSTAR
[Y.S. Bae et al 2009 Nucl. Fusion 49 022001]
Frascati Tokamak Upgrade
[G. Granucci et al, Nucl. Fusion 51 (2011) 073042]
Field null formation to maximize connection length and consequently sufficient EtBt/Bp
Null quality : size, sustaining time and location loop voltage and pre-ionization
Notes:
KSTAR: not much help with higher ECH power in the ECH-assisted start-up
DIII-D: vertical field pre-bias helps ohmic start-up with ECH pre-ionization
DIII-D
[NUCLEAR FUSION, Vol. 31. No. 11 (1991)]
8/36
Start-up experiments
Trapped Particle Configuration (TPC)
Efficient and robust tokamak start-up demonstrated
Y. An et al., Nucl. Fusion 57 016001 (2017)
Trapped particle Field null
• Mirror like magnetic field configuration – Enhanced particle confinement – Inherently stable decay index structure – Wider operation range of ECH power, pressure and low Vloop
Start-up Experiment in VEST
Robust and Reliable TPC Start-up Applied to KSTAR Successfully
Feasibility study of TPC in KSTAR
• Even though low mirror ratio than ST, achieving efficient start-up with TPC
• 2nd harmonic delay of 20 ms and ECH plasma density of 4x1018 m-2
• Ip formation with low Et less than 0.2 V/m
Earlier plasma colomn formation
than field null
J. Lee, et al., Proc. IAEA-FEC 2016 EX/P4-14
10/36
Start-up Experiment in VEST
Low Loop Voltage Start-up with different ECH Power at Low TF Field in TPC
Earlier Ip initiation at low loop
voltage with larger ECH power in
double swing TPC start-up
Smaller resistivity initiate higher
initial current to form sufficient
size of closed flux surface
Threshold current for successful
inductive start-up (reference for
outer PF start-up)
H.Y. Lee
11/36
EC / EBW pre-ionization
VEST pre-ionization with LFS XB mode conversion
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)
2.45GHz
6kW CW
ECH
J.G. Jo 30 35 40 45 50 55 60 65 70 75 80 85
3
6
9
12
15
18
21
24
Te [
eV
]
R [cm]
X-mode_2kW
X-mode_3kW
X-mode_4kW
X-mode_6kW
ECR
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
nUH
nR
ne [10
17m
-3]
R [cm]
X-mode_2kW
X-mode_3kW
X-mode_4kW
X-mode_6kW
ECR
12/36
Start-up experiments in VEST
Pre-ionization with Low-Field-Side Launch EC/EBW Heating
• LFS perpendicular injection of ECW 10 kW – Over-dense plasma (X-wave tunneling) – Parametric decay with mode conversion – High density with collisional heating near UHR
J.G. Jo et al., Phys. Plasmas 24 012103 (2017)
• TPC configuration for Pre-ionization – Higher density plasma with broad density profile – Different density profile with Bt strength
Low TF (Bo ~ 0.5 kG) High TF (Bo ~ 1 kG)
13/36
DC Helicity Injection
DC Helicity Injection Start-up
J.Y. Park
Lower Gun Location • R = 0.25 m
• Z = - 0.804 m
Magnetic Field Structure • Stacking ratio, G = 7 ~ 8
• Multiplication, M = ~ 16
The Electron Gun for DC Helicity Injection • High current electron beam based on arc discharge w/ washer stacks
• Low impurity
A toroidal current of up to ~20 kA has been generated by the electron gun • Increased multiplication factor confirmed as current streams reconnect
• Very dynamic states of current sheet observed
401 ms 402 ms 403 ms 400.4 ms
The electron gun
400 402 404 406 408 410 412 414 416-10
-9
-8
-7
-60
2
4
6
80.0
0.5
1.0
1.5
2.00.0
-0.2-0.4-0.6-0.8-1.0
0
5
10
15
20
Vacuum field @ Z = 0, R= 0.089
Bz [
mT
]
Time [ms]
Plasma field @ Z = 0, R= 0.089
Bz [
mT
]
I inj [
kA
]
Vin
j [kV
]
I toro
idal [
kA
]
14/36
Preparation for Advanced Tokamak Studies
Studies for Advanced Tokamak
15/36
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
16/36
Preparation for Advanced Tokamak Studies
Experimental Research Direction for VEST
Advanced Tokamak Study Scenario
Powerful Core Neutral Beam Injection (NBI) resembling alpha heating
tackles simultaneous achievement of high beta and high bootstrap current
High beta limit in spherical torus favors high beta operation
+
High bootstrap current fraction reversed shear configuration
Spherical Torus with Reversed Shear (RS) may provide AT regime !
• Sufficient confinement from ITB formation by RS
• Possible high beta even with low li in RS due to low aspect ratio
Advanced Tokamak Regime in VEST*
Low toroidal field(0.1T) with high βN(~7)< βN,Menard(8.7) and Ip(0.08 MA).
Fully non-inductive CD with 80% bootstrap fraction may be possible with ~500kW.
High H factor(~1.2) needs to be attempted by forming ITB with MHD stability.
* Estimated by 0-D system code
High power NBI to center, forming RS in VEST!
17/36
Preparation for Advanced Tokamak Studies
Simulations for the VEST Advanced Tokamak Scenario
Total power loss is calculated using NUBEAM simulations in various conditions (𝐈𝐏 = 𝟏𝟎𝟎 𝐤𝐀)
• Based on the result, R = 35 cm equilibrium is chosen for better NBI power absorption.
• 15 degree of the injection angle is chosen considering both power loss and engineering issue in VEST and its heating power profiles are calculated.
S.M. Yang, C.Y. Lee and Yong-Su Na
𝑬𝒃𝒆𝒂𝒎 R = 35 cm R = 40 cm
10𝑘𝑒𝑉 27% 33%
20𝑘𝑒𝑉 43% 70%
Total loss ( ne0 = 2 ∗ 1019 𝑚−3)
𝑃𝑒 , 𝑃𝑖 at 15° injection
𝑟/𝑎
Hea
tin
g p
ow
er
(
(ne0 = 2 ∗ 1019 𝑚−3)
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 Pe-20keV,600kW
Pe-10keV,150kW
Pi-20keV,600kW
Pi-10keV,150kW
18/36
Preparation for Advanced Tokamak Studies
Simulations for the VEST Advanced Tokamak Scenario
0-D system code analysis
Low toroidal field 0.1 T High beta N 7 Plasma current 80 kA
Non-inductive 100% Bootstrap fraction 80%
Required NB power 500 kW
1-D transport simulation NBI heating with NUBEAM code
(Density and equilibriums are given)
1D integrated modelling with ASTRA
code with TGLF module
(Density profile is given)
L-mode (edge at r=0.9) and H-mode
cases are compared
S.M. Yang, C.Y. Lee and Yong-Su Na
19/36
Preparation for Advanced Tokamak Studies
Preparation for Long Pulse Ohmic Discharges
S.C. Hong and J.Y. Park
TF with Ultra Capacitor from Battery PF in H-bridge circuit with Extra Capacitors
20/36
Preparation for Advanced Tokamak Studies
Development of Long Pulse Ohmic Discharges J.H. Yang and S.C. Kim
+10ms +20ms +30ms
-10ms 0ms
CE = 0.4
21/36
Preparation for Advanced Tokamak Studies
Discharge Improvement with Boronization
• Boronization during Helium Glow discharge cleaning – Carborane (C2B10H12) is used for non-toxic boronization – Water concentration recovery slowed down (RGA)
• Oxygen atom line emission (777.0 nm) reduced by 3 fold
– High density expected: More prefill gas (Hα line emission)
Diaphragm valve
Top flange (CF 6 inch)
Caborane oven
(baked 50°C)
Helium flow (MFC 2.0
sccm)
With great help from Dr. S.H. Hong (NFRI)
22/36
Neutral Beam Injection System
with ~500kW at 15keV
from KAERI
Thomson Scattering
with NdYAG 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 ~20kW 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
23/36
High power central heating
High Power NBI System : Dual Beam to Single Beam
NBI beam loss calculation with NUBEAM code
Too large orbit loss for counter-injection
Dual beam to single beam
Large shine through loss even for co-injection
Require high plasma density
5.0x1018
1.0x1019
1.5x1019
2.0x1019
2.5x1019
3.0x1019
0
10
20
30
40
50
60
70
80
90
100
Lo
ss R
ati
o [
%]
Target Plasma Density(#/m3)
counter-injection
co-injection
Beam at 20keV
S.H. Chung (KAERI)
24/36
High power central heating
High Power NBI System : System Design
3.1 m
Max Power
: 32.72 MW/m2
2.6 m
Max Power
: 35.16 MW/m2
1.8 m
Max Power
: 36.16 MW/m2
1.4 m
Max Power
: 32.77 MW/m2
0.7m
Max Power
: 28.12 MW/m2
Defl. angle, # of slits - 2 degree 29
- 1 degree 3
0 degree 13
1 degree 3
2 degree 29
Focusing beam profile calculation Beam power density at beamline
Calculated by BTR code
Slit beam focusing 3D beam line design
0
5
10
15
20
25
30
35
40
0 1 2 3
Maxim
um
Beam
Pow
er
Densi
ty [
MW
/m2]
Distance from Beam Exit [ m ]
2016-10-22
2016-09-23
2016-08-24
2016-07-25
2016-06-26
2016-05-27
2016-04-28
2016-03-29
25/36
High power central heating
High Power NBI System : Ion Source Test Completed
• Neutral beam injector – Co-directional – Design power: 500 kW
• Ion source test completed
– 750 kW at 15 keV • Under commissioning
– Power supply – Beamline
• Installation schedule
– March~April 2017
26/36
For high density plasma in fusion reactor
• Slow wave branch of LHW
→ Absorbed at the edge region
• Fast wave branch of LHW
→ Possible absorption at the core region
Proof of principle for current drive scheme
by fast wave branch of LHW in VEST
Current density profile control
Core Heating and Current Drive with LHFW
S.H. Kim (KAERI)
Possible propagation regime in CMA diagram
- FW launching density ~ f(n||, w, wce)
- FW-SW confluence density ~ f(n||, wce)
FW path
Accessibility for Lower Hybrid Fast Wave(LHFW)
27/36
In collaboration with KAERI and Kwangwoon Univ.
• LH Fast Wave – Good core
accessibility – Propagation at
low density
• Hardware – UHF Klystron – Combline antenna
N|| = 3-5 / 500 MHz
Current density profile control
LHFW H/CD Simulation and Component Preparation
28/36
Current density profile control
EBW Heating and Current Drive with XB Mode Conversion
GENRAY, CQL3D, mode conversion codes are used
Perpendicular, LFS, X-mode injection
• Short distance between R(X) cut-off and UHR
• High XB mode conversion efficiency with low n||
• Good central heating and current drive expected
► Core heating and current drive with XB mode conversion
Absorption near EC fundamental resonance
EBW propagation
ECR
S.H. Kim (KAERI)
29/40 Status 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 along the electron density profile
with additional MW – no collisional damping
• Plasma pulse duration is increased
• Electron temperature rises two times near 3rd
harmonics ECR resonance : the first
resonance position after mode conversion
30/36
Preparation for Advanced Tokamak Studies
VEST Diagnostic Systems
Diagnostic Method Purpose Remarks
Magnetic Diagnostics
Rogowski Coil Plasma current & eddy
current 3 out-vessel & in-vessel coils
Pick-up Coil & Flux Loop
Bz, Br & Loop voltage, flux
56 pick-up coils 9 loops
Magnetic Probe Array
Bz, Br measurement inside plasma
Movable single array
Probes Electrostatic Probe Radial profile of Te, ne
2 Triple Probes Mach probe
Optical Diagnostics
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
Reflectometry Radial profile of ne Edge density profile
EBE radiometer Core, edge Te BX mode conversion
Charge Exchange Spectroscopy
Rotation and Ti DNB
Thomson Scattering Te, ne profile NdYAG laser
31/36
Profile diagnostics
Movable Magnetic Probe Array System
J.H. Yang
• Equilibrium reconstruction with internal probe data • Direct derivation of JT(R) – w/o vertical displacement – Plasma degradation ~ 10%
J.H. Yang et al., Rev. Sci. Instrum. 83 10D721 (2012)
J.H. Yang et al., Rev. Sci. Instrum. 85 11D809 (2014)
32/36
Profile diagnostics
94 GHz Multi-channel Interferometer
J. Wang
• Microwave system with heterodyne interferometer
• 3.2 GHz IF (avoid EC noise)
– EC 2.45 GHz
• Single chord multi-chord
In collaboration with UNIST and KNU
33/36
Profile diagnostics
Thomson Scattering System
D.Y. Kim and Y.G. Kim Y.-G. Kim et al., Fus. Eng. Des. 96-97 882-885 (2015)
• Measurement target – ne: 5×1018 m-3
– Te: 10-200 eV • Laser energy: 0.65 J/pulse
In collaboration with NFRI and Seogang Univ.
34/36 Y.S. Kim, S.G. Ham and K.H. Lee
• Ion temperature and rotation measurement
– Dichronic beam splitter: Hα and HeII share collecting optics
– Transmission grating
• He2+ density measurement
• w/wo DNB
• Fabry-Perot interferometry
Profile diagnostics
Charge Exchange Spectroscopy/Beam Emission Spectroscopy
35/36
Future Research Plans
Long-term Research Plans
Reactor-relevant Advance Tokamak research
AT scenario for high beta and steady-state operation
Innovative divertor to handle long-pulse high-performance operation
Energetic particle transport …
36/36
Summary
VEST has achieved successful ohmic operation with plasma currents of up to ~100 kA, elongation
of ~ 1.6 and safety factor of ~6 with efficient ECH pre-ionization.
Efficient start-up method is developed with TPC(Trapped particle configuration) and EBW heating.
EBW heating with direct XB mode conversion from LFS launching by generating over-dense plasma in
the pre-ionization phase.
Enhanced pre-ionization with TPC improves start-up with low loop voltage, low ECH power, and wider
pressure window.
DC helicity injection startup experiments generate plasma current of ~20 kA.
Preparation for the study of Advanced Tokamak is progressing
VEST Advanced Tokamak scenario is simulated with transport codes.
Long-pulse ohmic discharges are being prepared by improving PF coil power system.
High power (~ 500 kW) NBI system is under development.
EBW/LHFW heating and current drive experiments are under preparation by performing simulations and
constructing hardware systems.
Profile diagnostics are under preparation
Reactor-relevant AT research by developing both AT scenario and innovative divertor concept will
be continued.
37/36
19th International Spherical Torus Workshop (ISTW 2017) will be held at
Seoul National University, 19-22 September 2017
Previous ISTWs were held in Oak Ridge (1994), Princeton (1995), Culham (1996), St. Petersburg (1997), Tokyo (1998),
Seattle (1999), Sao Jose dos Campos (2001), Princeton (2002), Culham (2003), Kyoto (2004), St. Petersburg (2005),
Chengdu (2006), Fukuoka (2007), Frascati (2008), Madison (2009), Toki (2011), York (2013), and Princeton (2015).
38/36
Thank you for your attention !
39/36
VEST Discharge Status : 100kA with Boronization (#14945) Discharge Status (Shot #14945)
• Kinetic plasma parameters from magnetics
– Spitzer temperature*: ~200 eV
– Diamagnetic flux Stored energy (perpendicular) <1 kJ
– Estimated plasma density: ~2×1019 m-3
• Estimation from triple probe measurements
– ~ 1.5×1019 m-3 at 100 kA from ~ 8×1018 m-3 at 60 kA
*Zσ = 3 / trapped particle fraction 88%
Greenwald density limit: ~3×1019 m-3
40/36
Start-up experiments
Current initiation and field structure
-3
-2
-1
0
0
2
4
6
8
10
390 392 394 396 398 400 402 404 406 408 410
-1
0
1
2
PF1 Current
PF3&4 Current
PF9 Current
PF
Cu
rre
nt
[kA
]
w/ Trapped Particle Configuration (PF3&4)
w/o Trapped Particle Configuration (PF3&4)
Pla
sm
a C
urr
en
t [k
A]
w/ Trapped Particle Configuration (PF3&4)
w/o Trapped Particle Configuration (PF3&4)
Flu
x L
oo
p [
V]
Time [ms]
The field structure with no Ip does not provide stable field structure.
#5252 / #5255 Ip~8kA No Ip
41/36
Start-up experiments
Current initiation and field structure
-3
-2
-1
0
0
2
4
6
8
10
390 392 394 396 398 400 402 404 406 408 410
-1
0
1
2
PF1 Current
PF3&4 Current
PF9 Current
PF
Cu
rre
nt
[kA
]
w/ Trapped Particle Configuration (PF3&4)
w/o Trapped Particle Configuration (PF3&4)
Pla
sm
a C
urr
en
t [k
A]
w/ Trapped Particle Configuration (PF3&4)
w/o Trapped Particle Configuration (PF3&4)
Flu
x L
oo
p [
V]
Time [ms] #5252 / #5255 Ip~8kA No Ip
It was observed that negligible plasma current even with better Et*Bt/Bp and sufficient Vloop.
42/36
Start-up Experiment in VEST
Decay Index for each scenario
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
Trapped Particle Configuration Stable Bv only Field Null
• Conventional field null configuration
does not provide stable Bv structure
at the onset of loop voltage
• The other scenarios provide stable Bv
structure at the onset of loop voltage
0 3 / 2
z
z
n
BRn
B R
43/36
Start-up Experiment in VEST
Efficient volt-second consumption
0
5
10
15
20
25
400.0 400.2 400.4 400.6 400.8 401.0 401.2 401.40
2
4
6
8
Pla
sm
a C
urr
en
t [k
A] #8556
Vo
lt-s
ec
on
d [
mV
s]
Time [ms]
0
5
10
15
20
25
400.0 400.5 401.0 401.5 402.00
2
4
6
8
Pla
sm
a C
urr
en
t [k
A] #8553
Vo
lt-s
ec
on
d [
mV
s]
Time [ms]
4 mV∙s
6.9 mV∙s
1. When sufficient pre-ionization and proper field structure is provided wasted volt-second is
not shown in field null configuration.
2. Use of trapped particle configuration further reduces the required volt-second
1. Enhanced pre-ionization by TPC
2. Stable Bv at the onset of Vloop
1. Stable Bv at the onset of Vloop
Trapped Particle Configuration Stable Bv
0
5
10
15
20
25
401.5 402.0 402.5 403.0 403.5 404.0 404.5 405.00
2
4
6
8
Pla
sm
a C
urr
en
t [k
A] #8230
Vo
lt-s
eco
nd
[m
Vs]
Time [ms]
8.5 mV∙s
Wasted V∙s
1. Degraded pre-ionization
2. Not stable Bv at the onset of Vloop
Conventional
44/36
Start-up experiments
Trapped Particle Configuration (TPC)
Efficient and robust tokamak start-up demonstrated
Y. An et al., Nucl. Fusion 57 016001 (2017)
Trapped particle Field null
• Mirror like magnetic field configuration – Enhanced particle confinement – Inherently stable decay index structure – Wider operation range of ECH power, pressure and low Vloop
45/36
Ohmic Start-up with ECH Pre-ionization
Higher dIp/dt and Wider Operation Regimes with TPC
1 2 3 4 5 6
0
1
2
3
4
5
6
7
dI p
/dt
[kA
/ms
]
ECH Power [kW]
with TPC
without TPC
No current initiation
10-5
10-4
-1
0
1
2
3
4
5
6
7
8
dI p
/dt
[kA
/ms]
Filling Pressure [Torr]
With TPC
Without TPC
No current initiation
20 40 60 80 100 120 140 160 180
1
2
3
4
5
6
7
8
9
10 with TPC
without TPC
dI p
/dt
[kA
/ms]
Et*B
t/B
p [V/m] at R=0.4m
Higher dIp/dt under TPC with identical Vloop and EtBt/Bp
Wider operating windows for gas pressure and ECH power
Y.H. An
46/36
Start-up Experiment in VEST
Solenoid-free Start-up from Outboard with Outer Poloidal Field Coils H.Y. Lee
The closed flux surfaces have not been formed in the cases of
both 6kW and 16 kW due to their high resistivity of > 1.0 E-5.
In the case of the resistivity of 1.0 E-5, the closed flux surface has
small minor radius 3~4 cm with very low Ip.
Once current column is formed, inward moving current can grow
by utilizing decreasing external inductance even with small E field.
Plasma current may be initiated with resistivity of less than 1.0 E-5.
To reach target resistivity, higher ECH power of ~30kW
may be needed to have sufficient density and
temperature in the pre-ionization phase.
Resistivity
1E-5
Resistivity
5E-6
47/36
Start-up experiments
Widely used Field Null Configuration (Breakdown/avalanche)
The field null configuration has been generally used even for the ECH-assisted start-up of tokamaks.
KSTAR
[Y.S. Bae et al 2009 Nucl. Fusion 49 022001]
Frascati Tokamak Upgrade
[G. Granucci et al, Nucl. Fusion 51 (2011) 073042]
Field null formation to maximize connection length and consequently sufficient EtBt/Bp
Null quality : size, sustaining time and location loop voltage and pre-ionization
Notes:
KSTAR: not much help with higher ECH power in the ECH-assisted start-up
DIII-D: vertical field pre-bias helps ohmic start-up with ECH pre-ionization
DIII-D
[NUCLEAR FUSION, Vol. 31. No. 11 (1991)]
48/36
Start-up Experiment in VEST
Mirror Ratio and Decay Index for Efficient Start-up in TPC
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
Stable Bv only Field Null
• Conventional field null configuration
does not provide stable Bv structure
at the onset of loop voltage
• Other scenarios provide stable Bv
structure at the onset of loop voltage
0 3 / 2
z
z
n
BRn
B R
• Efficient start-up with better pre-
ionization at high mirror ratio
Trapped Particle Configuration (TPC)
49/36
Ohmic Start-up with ECH Pre-ionization
Double Swing Start-ups with TPC and FNC
J.W. Lee
PF#1 Double swing TPC
402 404 406 408 410 412 414
0
20
40
60
80
100
Pla
sm
a c
urr
en
t [k
A]
Time [ms]
shot #14458, Full flux TPC
shot #14597, Full flux FNC, same day
PF#1 full flux startup using field null vs. TPC
Null: PF#1+PF#10, TPC: PF#1+PF#5
Fast plasma current ramp-up
Efficient ECH-assisted start-up using trapped particle configuration in VEST & KSTAR
and application for ITER
J. Lee, J. Kim, Y.H. An, M.-G. Yoo, Y.S. Hwang and Y.-S. Na (SNU and NFRI in Korea)
Reliable ECH-assisted start-up with
ITER-relevant Et in KSTAR
• BRIS-less start-up to counter Ip
operation for various physics
studies in KSTAR
• ITER-relevant Et level of less
than 0.3 V/m
• Unstable start-up with field null
even with ECH-assistance
• Reliable low Et start-up with pre-
ionization plasmas under TPC
Preliminary ITER scenario development using TPC
• Scenario development based on ITER_c_33NHXN_v3_4, appendix 6, half charging CS and P/S configuration B
• With consideration of eddy current evolution driven by PF coils on conducting structure
• Almost identical operation of solenoid coils, and newly determining the outer PF currents for TPC configuration
• About 0.2 s after solenoid swing, achieving the Et for Ip formation based on KSTAR result
• Save V∙s and avoid operating solenoid coils up to its engineering limits, Et of 0.3 V/m
Bp and pol. flux at 0.2 s
51/36
Current density profile control
Simulation Results for LHFW Heating and Current Drive S.H. Kim (KAERI)
TORIC Full Wave simulation : Absorption
GENRAY-CQL3D simulation : Current Drive
Ez field on Poloidal C.X. Power absorption dist. Driven current profile
LHFW ray tracing LHSW ray tracing Velocity dist. on phase space by CQL3D
Driven current profile
1D full wave coupling code
LHFW RF system and
operation parameters
Coupling cal. condition Coupling efficiency vs. den.
- Frequency = ~ 500 MHz (UHF)
- N∥ = 3 ~ 5, B0 = 0.2 T
- ne0 ~ 1x1018#/m3, Te0 ~ 1 keV
☆ In addition, 1D full wave code is developed to calculate
condition for efficient LHFW coupling: nb~3x1017#/m3
52/36
Current density profile control
LHFW Components
Klystron
The klystron is prepared by refurbishing an
old UHF broadcasting system of Korea which
has been hold by SNU and KAPRA.
Combline Antenna
S.H. Kim (KAERI) and B.J. Lee (KwangWoon Univ.)
Frequency 470~520 MHz
VSWR < 3:1
Peak n|| 3.25
Antenna size length = 486 mm, thickness = 36mm
Ey/Ez ratio 4~30 (average of 71.35%)
Peak E-field < 24.4kV/cm
53/22
LHY_JMVQ_DEC17
Joint meeting on VEST & QUEST on December 17th 2015, at Kyushu Univ.
-6 -4 -2 0 2 4 6
700
750
800
850
900
950
Ma
gn
etic
Fie
ld (
G)
Distance (cm)
Radial FieldQuartz Quartz
HFS LFS 875G
`
W
R
2
8
4
Pump
System
3 Stub
Tuner
2.45 GHz
Magnetron
Quartz Tube
Stainless Steel Wall
MW Loss
Protection
Bent
Magnetic
Field(BT)
LFS
Injection
HFS Injection
EBW Heating Experiments in linear device
Experimental Setup
Objectives : To find the feasibility of ECH from EBW
via XB mode conversion.
Small cylindrical device with axial magnetic field bent
similar to tokamak
Possible to change the direction of MW injection mode
– LFS&HFS
Easily changeable of magnetic field and MW power –
movable location of ECR
Antenna : open waveguide(WR284)
54/22
LHY_JMVQ_DEC17
Joint meeting on VEST & QUEST on December 17th 2015, at Kyushu Univ.
Diagnostics – measured at r = 0
The plasma with higher electron density generates from
LFS X mode than from HFS X mode.
As shown in CMA diagram, LFSX/HFSX injection does not
exceed R/L cutoff but in the experiment LFSX injection
overcomes the R cutoff via mode conversion.
Feasibility of EBW via direct XB mode conversion
300 400 500 600 700 800 9000.0
1.5
3.0
4.5
6.0
7.5
ne
[1
01
7m
-3]
Microwave Power [W]
High Field Side X-mode
Low Field Side X-mode
L-cutoff density
1.45 x 1017
m-3
ECH Experiment in linear device
Over-dense plasma generation
55/36
30 35 40 45 50 55 60 65 70 75 80 853
6
9
12
15
18
21
24
Te [
eV
]
R [cm]
X-mode_2kW
X-mode_3kW
X-mode_4kW
X-mode_6kW
ECR
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
nUH
nR
ne [10
17m
-3]
R [cm]
X-mode_2kW
X-mode_3kW
X-mode_4kW
X-mode_6kW
ECR
EC / EBW pre-ionization
Analysis on VEST pre-ionization plasma
UHR
Power absorption near the UHR(ne) by collisional damping and ECR(Te) by collisionless damping respectively.
Shorten density scale length by steep density gradient near outer limiter
1D full wave code has been developed and XB mode conversion efficiency is estimated to be ~20%.
ECR
J.G. Jo and S.H.Kim
56/36
EC / EBW pre-ionization
1D full wave simulation for QUEST
J.G. Jo and S.H.Kim
1D Full wave simulation
XB Mode conversion efficiency calculation
Condition : MW frequency, toroidal magnetic field, density profile
Kim, S. H., et al. Physics of Plasmas 21(6) (2014).
"One-dimensional full wave simulation on XB
mode conversion in electron cyclotron heating."
Example : 1D Full wave simulation
QUEST Simulation (8.2 GHz)
Density profile: Assumption
- Peak density (2e18, 5.5e18 #/m3)
XB Mode conversion coefficient
- 2e18 #/m3: ~1%
- 5.5e18 #/m3: ~38% 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.00E+000
5.00E+017
1.00E+018
1.50E+018
2.00E+018
2.50E+018
3.00E+018
3.50E+018
4.00E+018
4.50E+018
5.00E+018
5.50E+018
6.00E+018
8.2 Ghz
2nd
8.2 Ghz
1st
De
nsity
Distance (m)
R cutoff
UHR
L cutoff
O Cutoff
2e18
5e18
28 Ghz
2nd
57/36 Y.S. Kim and S.G. Ham
Profile diagnostics
Imaging Fabry-Perot interferometry
• Gas and Ion temperature measurement
– Hα : Gas temperature
– Ov : Ion temperature at core
• High resolution without scanning
• Preliminary experiment is in progress
• Measure Isotope shift (Hα Dα lamp)
– Take pictures by Nikon camera
(f = 120 mm, mirror gap = 1.5 mm)
– Real value : 0.178 nm
– Measured value : 0.172 ± 0.014 nm
58/36
Future Research Plans
Innovative Divertor Studies
K.S. Chung and K.J. Chung
Direct Energy Conversion concept applied in VEST divertor
Conceptual design of DEC applied on VEST