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GUTTA
Control system and breakdown studies
on a small spherical tokamak Gutta.
G.M. Vorobyov, D.A. Ovsyannikov, A.D. Ovsyannikov, E.V.
Suhov, E. I. Veremey, A. P. Zhabko
St. Petersburg State University
Zubov Institute
of Computational Mathematics and Control Processes,
Faculty of Applied Mathematics and Control Processes
Acknowledgements
This work was partly funded by the IAEA CRP “Joint Research Using Small Tokamaks”
This work is carrying out in the framework of Saint-Petersburg State University project
“Innovation educational environment in a classical university
G Vorobjev, GUTTA, Chengdu
Saint-Petersbrg
State University
GUTTA
OUTLINE
• History and main parameters of Gutta
• Main diagnostics and data acquisition
• Plasma position control systems
• Main experimental results
– ECR breakdown studies
– b/d using reversed current
– Iron core
– Horizontal position control studies
• Conclusions and future plans
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GUTTA
GUTTA, IOFFE, USSR (1980-1986)
GUTTA was one of the first attempts to built
a spherical tokamak,
G.M. Vorobyev et al, Ioffe Institute, 1980-86
Main parameters:
major radius R, cm
16
minor radius a, cm
8
aspect ratio A
2
vessel elongation k 2
toroidal field, T
1.5
plasma current Ip, ka 100
GUTTA at Ioffe Institute, 1984
GUTTA is now fully operational at St.
Petersburg State University, Russia
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MAIN DIAGNOSTICS
• Magnetics: 2 Rogowski coils for Ip, Rogowski coils for PF and TF
currents, 2 flux loops at midplane;
• Z and R position control, shape control: array of 24 pick-up coils (2
components at one toroidal position), 6 Mirnov coils - toroidal array at
midplane;
• Photomultiplier
• 94 GHz interferometer
• Spectrometer/monochromator with CMOS camera
• RF power detector at 900 in toroidal direction at midplane
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DATA ACQUISITION AND
PROCESSING
ADC boards
Control and
diagnostics complex
Measurement channels number
96
Input voltage range, В
±1,25
Input resistance, Ом
100
Sampling interval, μs
2,4,6,8,10,12,14,16
Input signals sampling
5461
digital capacity
11bit + sign
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Spectroscopic diagnostics
Spectroscopic diagnostics block-scheme
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Optical diagnostics
Spectrograph SpectraPro SP-2358:
Spectrograph SpectraPro SP-2358
Specifications (1200g/mm Grating):
Focal length: 300mm
Aperture Ratio: f/4
Optical Design: Imaging Czerny-Turner with
original polished aspheric mirrors
Optical Paths: 90° standard, 180° and multi-port
optional
Scan Range: 0 to 1400nm mechanical range
Operating Range: 185nm to the far infrared with
available gratings and accessories
Resolution: 0.1nm at 435.8nm
Dispersion: 2.7nm/mm (nominal)
Accuracy: ±0.2nm
Repeatability: ±0.05nm
Drive Step Size: 0.0025nm (nominal)
Focal Plane Size: 27mm wide x 14mm high
pco.1200 hs CMOS detector
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Plasma control systems
Plasma control systems on Gutta consists of:
•
Vertical and horizontal position feedback control systems.
•
Horizontal plasma position pre-programmed control.
Horizontal control system was build, tested and commissioned
Testing and tuning of vertical control system are in progress.
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Horizontal feedback control system
Main parameters of horizontal feedback control system:
Power switch
Voltage: 500V
Current: 400A (1,2 kA in pulse)
Frequency: 100 kHz
Capacitor bank:
Voltage: 450V
Current: 39600 µF
Capacitor bank
Start pulse
Integrator
Charge and
voltage control
system
Diagnostics
Displacemet
signal
Comparator
Control
signal
Magnetic
flux
changing
Power switch
Current
Diagnostic coils
Vertical field coil
Magnetic flux
Vertival magetic
field
Plasma column
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Horizontal program control
Main parameters of horizontal pre-program control system:
Power switch:
Voltage: 500V
Current: 400A (1,2 kA in pulse)
Frequency: 100 kHz
Capacitor bank:
Voltage: 450V
Current: 39600 µF
Digital controller:
PIC 16F876
Start pulse
Communications: UART
Digital controller
Control
signal
Charge and
voltage control
system
Capacitor bank
Power switch
Settings
Vertical field coil
PC
Plasma column
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Vertical feedback control system
Main parameters of vertical control
system:
Power switch:
Voltage: 1000V
Current: 200A (400 A in pulse)
Frequency: 100 kHz
Capacitor bank:
Voltage: 1000V
Current: 19800 µF
Start pulse
Integrator
Magnetic
flux
changing
Charge and
voltage control
system
Capacitor bank
Diagnostics
Summation
unit
Control
Comparator signal
Displacemet
signal
Diagnostic coils
Magnetic
flux
Vertival
magetic
field
Power
switch
Current
Vertical field coil
Plasma column
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Horizontal control system
Green- Magnetic flux through midplane
Green- Magnetic flux through midplane
Yellow- Control pulses
Yellow- Control pulses
Red-magnetic flux zero level
Red-magnetic flux zero level
White-control system threshold value
White-control system threshold value
Control feedback system OFF
Control feedback system ON
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ECR discharge, experiment set-up.
MICROVAWE POWER
WAVE LENGTH 30mm
FUNDAMENTAL RESONANCE FOR B0=0.15T
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ECR breakdown in pure Toroidal field
400
20 kW
10 kW
5 kW
300
200
100
0
20 kW 1st peak
20 kW between peaks
10 kW 1st peak
10 kW between peaks
300
H, a.u.
b/d delay, s
400
200
100
0
1
2
3
-3
Pressure, x 10 mm
0
0
1
2
3 4 5 6 7
-3
Pressure, x 10 mm
8
9
• breakdown delay increases at low pressure
• no dependence of b/d delay on RF power at 5 - 20 kW
• H intensity reduces with RF power
• very similar dependence of H intensity on pressure to what
observed on START
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Comparison of ECR b/d on START and GUTTA
START: 2.45GHz ~1.0 kW, 3.5ms
TF < 0.2 T, O- and X-mode launch
2nd harmon., XM
fundamental, XM
50
300
0
0.0 0.5 1.0 1.5 2.0-4 2.5
Pressure, mb x 10
100
2nd harmon., OM
fundamental, OM
H, a.u.
20 kW 1st peak
10 kW 1st peak
5 kW 1st peak
400
H, a.u.
H, a.u.
100
GUTTA: 9.4 GHz, 5 - 20 kW, 0.4 ms
TF ~ 0.15 T, O-mode launch
200
100
0
0
2
4
6
-3
Pressure, x 10 mm
8
• H intensity reduces with RF power
50
• very similar dependence of H intensity on
pressure to what observed on START
0
0.0 0.5 1.0 1.5 2.0 -42.5
Pressure, mb x 10
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• no pronounced maximum of HSaint-Petersbrg

dependence at 5 kW
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ECR Discharge.
Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle
Gas pressure
Microwave power
1.75*10-4 torr
20kW
Gas pressure
Microwave power
1.75*10-4 torr
20kW
During ECR discharge with constant microwave power and
some specific conditions (such as middle gas pressure, high
microwave power, not very good conditioned wall) regular
self-oscillations of visible light emission appear
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ECR Discharge.
Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle
Gas pressure
Microwave power
3.75*10-5 torr
20kW
Gas pressure
Microwave power
2.5*10-5 torr
20kW
At even lower filling pressure breakdown delay increases
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ECR Discharge. UV lamp assisted b/d
Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle
Gas pressure
Microwave power
2*10-5 torr
4 kW
Ultra-violet off – no b/d
Gas pressure
Microwave power
2*10-5 torr
4 kW
Ultra-violet on – clear b/d
Ultra-violet lamp assists breakdown at low pressure
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Self-oscillations of light emission –
old results
Light emission during ECR discharge in
tokamak
Light emission during electrode
discharge in linear device
B.N. Shustrov, A I. Anisimov, N. Blashenkov. G.Y. Lavrentyev. G.G. Petrov, “Self-organizing in gas discharge”,
Preprint Ioffe Institute, Leningrad,1988
The same processes observed in another devices and even
in electrode discharges
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Why there is a breakdown delay?
Common view is that after microwave power is ON, electron density rises to
threshold value, after breakdown occurrence. Delay may depend on gas pressure,
microwave power and poloidal fields.
1 ms
1 ms
5 ms
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Top, yellow – visible light; bottom, green – microwave power
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Reverse current preionization
Top, yellow – visible light; bottom, green – Loop voltage
• Reverse current preionization experiments were carried out.
• Preionization using plasma current reversal is as effective as
ECR preionisation (same light emission level)
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ECR preionization
Breakdown does not occur without microwave power.
Top, yellow – visible light; bottom, green – microwave power, red-loop voltage
Standard breakdown order
1 ms
Delay between ECR and ohmic field
breakdown is increasing up to 1ms.
ECR breakdown not happens, however
ohmic field breakdown occurs.
4 ms
Delay between ECR and ohmic field
breakdown is increasing up to 4ms.
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ECR preionization
Top, yellow – visible light; bottom, green – microwave power, red-current in TF coils
8 ms
Delay between ECR and ohmic field
breakdown is increasing up to 8ms.
30 ms
Delay between ECR and ohmic field breakdown is
increasing up to 30ms. Toroidal field between
breakdowns is absent.
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15 ms
Delay between ECR and ohmic field breakdown is
increasing up to 15ms. Toroidal field between
breakdowns is absent.
50 ms
Delay between ECR and ohmic field breakdown is
increasing up to 50ms. Toroidal field between
breakdowns is absent.
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ECR preionization experiments
• Delay in light oscillations at constant microwave power
during ECR discharge, ECR and Ohmic field breakdown
depends not only on processes in vacuum chamber, but
on vacuum vessel wall conditions
• Preliminary cleaning methods, ultraviolet radiation before
breakdown, ECR preionization (even without breakdown)
affects these conditions.
• Consequence of such influence stay for a long time,
which is typical not for charged particles lifetime, but for
chemical processes on vacuum vessel walls.
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Plasma Formation in CTF
• No central solenoid in CTF concept design
requires alternative formation schemes
Ferrite steel shielding of the central post and ferrite central rod can
provide enough flux for breakdown and initial current formation
for use of ferrite steel in JTF-2M see: M Sato, et al., Fusion Eng. Des., 51-52 2000 1073
CTF, Culham design with iron pin
Fe pin radius = 0.18m gives 100 mVsec
which is enough to ramp Ipl to 300kA.
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Plasma Formation in CTF
Inspired by Culham’s new CTF
design with the use of Ferritic
steel central rod, 1:5 (scale)
model of the CTF central post
has been installed in GUTTA
We plan to use GUTTA
tokamak for proof-of-principle
demonstration
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Plasma Formation in CTF: GUTTA 1:5 model
Soft iron rod and Al imitation of TF coil (not shown in photo)
Induction coils: 50Hz, 4A x 1000turns
z
measuring coils
plasma
measured flux structure
• Flux measurements have been done with and without TF coil
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Plasma Formation in CTF: GUTTA 1:5 model
• How much flux at midplane can be produced?
V
• flux loss by factor of 5 due to iron
saturation, some of it can still be
used during ramp-up
• solid TF coil requires radial cuts
for flux penetration
z, cm
Coil signal (flux) vs distance from induction coil:
red – without TF coil; black – with TF coil
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Future plans
• Developing and verification of plasma
mathematical models and control
methods.
• Studies of plasma vertical instability
dynamics.
• Optical measurements to determine
plasma temperature.
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