Control system and breakdown studies on a small spherical tokamak Gutta'

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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
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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, 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
Control and diagnostics complex
ADC boards
Measurement channels number 96 Input
voltage range, ? 1,25 Input resistance, ??
100 Sampling interval, µs 2,4,6,8,10,12,14,16 Inp
ut 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 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
Spectrograph SpectraPro SP-2358
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
Charge and voltage control system
Capacitor bank
Start pulse
Diagnostics
Displacemet signal
Control signal
Power switch
Integrator
Comparator
Magnetic flux changing
Current
Diagnostic coils
Vertical field coil
Vertival magetic field
Magnetic flux
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
Communications UART
Charge and voltage control system
Start pulse
Capacitor bank
Control signal
Digital controller
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
Charge and voltage control system
Capacitor bank
Start pulse
Diagnostics
Control signal
Comparator
Summation unit
Power switch
Integrator
Magnetic flux changing
Displacemet signal
Current
Diagnostic coils
Vertical field coil
Vertival magetic field
Magnetic flux
Plasma column
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Horizontal control system
Green- Magnetic flux through midplane Yellow-
Control pulses Red-magnetic flux zero
level White-control system threshold
value Control feedback system OFF
Green- Magnetic flux through midplane Yellow-
Control pulses Red-magnetic flux zero
level White-control system threshold
value Control feedback system ON
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ECR discharge, experiment set-up.
MICROVAWE POWER WAVE LENGTH 30mm
FUNDAMENTAL RESONANCE FOR B00.15T
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ECR breakdown in pure Toroidal field

• breakdown delay increases at low pressure
• no dependence of b/d delay on RF power at 5 - 20
  kW
• Ha intensity reduces with RF power
• very similar dependence of Ha 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 lt 0.2 T, O- and
X-mode launch
GUTTA 9.4 GHz, 5 - 20 kW, 0.4 ms TF 0.15 T,
O-mode launch

• Ha intensity reduces with RF power
• very similar dependence of Ha intensity on
  pressure to what observed on START
• no pronounced maximum of Ha 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 1.7510-4 torr Microwave power 20kW
Gas pressure 1.7510-4 torr Microwave power 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 3.7510-5 torr Microwave power 20kW
Gas pressure 2.510-5 torr Microwave power 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 210-5 torr Microwave power 4
kW Ultra-violet on clear b/d
Gas pressure 210-5 torr Microwave power 4
kW Ultra-violet off no 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
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
ECR breakdown not happens, however ohmic field
breakdown occurs.
1 ms
4 ms
Delay between ECR and ohmic field breakdown is
increasing up to 4ms.
Delay between ECR and ohmic field breakdown is
increasing up to 1ms.
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ECR preionization
Top, yellow visible light bottom, green
microwave power, red-current in TF coils
15 ms
8 ms
Delay between ECR and ohmic field breakdown is
increasing up to 15ms. Toroidal field between
breakdowns is absent.
Delay between ECR and ohmic field breakdown is
increasing up to 8ms.
50 ms
30 ms
Delay between ECR and ohmic field breakdown is
increasing up to 30ms. Toroidal field between
breakdowns is absent.
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
Fe pin radius 0.18m gives 100 mVsec which is
enough to ramp Ipl to 300kA.
CTF, Culham design with iron pin
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Plasma Formation in CTF
Inspired by Culhams new CTF design with the use
of Ferritic steel central rod, 15 (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 15 model
Soft iron rod and Al imitation of TF coil (not
shown in photo) Induction coils 50Hz, 4A x
1000turns
plasma
measured flux structure

• Flux measurements have been done with and
  without TF coil

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Plasma Formation in CTF GUTTA 15 model

• How much flux at midplane can be produced?

• 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

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.