control system and breakdown studies on a small spherical tokamak gutta

G Vorobjev, GUTTA, Chengdu

GUTTA

Saint-Petersbrg

State University

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 UniversityZubov Institute

of Computational Mathematics and Control Processes,Faculty of Applied Mathematics and Control Processes

AcknowledgementsThis 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


GUTTA

Saint-Petersbrg

State University

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


GUTTA

Saint-Petersbrg

State University

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 1.5plasma current Ip, ka 100

GUTTA, IOFFE, USSR (1980-1986)

GUTTA is now fully operational at St. Petersburg State University, Russia

GUTTA at Ioffe Institute, 1984


GUTTA

Saint-Petersbrg

State University

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


GUTTA

Saint-Petersbrg

State University

DATA ACQUISITION AND PROCESSING

Measurement channels number 96Input voltage range, В ±1,25Input resistance, Ом 100Sampling interval, μs 2,4,6,8,10,12,14,16Input signals sampling 5461digital capacity 11bit + sign

ADC boards Control and diagnostics complex


GUTTA

Saint-Petersbrg

State University

Spectroscopic diagnostics

Spectroscopic diagnostics block-scheme


GUTTA

Saint-Petersbrg

State University

Optical diagnostics

pco.1200 hs CMOS detector

Spectrograph SpectraPro SP-2358:

Specifications (1200g/mm Grating):Focal length: 300mmAperture Ratio: f/4Optical Design: Imaging Czerny-Turner with original polished aspheric mirrorsOptical Paths: 90° standard, 180° and multi-port optionalScan Range: 0 to 1400nm mechanical rangeOperating Range: 185nm to the far infrared with available gratings and accessoriesResolution: 0.1nm at 435.8nmDispersion: 2.7nm/mm (nominal)Accuracy: ±0.2nmRepeatability: ±0.05nmDrive Step Size: 0.0025nm (nominal)Focal Plane Size: 27mm wide x 14mm high

Spectrograph SpectraPro SP-2358


GUTTA

Saint-Petersbrg

State University

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.


GUTTA

Saint-Petersbrg

State University

Horizontal feedback control system

Integrator Comparator Power switch

Diagnostic coils

Displacemet signal

Control signal

Current

Vertival mageticfield

Diagnostics

Vertical field coil

Plasma column

Magnetic flux

Start pulse

Magnetic flux changing

Capacitor bank

Charge and voltage control

system

Main parameters of horizontal feedback control system: Power switch

Voltage: 500VCurrent: 400A (1,2 kA in pulse)Frequency: 100 kHz

Capacitor bank:Voltage: 450VCurrent: 39600 µF


GUTTA

Saint-Petersbrg

State University

Horizontal program control

Digital controller Power switch

Control signal

Vertical field coil

Plasma column

Start pulseCapacitor bank


system

PC

Settings

Main parameters of horizontal pre-program control system:Power switch:

Voltage: 500VCurrent: 400A (1,2 kA in pulse)Frequency: 100 kHz


Digital controller:PIC 16F876 Communications: UART


GUTTA

Saint-Petersbrg

State University

Vertical feedback control system

Integrator Comparator Power switch

Diagnostic coils

Displacemet signal

Control signal

Current

Vertival mageticfield

Diagnostics

Vertical field coil

Plasma column

Magnetic flux

Start pulse

Magnetic flux changing

Capacitor bank


system

Summation unit

Main parameters of vertical control system: Power switch:

Voltage: 1000VCurrent: 200A (400 A in

pulse)Frequency: 100 kHz



GUTTA

Saint-Petersbrg

State University

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


GUTTA

Saint-Petersbrg

State University

ECR discharge, experiment set-up.

FUNDAMENTAL RESONANCE FOR B0=0.15T

MICROVAWE POWERWAVE LENGTH 30mm


GUTTA

Saint-Petersbrg

State University

0 1 2 3 4 5 6 7 8 90

100

200

300

400

H,

a.u

.

Pressure, x 10-3mm

20 kW 1st peak 20 kW between peaks 10 kW 1st peak 10 kW between peaks

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

• H intensity reduces with RF power

• very similar dependence of H intensity on pressure to what

observed on START

0 1 2 30

100

200

300

400

b/d

dela

y,

s

Pressure, x 10-3mm

20 kW 10 kW 5 kW


GUTTA

Saint-Petersbrg

State University

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

GUTTA: 9.4 GHz, 5 - 20 kW, 0.4 ms TF ~ 0.15 T, O-mode launch

0 2 4 6 80

100

200

300

400

H,

a.u

.

Pressure, x 10-3mm

20 kW 1st peak 10 kW 1st peak 5 kW 1st peak

• H intensity reduces with RF power

• very similar dependence of H intensity on pressure to what observed on START

• no pronounced maximum of H dependence at 5 kW


GUTTA

Saint-Petersbrg

State University

ECR Discharge.

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

Gas pressure 1.75*10-4 torr Microwave power 20kW


Top, green – visible light; bottom, yellow – RF power at 900 in toroidal angle


GUTTA

Saint-Petersbrg

State University

ECR Discharge.



At even lower filling pressure breakdown delay increases



GUTTA

Saint-Petersbrg

State University

ECR Discharge. UV lamp assisted b/d

Ultra-violet lamp assists breakdown at low pressure

Gas pressure 2*10-5 torr Microwave power 4 kW

Ultra-violet off – no b/d

Gas pressure 2*10-5 torr Microwave power 4 kW

Ultra-violet on – clear b/d



GUTTA

Saint-Petersbrg

State University

Self-oscillations of light emission – old results

Light emission during ECR discharge in tokamak

Light emission during electrode discharge in linear device

The same processes observed in another devices and even in electrode discharges

B.N. Shustrov, A I. Anisimov, N. Blashenkov. G.Y. Lavrentyev. G.G. Petrov, “Self-organizing in gas discharge”,

Preprint Ioffe Institute, Leningrad,1988


GUTTA

Saint-Petersbrg

State University

1 ms1 ms

5 ms

Top, yellow – visible light; bottom, green – microwave power

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.


GUTTA

Saint-Petersbrg

State University

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)


GUTTA

Saint-Petersbrg

State University

ECR preionization

1 ms 4 ms

Top, yellow – visible light; bottom, green – microwave power, red-loop voltage

Standard breakdown order

Breakdown does not occur without microwave power.

ECR breakdown not happens, however ohmic field breakdown occurs.

Delay between ECR and ohmic field breakdown is increasing up to 1ms.



GUTTA

Saint-Petersbrg

State University

ECR preionization

8 ms 15 ms

30 ms50 ms

Top, yellow – visible light; bottom, green – microwave power, red-current in TF coils


Delay between ECR and ohmic field breakdown is increasing up to 15ms. Toroidal field between breakdowns is absent.




GUTTA

Saint-Petersbrg

State University

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.


GUTTA

Saint-Petersbrg

State University

Plasma Formation in CTF

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 1073520

Inboard shieldF82H

soft ironCentre pin

GlidcopCentre rod

Stainless steelRod casing

Fe pin radius = 0.18m gives 100 mVsec which is enough to ramp Ipl to 300kA.

• No central solenoid in CTF concept design requires alternative formation schemes

CTF, Culham design with iron pin


GUTTA

Saint-Petersbrg

State University

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


GUTTA

Saint-Petersbrg

State University

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

• Flux measurements have been done with and without TF coil

measured flux structure

measuring coils

z

plasma


GUTTA

Saint-Petersbrg

State University

Plasma Formation in CTF: GUTTA 1:5 model

z, cm

V

Coil signal (flux) vs distance from induction coil:

red – without TF coil; black – with TF coil

• 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


GUTTA

Saint-Petersbrg

State University

Future plans

• Developing and verification of plasma mathematical models and control methods.

• Studies of plasma vertical instability dynamics.

• Optical measurements to determine plasma temperature.

control system and breakdown studies on a small spherical tokamak gutta

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