G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Educational and research programme on small 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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Scientific scope and Working Plan for the 2nd year of the Project include:• Upgrade Data acquisition and processing systems

• New Plasma diagnostics

• Plasma control

• Measuring of plasma characteristics, testing of different control codes,

experiments on creating vertical-unstable plasma column and controlling

such configuration.

• Tokamak startup studies

• Heating of plasma (ECRH), in collaboration with T-10.

• Education program for undergraduate and postgraduate students.

• University tokamak “MINI” design studies

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Upgrade to the Data acquisition and processing

systems:

• Computer-based data acquisition system was set up, tested and

calibrated. It consists of 96 fast ADC.

• This system allows to collect all experimental data with high time

resolution (500kHz).

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

New Plasma diagnostics

• New optical diagnostics, SpectraPro high-resolution spectrometer (SP-

2358i) with attached high-speed detector (CMOS pco.1200hs), has been

commissioned. It includes a direct digital grating scan mechanism with

full wavelength scanning capabilities.

• Detector is equipped with 1280х16 pc2 CMOS camera and can process

40000 spectrum measurements per second. Additionally, intensity of

fixed spectral line can be measured with photomultiplier attached to the

second output of the spectrometer.

• Experiments with 48-channels electro-magnetic diagnostic for plasma

shape reconstruction have started.

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

DATA ACQUISITION AND PROCESSING COMPLEX

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Plasma control

Plasma parameters required for control purposes were

measured. Plasma response time on different control

algorithms and vessel influence on control dynamics was

measured. Vertical and horizontal feedback control

systems were commissioned and tested.

An experiment to create vertical-unstable plasma column and

to control such configuration was carried out.

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Horizontal program control

Digital controller Power switch

Control signal

Vertical field coil

Plasma column

Start pulseCapacitor bank

Charge and voltage control

system

PC

Settings

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

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

Capacitor bank:Voltage: 450VCurrent: 39600 µF

Digital controller:PIC 16F876 Communications: UART

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

Charge and voltage control

system

Summation unit

Main parameters of vertical control system: Power switch:

Voltage: 1000VCurrent: 200A (400 A in

pulse)Frequency: 100 kHz

Capacitor bank:Voltage: 1000VCurrent: 19800 µF

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

Flux w/o feedbackFlux with feedback

f/b input f/b input

f/b request level f/b request

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Tokamak startup studies

• Non solenoid startup was investigated:

- 300A achieved, duration 0.3ms, using 30kW 9.4GHz ECR.

- Dependence of current amplitude on RF power and vertical

fields was examined.

• Scenario with increasing toroidal field to provide constant

stability factor (qa(t) = const) during current ramp-up was

developed.

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

ECR discharge, experiment set-up.

FUNDAMENTAL RESONANCE at R = 16cm for B0=0.15T

MICROVAWE POWERWAVE LENGTH 30mm

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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 results to what observed on START tokamak at Culham

0 1 2 30

100

200

300

400

b/d

dela

y,

s

Pressure, x 10-3mm

20 kW 10 kW 5 kW

b/d delay dependence on filling pressure H intensity dependence on filling pressure

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

• no pronounced maximum of H dependence at 5 kW – new result

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

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

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

RF probeRF probe

H H

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

ECR Discharge.

Gas pressure 3.75*10-5 torr Microwave power 20kW

Gas pressure 2.5*10-5 torr Microwave power 20kW

At even lower filling pressure breakdown delay increases

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

H

RF probe RF probe

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

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

H

RF probe

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Why there is a breakdown delay?• Common view is that when microwave power is ON, electron density rises to threshold value, then breakdown happens. This b/d delay depends on gas pressure, microwave power and poloidal fields.• To check, second RF pulse was applied with some delay to the first one• No b/d delay was observed during second pulse at same pressure, RF power, magnetic field (even if there was no light emission during 1st pulse)

HH

RF probe

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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)

Uloop

H

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

ECR preionization with applied Uloop

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 at Uloop ~15V

ECR breakdown not happens, however ohmic field breakdown occurs.

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

Delay between ECR and ohmic field breakdown is increasing up to 4ms.

Uloop Uloop

H

H

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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 8ms.

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 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.

Btor

H

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

ECR preionization experiments

• Delay in light emission 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.

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Heating of plasma (ECRH)

• ECRH experiments, with RF power source with power 30kW and 9.4 GHz

were carried out.

• Experiment with RF power up to 200 kW is now under preparation in

collaboration with T-10 team.

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Education program for undergraduate and postgraduate students

• Undergraduate students participated in computation of magnetic

configurations, equilibrium and stability conditions and experimental

verification of theoretical calculations on GUTTA tokamak and in control

and data manipulation software developing.

• Laboratory work ”Plasma equilibrium control in a tokamak” was

prepared and tested.

• Two graduate students and one post-graduate student participated in

the 2nd JE at T-10

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

University tokamak “MINI” design studies

Main design features of tokamak “Mini” were chosen. Calculations have

been performed to verify these parameters.

Major radius, cm 20 Plasma current, kA 200

Minor radius, cm 10 Plasma density, cm -3 3·1014

Aspect ratio 2 Electron temperature, eV 200

Elongation 3 Ion temperature, eV 150

Toroidal magnetic field, T 2 Energy confinement time,ms 2

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

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

G Vorobjev, GUTTA, 2nd RCM, Beijin, 2006

GUTTA

Saint-Petersbrg

State University

Future plans Plasma modeling and controlDevelopment and verification of new mathematical models are scheduled. Improvement and adaptation of different control methods under control system capabilities will be performed. Development of control hardware is planned.

Reverse current preionizationExperiments on preionization with reverse current are scheduled. Dependenses on vertical, toridal, ohmic fields and RF power will be studied.

Optical diagnosticsExperiments to determine plasma temperature and density with optical diagnostics are scheduled.

Outer magnetic surface shape reconstructionTokamka Gutta is equipped with 48-channel electro-magnetic diagnostic for plasma shape reconstruction. Development, approbation and comparison of different mathematical methods of shape reconstruction are scheduled. Postgraduate students will participate in this activity.