Magnetic Diagnostics for GLAST-III Tokamak M. A. Naveed, Aqib javeed and GLAST Team National Tokamak Fusion Programme Islamabad Pakistan IAEA First Technical

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Magnetic Diagnostics for GLAST-III Tokamak

M. A. Naveed, Aqib javeed and GLAST Team

National Tokamak Fusion Programme Islamabad Pakistan

IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015

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Outline of the Talk 1. GLAST Tokamak2. Magnetic Diagnostics3. Rogowski Coils 4. Magnetic Probes5. Flux Loops6. Error fields7. Initial Results8. Summary


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GLAST-III (Glass Spherical Tokamak) Its Vessel is of Pyrex Major radius =20cm Minor Radius=10 cm 12 TF Coils and Central Solenoid Capacitor Banks are used for energizing different coils Central solenoid 2.2mF , 6kV, 40kJ TF coils 484mF, 450V, 49kJ Pre-Ionization Source is 2.45 GHz microwave oven

Magnetron( 800 Watt)


4IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015

GLAST-III

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Magnetic Diagnostics

Magnetic measurements are fundamental diagnostic for tokamak start up and operation .

GLAST with dielectric chamber facilitates us to install every pick up coil outside the vacuum. No current will be flowing in the chamber so a single Rogowski coil will be providing true plasma current.

Magnetic Probes, Flux loops and Rogowski coil are major magnetic diagnostics in GLAST-III.


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Magnetic Probes

Measured Parameters

Winding Wire

Diameter of Probe mm

Winding Lengthcm

Total turns

DC Resistance (Ohm)

Inductance(mH)

Big Probes 36 swg 12 4 2000 56.8 27.8

Small Probes 36 swg 7.5 mm 1.34 260 4 0.160


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Calibration of Magnetic Probes

• Helmholtz Coils• SWG 16(5A dc curret

Capacity)• Hall probe for

calibrating• 9.5 gauss per

ampere

The effective area ‘NA’ is given by


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Pulse Behavior of Magnetic Probes

Pulse generator which is capable of generation pulses of different time durations.


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Pulse Behavior of Magnetic Probes

For finding Magnetic field the differential signals are integrated.

Simplest are the RC Integrators.

In these studies 10ms, 22 ms and 47 ms time constant integrators are used.


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Fabrication of Rogowski Coil

RG-11 coaxial cable is used. The central conductor serves as return path.

Uniform winding is required so groves are made with dye.

Measured Parameters

L(µH)

R(Ω) Winding wire

Turns per cm

Length(cm)

Diameter(mm)

SensitivityVs/A

Rogowski-III 81.3 24 Swg 36 10 169 7 3.35x10-8


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Calibration of Rogowski Coil Calibration Setup

AC and pulsed sources are used for exiting a multi turn coil having number of turns up to 200 so that Rogowski coil when passed through this multi turn coil encounters a reasonable current to be measured. The LEM current monitor is used as a reference for calibration of Rogowski coils.


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0 10 20 30 40 50 60 700

5

10

15

20

25

30

35

40

Rogow

ski c

oil

outp

ut(m

v)

Current (kA)

Rogowski coil Signal recoded with 47 ms RC integratorChanel-1: Current signal andChanel -2: Rogowski signal


Behavior of Rogowski coil along with passive integrator at different values of current.

Calibration of Rogowski Coil

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Flux loops on GLAST-III

In GLAST-III loop voltage is measure horizontally at the mid plane of the chamber. Three differential pairs of poloidal flux loops are installed for the poloidal flux measurement at different locations. One of these pairs as shown in Fig is installed on the mid planed with a single turn of insulated wire installed


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Error Field corrections using flux loops

There are some error fields generated in Tokamak because of misalignments and winding inaccuracy of Toroidal field and Ohmic current drive coil systems.

These error fields create problems not only in the startup of Tokamak but also limit the pulse duration.

These fields must also be measured and compensated


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Error field minimization

Two loops are wrapped in toroidal direction at the mid plane with radii 9.2 cm and 31.8 cm. These loops are connected to each other in such a way that average vertical field in the region where plasma is to be generated is measured. The average vertical magnetic field will be given as:Average vertical Field= Vertical Flux / π( R2

2-R12)

Where R2 = 31.8 cm

R1= 9.2 cm

0.0 1.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0 13.5 15.0 16.5 18.0 19.5-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

CS and TF combine

TF

Ver

tical

Mag

netic

Fie

ld(G

auss

)

Time(mS)

CS


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Initial Shots of GLAST-III

-2 0 2 4 6 8 10

-6

-4

-2

0

2

-10

0

10

20

30-2 0 2 4 6 8 10

Pla

sma

Curr

ent(kA

)

Time(mS)

Loop V

oltag

e(V

olts)

0 4 8

-4

-2

0

2

4

-10

0

10

200 4 8

Pla

sma

Curr

ent(kA

)

Time(mS)

Loop V

oltag

e


(a)Vertical field applied (b)No vertical field applied

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The magnetic diagnostics installed presently on GLAST-III tokamak worked well for the initial operation.

The Rogowski coils used with RC integrators worked well for short duration pulses of plasma current.

The pulse source developed for on bench calibration of diagnostics coils helped to evaluate the response of these coils for pulses, nearly of same time scale and shape which are to be measured on the GLAST-III tokamak.

In future magnetic diagnostics for GLAST-III will be improved so that long duration plasma operation becomes possible. For this purpose long integration time constant active integrators will be developed.

Summary


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Thanks


Documents

Magnetic Diagnostics for GLAST-III Tokamak M. A. Naveed, Aqib javeed and GLAST Team National Tokamak Fusion Programme Islamabad Pakistan IAEA First Technical