Magnetic Diagnostics for GLAST-III Tokamak

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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  
2
Outline of the Talk

• GLAST Tokamak
• Magnetic Diagnostics
• Rogowski Coils
• Magnetic Probes
• Flux Loops
• Error fields
• Initial Results
• Summary

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
3
GLAST-III (Glass Spherical Tokamak)

• Its Vessel is of Pyrex
• Major radius 20cm
• Minor Radius10 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)

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
4
GLAST-III
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
5
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.

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
6
Magnetic Probes
Measured Parameters Winding Wire Diameter of Probe mm Winding Length cm 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
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
7
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
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
8
Pulse Behavior of Magnetic Probes

Pulse generator which is capable of generation
pulses of different time durations.
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
9
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.

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
10
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(O) Winding wire   Turns per cm Length (cm) Diameter (mm) Sensitivity Vs/A
Rogowski-III 81.3 24 Swg 36 10 169 7 3.35x10-8
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
11
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.
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
12
Calibration of Rogowski Coil
Rogowski coil Signal recoded with 47 ms RC
integrator Chanel-1 Current signal and Chanel
-2 Rogowski signal
Behavior of Rogowski coil along with passive
integrator at different values of current.
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
13
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 plane d with a
single turn of insulated wire installed
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
14
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

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
15
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 / p( R22-R12)
• Where R2 31.8 cm
• R1 9.2 cm

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
16
Initial Shots of GLAST-III
(a)Vertical field applied
(b)No vertical field applied
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
17
Summary

• 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.
•  

IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015  
18
Thanks
IAEA First Technical Meeting on Fusion Data
Processing, Validation and Analysis 1 3 June
2015