1. 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 1

2. Outline of the Talk 1.GLAST Tokamak 2.Magnetic Diagnostics 3.Rogowski Coils 4.Magnetic Probes 5.Flux Loops 6. Error fields 7.Initial Results 8.Summary IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015 2

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

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

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 5

6. Magnetic Probes Measured Parameters Winding Wire Diameter of Probe mm Winding Length cm Total turns DC Resistance (Ohm) Inductance (mH) Big Probes 36 swg12 4200056.827.8 Small Probes 36 swg7.5 mm1.34 26040.160 IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015 6

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 7

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 8

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 9

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(Ω) Winding wire Turns per cm Length (cm) Diameter (mm) Sensitivity Vs/A Rogowski-III81.324Swg 361016973.35x10 -8 IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015 10

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 11

12. Rogowski coil Signal recoded with 47 ms RC integrator Chanel-1: Current signal and Chanel -2: Rogowski signal IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015 Behavior of Rogowski coil along with passive integrator at different values of current. Calibration of Rogowski Coil 12

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 13

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 14 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 / π( R 2 2 -R 1 2 ) Where R 2 = 31.8 cm R 1 = 9.2 cm IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015 15

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

17.  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 IAEA First Technical Meeting on Fusion Data Processing, Validation and Analysis 1 – 3 June 2015 17

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