Capillary discharge system

The demonstration of a large soft X-ray amplification in discharge-created plasma in the year 1993 opened a new path towards the development of table top soft X-ray lasers.

Capillary discharge based XUV source can create long, dense and "stable" plasma column with the advantage of being compact and cost effective. The coherent X-ray source based on fast capillary discharge requires fast current rise-time ( ~2 - 4 x1012 A/s) in the capillary which provides a rapid detachment of the plasma from the capillary walls to limit the amount of material ablated from the walls. This is possible only in the gas filled capillaries with uniform pre-ionization.

Fig. 1 Capillary discharge scheme

One of the methods (shown in Fig. 1) to create fast capillary discharge is by using Marx generator to charge a fast waterline co-axial capacitor up to self breakdown voltage of a spark gap and fast capacitor is discharged through pre-ionized capillary.

A 450 kV, 40 kA capillary discharge plasma system is set up for study of x-ray lasers and waveguide for ultra-short laser pulses.

In this system, our section is involved in development of following systems:

A Marx generator to produce a voltage pulse of 100 to 450 kV and maximum stored energy of 700 J is designed and fabricated. This generator is used to charge fast waterline coaxial capacitor. This is a 10 stage generator consisting of a spark gap column (all the gaps are in one column so it can be pressurized using N2 to get higher erected voltages with same gap length), 60nF capacitors, charging resistors and trigger circuit. Charging and triggering of Marx is done using power conditioning system and controlled by an electronic control system.




2. Pulse forming line consisting of co-axial waterline capacitor, SF6 filled spark gap and capillary chamber. Water line capacitor is made of co-axial geometry its capacity by design is ~6 nF and equivalent series inductance ~70 nH. The spark gap chamber is made from two brass electrodes, in which one is connected to water line capacitor and other to capillary.

   

   A pre-pulse feed through is placed in spark gap chamber and it is connected to the high voltage end of the capillary. The spark gap chamber can be pressurized using SF6 up to 28 psi. Capillary chamber consist of a capillary and Rogowski coil. It can also be pressurized using SF6. Capillary is a hollow tube made of alumina which can be filled with N2 or argon gas at low pressure.

3. Pre-pulse generator is used to pre-ionize the gas filled in capillary. It generates a 20 kV, 1µs/100 µs double exponential pulse at the high voltage end of the capillary. The pre ionization of the capillary is done few microseconds prior to the main discharge current.

   

   The generator consist of a 20 kV SMPS supply, a 0.4 µF capacitor an air core inductor and a copper sulphate resistor. The capacitor is discharge through the inductor and the resistor using a trigatron.

   

Instrumentation for measurement of high voltages and currents. This includes development of copper sulphate divider for measurement of pulse voltages and Rogowski coil for pulse currents. Copper sulphate divider is a resistive divider which can absorb high pulsed energy without deteriorating its property. This divider is made from copper sulphate solution column of 1 kohm and lower resistance of 0.5 ohm across which voltage is measured. Lower resistance is made using 20 MFRs in parallel connected in squirrel cage fashion to minimize inductance and increase power rating. Rogowski coil is being developed in-house to measure current which is more than 20 kA (first peak), damped sinusoidal having rise time ~50ns and time period of ~250ns.
The capillary discharge system is operated by an electronic control system. The control system directs a power conditioning unit, which is used to charge the Marx generator and trigger pre-ionization pulse and main discharge pulse with user given delay in range of 1 to 50 µs. This control system is realized by a microcontroller unit and it is interfaced optically to the power conditioner and pulse circuits.



The user needs to set the parameters by software in a PC which sends commands to the microcontroller unit for various operations. A data acquisition system is also linked with the control system that acquires data after every discharge experiment.