Laser Plasma Section

X-ray generation from capillary discharge plasmas and facility of soft-x-ray laser operating at 46.9 nm in Ar capillary discharge plasma

This activity mainly involves research on generation of soft x-ray laser through fast capillary discharge scheme. The quest for getting lasing action in the x-ray region started right from the invention of laser. A soft x-ray laser has several applications in wide areas of science and technology e.g. Nano-imaging, lithography, X-ray holography, dense plasma diagnostics and many more. Fast capillary discharge scheme is an efficient technique to generate hot and dense plasma column having larger length to diameter ratio (typically 1000:1). Such long plasma columns acts as potential gain medium for soft x-ray amplification under suitable conditions. In our laboratory, soft x-ray laser at a wavelength of 46.9 nm has been successfully built. The laser has been characterized for its temporal, spatial and spectral properties. It has a pulse width of ~ 1.2 ns (FWHM), divergence ~ 3.5 mrad and energy ~ 2-4 μJ in single pulse.

The generation of this laser is based on fast pumping of electrical energy in the plasma by collisional electron excitation. This is achieved by passing a fast electric current (few tens of kA in few tens of nanoseconds) with high dI/dt (> 1011A/s) through a pre-ionized gas column of Ar filled inside a ceramic capillary (Fig.1). The magnetic field generated by the current exerts strong magnetic force acting radially inwards on the plasma column. As a result, the plasma column gets pinched to a highly ionized hot and dense column, which can have sufficient population of Ne-like Ar (Ar8+) ions under suitable conditions.These Ar8+ ions are the lasing species and collectively form the gain medium of x-ray laser. Collisional electron excitation of these ions creates population inversion between 3p 1S0 and 3s 1P0 energy levels, which leads to soft x-ray lasing at 46.9 nm wavelength (Fig.2).

Fig.1 Pumping mechanism of the scheme
Fig.1 Pumping mechanism of the scheme
Fig.2 Pumping mechanism of the scheme
Fig.2 Pumping mechanism of the scheme

Experimental System

A schematic diagram of the system is shown in Fig. 3 and a photograph of the actual system in Fig.4. To get a fast current pulse, a 450 kV 10- stage Marx generator is erected upto a voltage of ~ 300-450 kV, to charge a pulse forming line (PFL) in the form of a coaxial waterline capacitor (C = 6 nF, L = 70 nH and Z = 3.3 Ω ) filled with low conductivity de-ionized water. The PFL is discharged through a self-triggered spark gap pressurized by SF6 gas to finally deliver a discharge current in the range of 25 - 50 kA. A pre-pulse generator provides a suitable pre-pulse current required for creating a low temperature plasma before the main discharge. This current pre-pulse (15 – 40 A) is applied few microseconds (5- 10 μs) before the main pulse, to obtain uniform pre-ionization in the capillary. The x-ray lasing in the capillary discharge depends on various experimental parameters like discharge current, its half cycle duration, amplitude and temporal profile of the pre-pulse, delay between the pre-pulse and main discharge currents, and the initial argon gas pressure in the capillary. All these parameters control the final temperature, density and dimensions of the pinched plasma column, which finally leads to the x-ray lasing.

Fig.3 Schematic diagram of Capillary Discharge X-ray Laser System
Fig.3 Schematic diagram of Capillary Discharge X-ray Laser System
Fig.4 A photographic view of the system
Fig.4 A photographic view of the system

Experimental results - Diode (Temporal)

A typical profile of the current used for generation of plasma and the temporal waveform of the signal recorded on vacuum diode using a vacuum bi-planar diode is shown in Fig. 5a. Fib 5b shows different components of a quadrant diode developed in house. This diode is used for locating the centre of the x-ray laser and for aligning other diagnostics like transmission grating etc.,

Fig.5 A typical diode signal along with the capillary current
Fig.5 A typical diode signal along with the capillary current
Fig.5b Detailed view of the in-house developed quadrant diode
Fig.5b Detailed view of the in-house developed quadrant diode

Experimental results - TGS (Spectral)
br> The spectrum of the plasma emission from the capillary recorded using transmission grating spectrograph (TGS) indicated the presence of strong lasing line at 46.9 nm. Various diffraction orders of this lasing line were also observed (Fig. 6a). No lasing line was observed in the absence of prepulse (Fig. 6b).

Fig.6 Transmission grating spectrum of x-ray laser
Fig.6 Transmission grating spectrum of x-ray laser

Experimental results - Double Slit (Coherence)

Young’s double slit experiment was conducted with this x-ray laser to confirm the spatial coherence property of this laser beam (Fig. 7).

Fig.7 double slit Interference pattern obtained with x-ray laser
Fig.7 double slit Interference pattern obtained with x-ray laser

Ultra-compact & Portable X-ray Laser at 46.9 nm

An ultra-compact and portable x-ray laser based on fast capillary discharge scheme operating at 46.9 nm is presently under development. The system will have a size of 0.4 m × 0.4 m. The initial laser has been observed with this system and characterization of its various parameters are under process to obtain reliable laser action. The idea is to finally obtain repetitive x-ray laser pulses.

Our future goals

There has been significant interest worldwide in extending the capillary discharge scheme for x-ray laser at even shorter wavelengths. Efforts are presently going on in our laboratory to get x-ray lasing at 13.4 nm in nitrogen plasma through this scheme which has not been demonstrated anywhere till date. The experimental conditions in terms of fast and high current as well as high gas pressure required for this x-ray lasing as predicted by simulations are very stringent. The length of capillary (with 2.8 mm inner diameter) was reduced from 15 cm to 9.7 cm in order to reduce the inductance of the system which in turn made the current faster and much higher. With this, the discharge current could be raised significantly to ~ 80 kA compared to ~50 kA obtained in previous experiments. The quarter period of this current was measured to be ~ 48 ns. Also, the pulse power system for capillary discharge has been taken to a newer height where the Marx bank has been replaced with a newly developed Tesla Transformer. With this change, the discharge current has now been increased up to ~100 kA with ~48 ns quarter period. Such fast and high current will provide potential help in our research for x-ray lasing at shorter wavelength.

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