X-Ray Laser

The principles of laser operation are well understood. The major difficulty in producing an X-ray laser is the extreme threshold power level needed. This threshold is proportional to the sixth power of frequency, so a 0.1 nanometer laser would require approximately 10.sup.21 watts to operate under the conditions of the original ruby lasers. This specification describes an apparatus which will attain X-ray laser conditions efficiently and at relatively low energy inputs.

For a given energy, power is increased by shortening the duty cycle. As threshold power is proportional to cavity volume and inversely proportional to the effective optical length of the cavity, a minimum cavity cross-section and maximum optical path length must be engineered. As the only practical method of producing inverted X-ray states is photo-ionisation, a discrete X-ray driving source must be provided. These criteria may all be satisfied by exploiting the violent collapse of a bubble.

The present device consists essentially of a bubble subject to violent collapse. The environment of the bubble is most likely to be liquid, but may be a colloid, suspension, slurry, gel, fluidised bed, solid or other medium which is or becomes fluent under impact, referred to herein generically as a fluent medium. The bubble shape is most likely to approximate a cylinder of high aspect ratio, i.e. a disc-like cylinder, though a range of other bubble shapes may provide advantages. The collapse will be precipitated by externally applied pressure though this could conceivably be the ambient pressure of the environment.

In the preferred embodiment, a bubble in liquid metal is used because the vapour pressure in the bubble can be very low, the ratio of specific heats for the vapour can be high, repeated collapse cycles can occur without chemical decomposition and careful alloying can provide a range of operational X-ray transitions. The bubble is essentially cylindrical with possibly a slight taper to encourage uni-directional emission. This configufation may be stably maintained by control of the liquid flow pattern during collapse. This control may be exercised by the profiles of the inner surface of the container, by inducing rotational or other non radial flow patterns or by inducing electric currents or magnetic fields. It is envisaged that rapid collapse will be induced by the application of very high external pressures. The more rapid the collapse, the more adiabatic the heating processes.

The mode of action is as follows. The collapse of the bubble can provide a very hot core either by adiabatic compression of the residual vapour within the bubble or by impact of opposing bubble walls at the instant of collapse. By appropriate selection of bubble dimensions, applied pressure, bubble vapour pressure and collapse geometry, it is possible to arrange that the core will have a very small cross section and will be hot enough to provide an intense thermal X-ray source for a short instant of time. The emitted X-rays will be absorbed in the cold bubble wall surrounding the hot core. The emitted radiation will propagate at the speed of light for a transparent core, whereas the energetic electrons associated with the heat wave will travel significantly more slowly and can be impeded by circulating currents in the liquid. Preferential photoionisation can therefore occur in the bubble wall and give rise to an inverted X-ray transition. The inversion may be dynamic or intrinsic to the transition if the ground state lifetime is significantgly less than the excited state lifetime. The high density of the bubble wall will ensure rapid absorption of the incident X-rays providing a small effective cavity volume. Calculation indicates that laser conditions may easily be attained.

Super-radiant emission can occur in the bubble wall and the gain will be greatest parallel to the surface. However, X-rays have a refractive index of less than one in matter, so the reduction in density toward the centre of the core will cause self focusing of the super-radiant beam along the axis of the device. Inelastic scattering will delete the electron density along the central beam. This will further enhance self focusing and will provide a relatively loss free exit path for the super-radiant emission.

The high pressures at the instant of collapse will cause the liquid in the bubble wall, to be substantially compressed. This will raise the effective absorption coefficient, so reducing the effective cavity cross section.

The X-ray cavity will be a dynamically changing volume. As the heat wave expands, so the zone of inversion will expand with it until the conditions of stimulated X-ray emission can no longer be sustained.

While only a fraction of the total energy of the system will be emitted as X-rays, the remainder can be efficiently recouped mechanically and used for a subsequent shot. In this fashion, an oscillating system may be exploited to provide a very efficient conversion of energy into a super-radiant X-ray beam.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an axial section, on line I--I in FIG. 2, of an X-ray laser embodying the invention, and

FIG. 2 is a plan view of the cylinder part of the laser.

In FIG. 1 there is axial symmetry about the line A--A. A piston 1 fits a cylinder 2 with an O-ring seal 3. An inlet 4 in the piston is counterbored to form a seating 5 for a sealing ball 6. A clamping screw 7 is provided to tighten the ball on to its seat and, above the screw, is a well 8. The assembly may be filled with a preset amount of mercury 9 by a sequence of compressing the piston and cylinder assembly to expel air then expanding to draw in mercury from the well 8. The valve can be closed tight when all air has been expelled.

An annular spring 10 urges the piston out of the cylinder and thereby ensures that an equilibrium a bubble 11 exists in the mercury. The inner profiles of the piston and cylinder ensure that at equilibrium, a symmetrical annulus of mercury exists around an essentially cylindrical bubble of mercury vapour, i.e. a flat cylindrical, disc-like bubble 11. This is attained by providing a central portion 14 with small clearance and an outer portion 15 with much larger clearance, with a coned transitional zone 16 therebetween. Surface tension and gravity may be exploited to provide the uniform annulus.

To excite the system, a rapid compression of the piston and cylinder arrangement is undertaken with a large applied pressure. This may be generated by a hammer blow or detonation of an explosive charge. The compression causes the cylindrical bubble of mercury vapour to contract with increasing rapidity to the instant of collapse. A collimated X-ray beam is then formed along the axis as outlined above. This beam passes through the X-ray transparent window 12 in the centre of the cylinder 3. The concentration of energy in this window will doubtless produce a hole, but it may be arranged that leakage of air into the assembly or of mercury out of the assembly will be insignificant during the brief period of operation. A new window may be provided for each shot.

Asymmetric profiling of piston and cylinder faces may be exploited to produce undirectional emission by producing a tapered cylindrical bubble which will collapse progressively from the piston toward the X-ray window.

The rebound energy after peak compression will be converted into potential energy in the spring 10. This in turn will initiate a further compression cycle and as X-ray emission may be arranged to be the major source of loss, the initial impact energy may be efficiently converted to X-ray energy in a series of compression cycles. The cycles may be sustained by driving the system at its resonance frequency, e.g. using an electrically actuated hammer.

A rotation of the mercury about the axis of symmetry A--A may be be generated by vanes 13 or similar devices on the surface of the piston, cylinder or both, as shown in FIG. 2. The rotational flow imparted to the mercury may be exploited to stabilise the geometry of the bubble during collapse.

An axial magnetic field between piston and cylinder may be provided. The radial component of the mercury velocity will produce a circulating induced current which will also tend to stabilise the collapse geometry and will impede the propagation of fast electrons in the heat wave.