Patent Number: 
Section: claims

1. A method of producing a localised concentration of energy comprising using an external device to apply one or more shockwaves to a static volume of non-gaseous medium to create at least one shockwave propagating through the non-gaseous medium so as to be incident upon a pocket of gas within the medium, wherein the pocket of gas is attached to a surface comprising a depression, wherein the pocket of gas has a volume greater than or equal to a volume of the depression and the depression is shaped so as partially to receive the pocket of gas. 2. The method as claimed in claim 1, comprising controlling the formation of a transverse jet during collapse of the pocket of gas, wherein the surface depression is designed to receive the transverse jet impact such that at least some of pocket of gas is trapped between the impacting jet and the surface depression. 3. The method as claimed in claim 2, wherein the jet is arranged to strike an area of surface that has been prepared with a particular roughness or microscopic shape such that many small portions of the pocket of gas are trapped between the jet tip and the target surface. 4. The method as claimed in claim 1, wherein the surface depression and the pocket of gas are arranged such that the initial contact region is a curve which forms a closed loop. 5. The method as claimed in claim 4, wherein a section of the surface depression has a curvature greater than that of the tip of the jet and wherein the section of the surface depression is arranged such that the jet impacts therein. 6. The method as claimed in claim 1, wherein the surface depression tapers in cross-section away from the mouth of the surface depression. 7. The method as claimed in claim 1, wherein the surface depression has a depth greater than its width. 8. The method as claimed in claim 1, wherein the surface depression comprises a plurality of discrete portions. 9. The method as claimed in claim 8, wherein the discrete portions are piecewise polynomial. 10. The method as claimed in claim 1, comprising using an explosive device to create the shockwave with a pressure of between 0.1 GPa and 50 GPa. 11. The method as claimed in claim 1, wherein the pocket of gas is formed with the use of a pre-manufactured membrane that defines the boundary between the pocket of gas and the non-gaseous medium. 12. The method as claimed in claim 11, wherein the membrane is frangible and is arranged to break upon impact from the shockwave. 13. The method as claimed in claim 1, wherein a width of the pocket of gas is greater than equal to a corresponding width of the depression. 14. The method as claimed in claim 1, wherein a depth of the pocket of gas is greater than equal to a corresponding depth of the depression. 15. The method as claimed in claim 1, wherein the depression is continuously curved.