Induced m=0 instability in fast ablative capillary discharges and possibleutilisation for X-ray lasers

High current linear discharges confined by their own magnetic field are sub ject to magneto-hydrodynamic instabilities which perturb a straight plasma column. An example is the m=0 mode, which is characterised by the developme nt of necks contracting rapidly towards the axis with the ion sound speed. Discharges through capillaries were assumed to be stable hitherto, but by c utting capillaries lengthwise after a few shots hot spot traces clearly imp rinted on the inner wall of the capillary are observed. They are interprete d as marks of an m=0 instability, and this interpretation is substantiated by a series of time-gated pinhole images, which show that the hot plasma re gion is clearly detached from the wall at the second current maximum and co ncentrated on the axis thus making the development of the instability possi ble. The instability occurs only with a specific sample of polyacetal as wa ll material, and its axial wavelength increases with the length of the capi llary. By modulating respectively the inner wall of the capillary the wavel ength can be imposed within limits. This is exploited for a soft X-ray lase r scheme based on charge exchange pumping of bare carbon ions of hot plasma streaming from the necks and colliding with cold plasma outside the neck r egions. Exponential growth of the Balmer-alpha line of CVI at 18.22 nm is r ealised.