Study of a hydrogen capillary discharge for particle acceleration in the lower pressure regime

Summary form only given. Plasmas with densities in the range 10 ²⁴-10²⁵ m^-3 have been created by making use of capillary discharges. These capillary discharges are low-current (300-800 A) discharges produced in hydrogen filled alumina capillaries of 0.3-0.5 mm diameter with a lifetime of a few hundred nanoseconds. The plasma channel produced in such a discharge has a hollow electron density profile. The minimum electron density results in a maximum refractive index in the centre which makes such a plasma suitable as a waveguide for intense laser light. When the plasma channel is used to guide a high-power (TW) intense laser pulse, it can be used as an accelerator for injected electron bunches. Such a laser pulse creates a wakefield in the plasma that in turn accelerates the electrons to energies of several hundreds MeV. Creating a lower density plasma channel with a density around 10²³ m^-3 is rather attractive due to the simpler synchronization of the injection of the electron bunch and the created wakefield by the high power laser pulse. A serious experimental problem in creating a capillary plasma with a low density can be the desorption of the hydrogen from the capillary walls. Desorption of one monolayer of hydrogen can give a density around 10²⁴ m^-3. Our experimental results show that if there would be any desorption, the contribution is less than 10²² m^-3. Experimental results show a more fundamental problem for creating a suitable low density capillary plasma for wakefield acceleration of electrons. This is the relatively large matched spot size at lower pressures. That means that only large focused laser spot sizes can be guided properly resulting in lower energy density in the spot. This gives a weaker wakefield which in turn results in a smaller energy gain for the accelerated electrons. This problem can be solved by decreasing the capillary diameter, increasing the current (voltage) and using an axial magnetic field, resulting in a smaller matched spot size