Plasma sources, optics and diagnostics

 
 

As the name already suggests plasma acceleration as well as plasma optics require the generation of plasma background. This background density of especially the plasma electrons is a crucial parameter for the plasma acceleration process and requires careful adjustment to the specific needs of an experiment. Owing to the requirements of the individual experiments the plasma background density can differ by multiple orders of magnitude, thus requiring specific targets and diagnostics build for the individual experiments. In the following the currently existing plasma generation mechanisms, target structures and diagnostics utilised within the FLASHForward framework will be outlined.

Compact plasma lens setup for capture and transport of electrons beams from a gas jet. Via laser wakefield acceleration in a short 1mm long gas jet electron beams with a peak energy of about 100 MeV are generated (left side of the picture). The laser is then dumped onto a beam block (bright red part to the right) while the electron beam is transported through the plasma lens (purple glowing structure on the right). the diverging beams are captured by the lens and focussed to an electron spectrometer for characterisation. In this setup fast studies of beam emittance and temporal evolution of plasma lens parameters can be obtained.

To form a plasma in general energy is deposited in a specific specimen to allow for (at least partial) ionisation of the medium. For energy deposition currently two options are available, a high voltage discharge or an intense laser pulse. For the first case a roughly square wave, flat top pulse with voltages between 0 kV and 40 kV, a rise time of about 20 ns and a duration of up to 400 ns is applied to two electrodes resulting in breakdown of the gaseous medium in between. The resulting current can be adjusted via variable impedance matching with currents between 100 A and 1000 A for usual discharge conditions. In laser based plasma generation an 800 nm, 25 TW, 900 mJ laser system can be focussed into a target using different focussing geometries. A short focal length optic of 0.5 m or 0.7 m focal length allows for plasma generation and simultaneous wakefield generation in a short and narrow channel while an 18 m focal length setup allows for a long plasma column of more than 50cm with a width of 500 um and more to be produced.

Within the FLASHForward framework the medium from which the plasma is generated is always gaseous. In general two different types of target structure preparing the gases region for plasma generation can be distinguished: Pulsed gas jets and capillary structure targets.
With gas jets a high pressure gas is applied at the inlet and expanded through a small orifice. The small orifice size and geometry of the driver allow for opening durations of the orifice on few millisecond timescale. This results in a gas-puff which is shaped by the expansion section behind the small orifice and allows to do experiments at comparably high plasma electron densities on the order of 10^19 electrons per cubic-centimetre.

In the picture the magnetic field strength is plotted versus radius and time. The initially linear behaviour over time transforms to a non linear radial dependence which in experiments would lead to space dependent focussing of the lens and thus influence beam parameters behind these lenses. Changing from linear to non-linear field strength depends on the electron temperature equilibration time, which in depends on specific experiment parameters, e.g. backing pressure and gas species.