Figure 3: Determining the shape of the plasma channel.
From: Demonstration of a positron beam-driven hollow channel plasma wakefield accelerator
(a) A mosaic of images illustrating the principle behind the raster scan. Each subimage shows both the positron beam (with central hot spot) and laser profile (with ring) at low intensity reflecting off of a titanium foil. The laser is scanned in the transverse plane while remaining parallel to the beam trajectory. The arrows indicate the direction of force that the positron beam experiences for a plasma channel located at the position of the laser. (b) The kick map shows the magnitude and direction of the kick delivered to the beam averaged over ∼10 shots as the channel location is scanned with respect to the beam trajectory. A net kick of (43.6,48.7) μrad in (x,y) is subtracted from the data. We superimpose the intensity contour of the central Bessel peak of the laser measured upstream of the plasma as a red dashed line. (c) Area of the positron beam measured on a YAG screen downstream of the plasma averaged over ∼10 shots as the channel location is scanned with respect to the beam trajectory.
