Electron beam quality measurements on the ALPHA-X laser-plasma wakefield accelerator

The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme at the University of Strathclyde is developing laser-plasma wakefield accelerators to produce high energy, ultra-short duration electron bunches as drivers of radiation sources. Coherent emission will be produced in a free-electron laser by focussing the ultra-short electron bunches into an undulator. To achieve net gain, high peak current, low energy spread and low emittance are required. A high intensity, ultra-short pulse from a 30 TW Ti:sapphire laser is focussed into a helium gas jet to produce femtosecond duration electron bunches in the range of 80 - 200 MeV. Beam transport is monitored using a series of Lanex screens positioned along the beam line. Measurements of the electron energy spectrum, obtained using the ALPHA-X high resolution magnetic dipole spectrometer, are presented. The maximum central energy of the monoenergetic beam is 90 MeV and r.m.s. relative energy spreads as low as 0.8% are measured. The mean central energy is 82 ± 4 MeV and mean energy spread is 1.1 ± 0.4%. We also present pepper-pot measurements of the normalised transverse emittance where mono-energetic electrons are passed through an array of 52µm diameter holes in tungsten. The analysis of the pepper-pot results sets an upper limit for the normalised emittance at 5.5 ± 1π mm mrad for an 82 MeV beam. With further acceleration to 1 GeV, the relative energy spread will reduce giving beam parameters that indicate the feasibility of a compact X-ray FEL driven by a plasma-wakefield accelerator.