Future high-energy plasma accelerators may use a plasma source so the pulses (laser or particle beams) driving the accelerating plasma waves will interact with a preformed plasma. This plasma can have its density, composition or electric current dynamically tailored to produce a desired effect on the beam-plasma interaction or on the driving beam. For example,  a plasma with a radially parabolic density profile may act on a laser pulse as a guiding optical fibre suppressing diffraction and keeping the laser beam focused to high intensity. A preformed plasma is also needed when the driving beam intensity is not enough to ionize the accelerating medium. This is the case in the groundbreaking proton driven plasma wakefield acceleration experiment (AWAKE) in CERN. Here, the SPS 400 GeV proton beam (the LHC feeder) will be used to drive plasma waves in a 10 meter long plasma. A possible method to produce the adequate plasma for this experiment was proposed and is being developed here at Imperial College. It consist in an electric discharge made though a tube filled with argon at low pressure. A micro-second high-voltage pulse is used to ionize the gas and heat the plasma to a temperature close to 1 eV, where the argon is almost fully ionised. A 3 meter long prototype, shown in the figure, has proved to produce the adequate plasma densities for the AWAKE experiment opening the way the construction of a full length 10 meter plasma.

plasma_source
3 meter long, 2 cm diameter argon plasma source. An arc discharge between the electrodes at the tube extremities heat producing an ionisation rate close to 1.