Abstract. New acceleration technology is mandatory for the future elucidation of fundamental particles and their interactions. A promising approach is to exploit the properties of plasmas. Past research has focused on creating large-amplitude plasma waves by injecting an intense laser pulse or an electron bunch into the plasma. However, the maximum energy gain of electrons accelerated in a single plasma stage is limited by the energy of the driver. Proton bunches are the most promising drivers of wakefields to accelerate electrons to the TeV energy scale in a single stage. An experimental program at CERN -the AWAKE experiment -has been launched to study in detail the important physical processes and to demonstrate the power of proton-driven plasma wakefield acceleration. Here we review the physical principles and some experimental considerations for a future proton-driven plasma wakefield accelerator.

The electron energy distribution function (EEDF) in the E to H mode transition region of an inductively coupled argon discharge has been studied experimentally. The EEDF, which has a Maxwellian- or Druyvesteyn-like shape (depending on pressure) in both “pure” modes, shows a trend to a bi-Maxwellian shape in the vicinity of both the E to H and the H to E mode transitions. Moreover, the normalized electron energy probability functions closely before the E to H and the reverse H to E mode jumps are almost identical, indicating a similar power coupling at both transition points.

The input power supplied to an inductively coupled argon discharge has been periodically amplitude-modulated with different modulation shapes and frequencies in order to examine the dynamics of the E to H mode transition. Time-resolved measurements of the total light emission intensity, the electron density, the floating potential and the current and voltage in the induction coil have been performed. Various effects, which have not been reported in the literature so far, have been observed, indicating a two-step process of the E to H mode transition at rectangular shaped amplitude modulation. Non-continuous behaviour in the H mode, close to the H to E mode transition at continuous (triangular) modulation was observed as well.

Electrostatic probe measurements have been performed in order to derive plasma parameters in a pulsed-power discharge. The experiment has been designed to produce plasma filled arc-shaped magnetic flux tubes. The plasma is sustained by high current densities along the tube axis, which drive the arc-shaped structure to expand via the magnetic hoop-force. The electrostatic probe is located at a fixed position in space and scans over the minor diameter (≈3 cm) of the discharge arc while it passes. The probe is designed as an asymmetric triple probe in order to get instantaneous information on electron temperatures and densities. Peak values of up to 10 eV and about 2 × 1021 m−3 respectively were found. Owing to the high reproducibility of the experiment it was possible to take double probe characteristics in subsequent shots for comparison. In addition, the measurements of the line integrated density were performed by means of a CO2 laser interferometer. The results of the electrostatic triple probe in the investigated plasma regime are compared with the results of the laser interferometer. While the shapes of the density distribution are in reasonable agreement, the peak values derived from the triple probe underestimate the electron density by up to a factor of 5.