The ultra-short electron beams, produced through the velocity bunching compression technique at the SPARC_LAB test Facility (Frascati, Italy), are used to produce Coherent Transition Radiation in the terahertz (THz) range. This paper reports on the main features of this THz source, which have a spectral coverage up to 5 THz, a pulse duration down to 100 fs, and an energy per pulse on the order of tens of μJ. These figures of merits open the possibility to apply this source for nonlinear and THz pump-probe experiments in Solid-State Physics and material science.

The development of compact accelerator machines is leading towards the use of plasma-based devices that are able to sustain large acceleration gradients up to several tens of GV m ⁻¹ . The main issue, in this regard, is due to the necessity to produce the plasma shot-by-shot starting from neutral gases, since its lifetime is limited to a few tens of microseconds. This puts severe limitations on its resulting uniformity, stability and reproducibility, which in turn strongly affects the quality of accelerated beams by plasmas. In this work, we describe a gas-filled discharge-capillary where the plasma generation, achieved by ionizing Hydrogen gas with a high-voltage electrical discharge, is stabilized by triggering its ignition with an external laser pulse. Results show a noticeable stabilization of the resulting plasma density along the capillary and the discharge pulse. This enables the development of very long capillaries and in turn, acceleration lengths as required, for instance, by next-generation plasma-based facilities delivering beams in the GeV-scale.

Plasma confinement represents a crucial point for plasma-based accelerators and plasma lenses because it can strongly affect the beam properties. For this reason, an accurate measurement of the plasma parameters, as plasma temperature, pressure and electron density, must be performed. In this paper, we introduce a novel method to detect the plasma temperature and the pressure for gas-filled capillaries in use at the SPARC_LAB test facility. The proposed method is based on the shock waves produced at the ends of the capillary during the gas discharge and the subsequent plasma formation inside it. By measuring the supersonic speed of the plasma outflow, the thermodynamic parameters have been obtained both outside and inside the capillary. A plasma temperature around 1.4 eV has been measured, that depends on the geometric properties and the operating conditions of the capillary.

Towards the next generation of compact plasma-based accelerators, useful in several fields, such as basic research, medicine and industrial applications, a great effort is required to control the plasma creation, the necessity of producing a time-jitter free channel, and its stability namely uniformity and reproducibility. In this Letter, we describe an experimental campaign adopting a gas-filled discharge-capillary where the plasma and its generation are stabilized by triggering its ignition with an external laser pulse or an innovative technique based on the primary dark current (DC) in the accelerating structure of a linear accelerator (LINAC). The results show an efficient stabilization of the discharge pulse and plasma density with both pre-ionizing methods turning the plasma device into a symmetrical stable accelerating environment, especially when the external voltage is lowered near the breakdown value of the gas. The development of tens of centimeter long capillaries is enabled and, in turn, longer acceleration lengths can be adopted in a wide range of plasma-based acceleration experiments.

Plasma-based acceleration experiments require capillaries with a radius of a few hundred microns to confine plasma up to a centimeter scale capillary length. A long and controlled plasma channel allows to sustain high fields which may be used for manipulation of the electron beams or to accelerate electrons. The production of these capillaries is relatively complicated and expensive since they are usually made with hard materials whose manufacturing requires highly specialized industries. Fine variations of the capillary shape may significantly increase the cost and time needed to produce them. In this article, we demonstrate the possibility of using 3D printed polymeric capillaries to drive a hydrogen-filled plasma discharge up to 1 Hz of repetition rate in an RF based electron linac. The plasma density distribution has been measured after several shot intervals, showing the effect of the surface ablation on the plasma density distribution. This effect is almost invisible in the earlier stages of the discharge. After more than 55000 shots (corresponding to more than 16 h of working time), the effects of the ablation on the plasma density distribution are not evident and the capillary can still be used. The use of these capillaries will significantly reduce the cost and time for prototyping, allowing us to easily manipulate their geometry, laying another building block for future cheap and compact particle accelerators.