Controlled density-downramp injection in a beam-driven plasma wakefield accelerator

This paper describes the utilization of beam-driven plasma wakefield acceleration to implement a high-quality plasma cathode via density-downramp injection in a short injector stage at the FLASHForward facility at DESY. Electron beams with charge of up to 105 pC and energy spread of a few percent were accelerated by a tunable effective accelerating field of up to 2.7 GV/m. The plasma cathode was operated drift-free with very high injection efficiency. Sources of jitter, the emittance and divergence of the resulting beam were investigated and modeled, as were strategies for performance improvements that would further increase the wide-ranging applications for a plasma cathode with the demonstrated operational stabilityComment: 11 pages, 9 figure

Electron bunch generation from a plasma photocathode

Plasma waves generated in the wake of intense, relativistic laser or particle beams can accelerate electron bunches to giga-electronvolt (GeV) energies in centimetre-scale distances. This allows the realization of compact accelerators having emerging applications, ranging from modern light sources such as the free-electron laser (FEL) to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre (GV m$^{-1}$) wakefields can accelerate witness electron bunches that are either externally injected or captured from the background plasma. Here we demonstrate optically triggered injection and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ''plasma photocathode'' decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical density down-ramp injection, is highly tunable and paves the way to generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultra-high brightness beams

Emittance growth and preservation in a plasma-based linear collider

Particle physics is addressing some of the grandest questions, armed with big science machines: high energy particle colliders. These machines have, however, ballooned in size, and new technologies for accelerating particles are therefore required. Plasma-based acceleration is a promising new concept in this regard, enabling higher-than-ever accelerating fields by surfing particles on plasma waves|or wakefields|promising smaller and potentially cheaper particle accelerators. Nevertheless, many challenges remain before plasma wakefield accelerators (PWFAs) can be used for the next linear electron-positron collider. One particularly important question is whether PWFAs can preserve the required beam quality - or emittance - to produce a sufficient collision rate. This thesis addresses questions about emittance growth in a plasma-based linear collider, specifically for three important aspects of such a machine. Firstly, staging of several plasma accelerator cells is a method suggested to reach high energies with moderate-energy drivers, but is made difficult by the large chromaticity and emittance growth induced during capture of highly diverging beams. Apochromatic corrective optics - where only linear optics elements are required - is proposed as a (partial) solution to this problem. Secondly, acceleration of positron beams is not trivial in a plasma accelerator, due to the charge asymmetry of ion-electron plasmas. Hollow channel plasmas have been proposed as a solution to this problem - symmetrizing the electron/positron plasma response. However, strong transverse wakefields in these hollow channels lead to rapid beam breakup, which was measured precisely in an experiment in the FACET facility at SLAC. Lastly, compact accelerating structures must be matched by similarly compact beam focusing devices. Active plasma lensing is a promising technique in this regard, but can suffer from aberrations and consequently emittance growth due to both nonuniform plasma temperatures and distortive plasma wakefields. This was studied experimentally at the CLEAR User Facility at CERN, where in particular it was found that the nonuniform plasma temperature aberration in an active plasma lens could be suppressed by changing from a light to a heavy gas species. As a consequence, emittance preservation in an active plasma lens was demonstrated for the first time

Long-range attraction of an ultrarelativistic electron beam by a column of neutral plasma

We report on the experimental observation of the attraction of a beam of ultrarelativistic electrons towards a column of neutral plasma. In experiments performed at the FACET test facility at SLAC we observe that an electron beam moving parallel to a neutral plasma column, at an initial distance of many plasma column radii, is attracted into the column. Once the beam enters the plasma it drives a plasma wake similar to that of an electron beam entering the plasma column head-on. A simple analytical model is developed in order to capture the essential physics of the attractive force. The attraction is further studied by 3D particle-in-cell numerical simulations. The results are an important step towards better understanding of particle beam–plasma interactions in general and plasma wakefield accelerator technology in particular

Electron bunch generation from a plasma photocathode

Plasma waves generated in the wake of intense, relativistic laser or particle beams can accelerate electron bunches to giga-electronvolt (GeV) energies in centimetre-scale distances. This allows the realization of compact accelerators having emerging applications, ranging from modern light sources such as the free-electron laser (FEL) to energy frontier lepton colliders. In a plasma wakefield accelerator, such multi-gigavolt-per-metre (GV m$^{-1}$) wakefields can accelerate witness electron bunches that are either externally injected or captured from the background plasma. Here we demonstrate optically triggered injection and acceleration of electron bunches, generated in a multi-component hydrogen and helium plasma employing a spatially aligned and synchronized laser pulse. This ''plasma photocathode'' decouples injection from wake excitation by liberating tunnel-ionized helium electrons directly inside the plasma cavity, where these cold electrons are then rapidly boosted to relativistic velocities. The injection regime can be accessed via optical density down-ramp injection, is highly tunable and paves the way to generation of electron beams with unprecedented low transverse emittance, high current and 6D-brightness. This experimental path opens numerous prospects for transformative plasma wakefield accelerator applications based on ultra-high brightness beams