Ebook: Laser-Plasma Acceleration

In the past decade, laser-plasma acceleration has shown an impressive progress, with outstanding achievements from both experimental and theoretical viewpoints. Exploiting closely the development of ultraintense, ultrashort pulse lasers, laser-plasma acceleration has rapidly developed, showing accelerating gradients of tens of GeV/m and making the vision of miniature accelerators more realistic than ever. Indeed, the growing interest of the wider scientific community is now leading to joint programmes that see two communities, the laser-plasma and the high-energy physics community working side by side for a common objective.

The idea of the Course emerged from this context, with the unique objective to give young scientists the opportunity to capture the best from both worlds of laser-plasma and accelerator physics, with intensive training and “hands-on” opportunities on the key aspects of laser-plasma acceleration. The lectures spanned from the secrets of lasers, to the power of numerical simulations, from the rock-solid know-how of beam dynamics, to the eluding world of laboratory plasmas, with the unique objective to establish a common knowledge for the future laser-plasma accelerator community.

The magnificent location of the School and the flawless and friendly local organization were key ingredients in successfully delivering such a demanding training programme to the audience, whose attention, interest and active participation was exceptionally high throughout the duration of the Course.

These Proceedings were prepared by the authors with the aim to provide a wider community with a reference in the wide range of topics, the knowledge of which will be needed in the future research on laser-plasma acceleration. This book also points at selected existing literature to which the reader is also redirected for further reading, from the founding papers of acceleration beyond the GeV (C.E. Clayton, Physical Review Letters, 2010).

F. Ferroni, L. A. Gizzi and R. Faccini

The following lecture notes offer a short, elementary introduction to the field of high-intensity laser interactions with matter. The material starts with basic phenomena such as ionization, plasma characterization and thresholds for non-linear behaviour. In the latter sections, topics relevant to laser-based particle acceleration are covered in detail, including wave propagation in underdense plasmas and fast electron heating in overdense (solid) plasmas. The notes are concluded with a short summary of plasma simulation techniques, including a work-through tutorial using a particle-in-cell code.

In this lecture we introduce from basic principles the main concepts of beam focusing and transport in modern accelerators using the beam envelope equation as a convenient mathematical tool. Matching conditions suitable to preserve the beam quality are derived from the model for significant beam dynamics regimes. An extension of the model to the plasma accelerator case is introduced. The understanding of similarities and differences with respect to traditional accelerators is also emphasized.

In this lecture we will review some of the requirements when designing and operating laser systems for laser-plasma acceleration. We will analyse the impact of the different components of the laser system on the final output and discuss how they can be mitigated against or prevented. We will study some of the existing laser facilities used for particle acceleration and the future prospects for the next generation of facilities.

Beam diagnostics and instrumentation are an essential part of any kind of accelerator. There is a large variety of parameters to be measured for observation of particle beams with the precision required to tune, operate and improve the machine. Depending on the type of accelerator, for the same parameter the working principle of a monitor may strongly differ, and related to it also the requirements for accuracy. This report will mainly focus on electron beam diagnostic monitors presently in use at 4th generation light sources (single-pass free-electron lasers), and present the state-of-the-art diagnostic systems and concepts.

In these lecture notes a general description of diagnostic techniques for investigating laser-matter interactions will be given. Focus will be on laser-gas interactions relevant to plasma acceleration and laser-solid interactions of interest for Inertial Confinement Fusion studies. A detailed discussion on the temporal and spatial features of laser pulses amplified by the Chirped Pulse Amplification (CPA) technique will be given, with a special attention to the properties that play an important role in experiments, including pulse duration, contrast, pre-pulses and focusing configurations. Plasma formation and plasma density evolution by optical interferometry techniques will be presented along with the techniques for recovering information when optical interferometry with ultrashort pulses is adopted. Further, X-ray imaging and spectroscopy of plasmas will be introduced with attention to novel monochromatic imaging technique applied to plasmas generated by ultrashort laser pulses for investigation of fast electron transport in solids.

An increasing number of high-power laser laboratories are being established with the aim of developing laser-plasma accelerators. Here a short overview is given on the preliminary results obtained in the self-injection test experiment as a part of the commissioning phase of the 220 TW FLAME laser facility. For laser energies on target below 1 J and pulse durations around 30 fs, accelerated electron bunches had an energy up the hundred MeV range, showing a significant degree of collimation below 10 mrad, in agreement with similar published results and confirming the expected laser performance at this energetic level. A correlation between electron collimation and upshift on laser transmitted light was also observed.

The advance in laser plasma acceleration techniques pushes the regime of the resulting accelerated particles to higher energies and intensities. In particular, the experiments with the 250 TW laser at the FLAME facility of the INFN Laboratori Nazionali di Frascati, will enter the GeV regime with more than 100 pC of electrons. At the current status of understanding of the acceleration mechanism, relatively large angular and energy spreads are expected. There is therefore the need for developing a device capable to measure the energy of electrons over three orders of magnitude (few MeV to few GeV), with still unknown angular divergences. Within the PlasmonX experiment at FLAME, a spectrometer is being constructed to perform these measurements. It consists of an electro-magnet and a screen made of scintillating fibers for the measurement of the trajectories of the particles. The large range of operation, the huge number of particles and the need to focus the divergence, present challenges in the design and construction of such a device. We present the design considerations for this spectrometer that lead to the use of scintillating fibers, multichannel photo-multipliers and a multiplexing electronics, a combination which is innovative in the field. We also present the experimental results obtained with a high-intensity electron beam performed on a prototype at the LNF beam test facility.

The acceleration of ions with high-power lasers has been a very active field of research during the past 10 years. This paper summarizes the main results obtained in the field, detailing the mechanisms of the acceleration process and the main observed beam characteristics. Perspectives for future development of the field and current and future applications are also discussed.

The physics of laser-plasma interactions has undergone dramatic improvements in recent years. By directing a multi-TW, ultrashort laser pulse onto a thin foil or a gas jet, it is nowadays possible to produce multi-MeV proton, ion and electron beams. Although much progress has been made in characterizing and improving the quality of such laser-generated beams, it is still an untouched issue whether the laser-generated beams are or can be spin polarized and, thus, whether laser-based polarized sources are conceivable. To this end, one may either think of a spatial selection of certain spin states through the huge magnetic field gradients that are inherently generated in the laser-generated plasmas, or of pre-polarized target particles which maintain their polarization during the rapid acceleration process. We have developed a method to measure the degree of polarization of protons that have been accelerated at the 300 TW laser facility ARCturus at Dusseldorf University.

The measurements reported here provide scaling laws for the ion acceleration process in the regime of ultrashort (50 fs), ultrahigh contrast (10¹⁰) and ultrahigh intensity (>10²⁰W/cm²), never investigated previously. The scaling of the accelerated ion energies was studied by varying a number of parameters such as target thickness (down to 10 nm), target material (C and Al) and laser light polarization (circular and linear) at 35° and normal laser incidence. A twofold increase in proton energy and an order of magnitude enhancement in ion flux have been observed over the investigated thickness range at 35° angle of incidence. Furthermore, at normal laser incidence, measured peak proton energies of about 20 MeV are observed almost independently of the target thickness over a wide range (50 nm–10 μ m).

A high-efficiency regime of acceleration in laser plasmas has been discovered recently, allowing table top equipment to deliver doses of interest for radiotherapy with electron bunches of suitable kinetic energy. A R&D program aimed to the realization of an innovative class of accelerators for medical uses, through radiobiological validation, is in progress at CNR, Pisa. Actually, biological effects of electron bunches from laser-driven electron accelerator are largely unknown. In radiobiology and radiotherapy, it is known that the early spatial distribution of energy deposition following ionizing radiation interactions with DNA molecule is crucial for the prediction of damages at cellular or tissue levels and during the clinical responses to this irradiation. The purpose of the present study is to evaluate the radio-biological effects obtained with electron bunches from a laser-driven electron accelerator compared with bunches coming from medical radio-frequency–based linac's. To this purpose a multidisciplinary team including radiotherapists, biologists, medical physicists, laser and plasma physicists is working at CNR Campus and University of Pisa. Dose on samples is delivered alternatively by the “laser-linac” operating at the ILIL lab of Istituto Nazionale di Ottica and an RF-linac operating at Pisa S. Chiara Hospital. Experimental data are analyzed on the basis of suitable radiobiological models as well as with numerical simulation based on Monte Carlo codes. Possible collective effects are also considered in the case of ultrashort, ultradense bunches of ionizing radiation produced with the laser technique. This lecture will describe and shortly discuss most of the points above.

Radiation particles, besides their application to fundamental research, are widely used in all fields of science (medicine, material science, chemistry, etc.). Up to now the radiations were produced by sources like accelerators, X-ray tubes, radioactive materials with the well-known problems of costs, parameters and safety. In the last few years, following the development of lasers able to focus ultrashort high-intensity pulses onto targets, the generation of ionizing radiation by intense lasers have become possible. This paper focuses on the radiological protection aspects, mainly licensing requirements, prompt and residual radiation fields, shielding of produced radiations, shielding materials, radiation monitoring, determination of any environmental impact, and other specific operational requirements of an “accelerator facility”, that a project manager should take into account in designing a facility for laser-based accelerators.