O. Lundh

Injection and acceleration of quasimonoenergetic relativistic electron beams using density gradients at the edges of a plasma channel

The injection of quasimonoenergetic electron beams into a laser wakefield accelerator is demonstrated experimentally using density gradients at the edges of a plasma channel. In the experiment, two laser pulses are used; the main laser pulse drives a wakefield, while a second countercrossing laser beam produces a plasma whose expansion creates a channel with significant density gradients. Local injection of electrons in the wakefield is triggered by wave breaking in the density ramp. The injection is localized spatially and leads to the generation of collimated and narrow energy spread relativistic electron beams at the 100 MeV level, with charges in the range of 20–100 pC. The stability of this injection process is compared to the stability of the colliding pulse injection process and is found to be inferior for our experimental conditions. On the other hand, it is found that as the electron beam divergence is smaller in the case of gradient injection, the transverse emittance might be better.

Exploring ultrashort high-energy electron-induced damage in human carcinoma cells.

Dear Editor, n nIn conventional cancer therapy or fundamental radiobiology research, the accumulated knowledge on the complex responses of healthy or diseased cells to ionizing radiation is generally obtained with low-dose rates. Under these radiation conditions, the time spent for energy deposition is very long compared with the dynamics of early molecular and cellular responses. The energy depositions occur concomitantly with primary radio-chemical events (radical reactions), multiple biomolecular damage (membrane and DNA lesions), and repair.1 Such interferences may significantly influence the efficiency of signalling channels and repair processes or the recruitment of transducer proteins for programmed cell death or senescence. They render more complex or uncertain (i) a precise understanding of time-dependent relationships between the initial ionization density profile and the integrated complex response of normal or malignant tumour cells, and (ii) a complete description of biomolecule modifications and genome alterations resulting from energy deposition. n nThe use of ultrashort pulsed radiation would offer new perspectives for exploring the ‘black box aspects of long irradiation profiles and favouring the selective control of early damage in living targets. Several attempts were previously performed using nanosecond or picosecond pulsed irradiations on various mammalian cells and radiosensitive mutants at high dose rate.2, 3, 4, 5 The effects of single or multi-pulsed radiations on cell populations were generally analyzed in the framework of dose survival curves or characterized by 2D imaging of γ-H2AX foci and no increase in cytotoxicity was shown compared with a delivery at a conventional dose rate. Moreover, when multi-shot irradiations were performed, the overall time needed to obtain an integrated dose of several Grays again overlapped with the multi-scale dynamics of biomolecular damage–repair sequences and cell signalling steps. n nIdeally, a single-shot irradiation delivering a well-defined energy profile, via a very short temporal window, would permit the approach of a real-time investigation of early radiation-induced molecular damage within the confined spaces of cell compartments. Owing to the potential applications of intense ultrashort laser for radiation therapy,6 the model of the A431 carcinoma cell line was chosen. An ultrafast single-shot irradiation strategy was carried out with these radio-resistant human skin carcinoma cells,7 using the capacity of an innovating laser-plasma accelerator to generate quasi mono-energetic femtosecond electron bunches in the MeV domain and to deliver a very high dose rate of 1013u2009Gyu2009s−1 per pulse.8, 9 The alkaline comet assay,10, 11 which is commonly used to quantify global DNA damage in individual cells (single-, double-strand breaks, and alkali-labile sites), was applied to detect the impact of the 100u2009fs single-shot 1u2009Gy exposure with electrons of mean energy 95u2009MeV (Figure 1). The initial distribution of irradiated cells as a function of the comet tail moment, which reflects the level of DNA damage, shows a shift towards a population of more damaged cells, as compared with the sham-irradiated cells. The fraction of cells with damage above a control tail moment value of 4 exhibited an eightfold increase over that of the control cells. When carcinoma cells were maintained for 60u2009min at 37°C before the comet assay to allow DNA repair, the distributions of irradiated and non-irradiated cells became similar, indicating repair of the DNA lesions. The recovery of a near homogeneous distribution of low comet tail moments argues for the reparability of the global DNA lesions triggered by a single femtosecond irradiation shot at 1u2009Gy. To assess the consequences of ultrafast electron-induced damage, the cytotoxicity was characterized in the same experiment, using a novel survival assay at a single-cell level.12 Two weeks after the femtosecond 1u2009Gy irradiation, a 95% survival was observed from a panel of 300 cells seeded in clonal microcultures. n n n nFigure 1 n nInduction of DNA damage in skin carcinoma cells irradiated at a very high dose rate by a single ultrashort bunch of high-energy electrons. (a) Image showing the overlap between the 2D dose deposition of a femtosecond quasi-monoenergetic electron bunch ... n n n nThis first investigation of a single-shot 1u2009Gy irradiation performed at high energy level and very high dose rate demonstrates that a measurable assessment of immediate and reversible DNA damage in carcinoma cells can be explored at the single-cell level. This breakthrough opens the possibility of a complete characterization of induced damage and repair, and notably of DNA double-strand breaks. In the framework of advanced spatio-temporal radiation biology concepts,13 one challenge of such a non-conventional irradiation concerns the complete understanding of the multi-scale events triggered by the initial energy deposition, starting from the production and amplification of the localized radical processes and the induction of the primary lesions. A second challenge is deciphering the integrated cell response to these primary events, including cell signalling, damage sensing and DNA repair, and characterizing their late effects in cells, such as cell death, gene mutation and genomic instability. The emergence of ultrafast high-energy radiation biology could foreshadow the time-dependent and nanometric spatially defined effects in biomolecular architectures, such as aqueous groove of DNA, nucleosomes, protein pockets and sub-cellular compartments. Establishing an innovating approach of real-time nanodosimetry represents a prerequisite for the control of irradiations of living cells at very high dose rates. Moreover, the influence of the quality of short-pulsed particle beams (electrons and protons) on the relative biological efficiency (RBE) needs to be carefully evaluated, in synergy with ultrafast dose-fractionating protocols. Such knowledge is a necessary step before medical applications of ultrashort laser-accelerated particle beams. Currently, X-rays in the few MeV energy range represent the majority of ionizing radiations used for cancer therapy. The dose deposited by very high-energy electron beams in the tissue depth is higher and could be beneficial to target deep tumors.14 Specific conditions afforded by very high dose rates and ultrashort dose fractionations would permit the real-time control of amplified radio-sensitivity during selective targeting protocols, in combination with the modern prodrug strategies developed in chemotherapy.15 Ultrafast radiation biology thus represents a newly emerging interdisciplinary field, in strong synergy with the most recent progresses of ultrashort radiation sources, high-energy bioradical femtochemistry, molecular biology and anti-cancer therapy.

Compact and high-quality gamma-ray source applied to 10 μm-range resolution radiography

Gamma-ray beams with optimal and tuneable size, temperature, and dose are of great interest for a large variety of applications. These photons can be produced by the conversion of energetic electrons through the bremsstrahlung process in a dense material. This work presents the experimental demonstration of 30 μm resolution radiography of dense objects using an optimized gamma-ray source, produced with a high-quality electron beam delivered by a compact laser-plasma accelerator.

Mapping the X-Ray Emission Region in a Laser-Plasma Accelerator

The x-ray emission in laser-plasma accelerators can be a powerful tool to understand the physics of relativistic laser-plasma interaction. It is shown here that the mapping of betatron x-ray radiation can be obtained from the x-ray beam profile when an aperture mask is positioned just beyond the end of the emission region. The influence of the plasma density on the position and the longitudinal profile of the x-ray emission is investigated and compared to particle-in-cell simulations. The measurement of the x-ray emission position and length provides insight on the dynamics of the interaction, including the electron self-injection region, possible multiple injection, and the role of the electron beam driven wakefield.

Characterization of the beam loading effects in a laser plasma accelerator

In this study, electrons were injected into a laser plasma accelerator using colliding laser pulses. By varying the parameters of the injection laser pulse, the amount of charge accelerated in the plasma wave could be controlled. This external control of the injected load was used to investigate beam loading of the accelerating structure and especially its influence on the electron beam energy and energy spread. Information on the accelerating structure and bunch duration was then derived from these experimental observations.

Comparison of measured with calculated dose distribution from a 120-MeV electron beam from a laser-plasma accelerator.

PURPOSEnTo evaluate the dose distribution of a 120-MeV laser-plasma accelerated electron beam which may be of potential interest for high-energy electron radiation therapy.nnnMETHODSnIn the interaction between an intense laser pulse and a helium gas jet, a well collimated electron beam with very high energy is produced. A secondary laser beam is used to optically control and to tune the electron beam energy and charge. The potential use of this beam for radiation treatment is evaluated experimentally by measurements of dose deposition in a polystyrene phantom. The results are compared to Monte Carlo simulations using the geant4 code.nnnRESULTSnIt has been shown that the laser-plasma accelerated electron beam can deliver a peak dose of more than 1 Gy at the entrance of the phantom in a single laser shot by direct irradiation, without the use of intermediate magnetic transport or focusing. The dose distribution is peaked on axis, with narrow lateral penumbra. Monte Carlo simulations of electron beam propagation and dose deposition indicate that the propagation of the intense electron beam (with large self-fields) can be described by standard models that exclude collective effects in the response of the material.nnnCONCLUSIONSnThe measurements show that the high-energy electron beams produced by an optically injected laser-plasma accelerator can deliver high enough dose at penetration depths of interest for electron beam radiotherapy of deep-seated tumors. Many engineering issues must be resolved before laser-accelerated electrons can be used for cancer therapy, but they also represent exciting challenges for future research.

Comparison of measured with calculated dose distribution from a 120-MeV electron beam from a laser-plasma accelerator

PURPOSEnTo evaluate the dose distribution of a 120-MeV laser-plasma accelerated electron beam which may be of potential interest for high-energy electron radiation therapy.nnnMETHODSnIn the interaction between an intense laser pulse and a helium gas jet, a well collimated electron beam with very high energy is produced. A secondary laser beam is used to optically control and to tune the electron beam energy and charge. The potential use of this beam for radiation treatment is evaluated experimentally by measurements of dose deposition in a polystyrene phantom. The results are compared to Monte Carlo simulations using the geant4 code.nnnRESULTSnIt has been shown that the laser-plasma accelerated electron beam can deliver a peak dose of more than 1 Gy at the entrance of the phantom in a single laser shot by direct irradiation, without the use of intermediate magnetic transport or focusing. The dose distribution is peaked on axis, with narrow lateral penumbra. Monte Carlo simulations of electron beam propagation and dose deposition indicate that the propagation of the intense electron beam (with large self-fields) can be described by standard models that exclude collective effects in the response of the material.nnnCONCLUSIONSnThe measurements show that the high-energy electron beams produced by an optically injected laser-plasma accelerator can deliver high enough dose at penetration depths of interest for electron beam radiotherapy of deep-seated tumors. Many engineering issues must be resolved before laser-accelerated electrons can be used for cancer therapy, but they also represent exciting challenges for future research.

Betatron emission as a diagnostic for injection and acceleration mechanisms in laser plasma accelerators

Betatron x-ray emission in laser plasma accelerators is a promising compact source that may be an alternative to conventional x-ray sources, based on large scale machines. In addition to its potential as a source, precise measurements of betatron emission can reveal crucial information about relativistic laser-plasma interaction. We show that the emission length and the position of the x-ray emission can be obtained by placing an aperture mask close to the source, and by measuring the beam profile of the betatron x-ray radiation far from the aperture mask. The position of the x-ray emission gives information on plasma wave breaking and hence on the laser non-linear propagation. Moreover, the measurement of the longitudinal extension helps one to determine whether the acceleration is limited by pump depletion or dephasing effects. In the case of multiple injections, it is used to retrieve unambiguously the position in the plasma of each injection. This technique is also used to study how, in a capillary discharge, the variations of the delay between the discharge and the laser pulse affect the interaction. The study reveals that, for a delay appropriate for laser guiding, the x-ray emission only occurs in the second half of the capillary: no electrons are injected and accelerated in the first half.

Femto-second ultrashort laser wakefield electron bunch-duration measurements: a prism-based dispersion visible-to-IR spectrometer

A wide-band spectral diagnostic system based on dispersion property of the Zinc Selenide prism, a crystalline material highly dispersive in the near-to-far infrared spectral range, has been studied and developed for the laser wakefield acceleration experiment at LOA for the measurement of few femto-seconds long electron beam. The extensive PIC simulation studies of the colliding-beam LWFA have shown very short electron beam duration of less than 10 femtoseconds. The prism spectrometer diagnostic with highly sensitive Mercury Cadmium Telluride infrared detector and the diffraction-grating spectrometer with a high-resolution imaging visible camera together have the spectral range coverage and resolution capable of detecting ultra-short Coherent Transition Radiation (CTR) generated by interaction of bunch charges with a 100 microns thickness aluminum foil. The beam profile of asymmetric shape then could be extracted from the CTR spectrum by inverse Fourier transformation with Kramers-Kronig relation. The diagnostic system has been tested and calibrated for characterization of blackbody source spectrum and spectral responsivity. The measurement of electron beam duration of few femtoseconds has yet been convincingly shown with high resolution, and the measurements of this kind have important implication in understanding and subsequent successful operation of the future FEL light source with a highly mono-energetic LWFA beam source.

Control and mapping of x-ray emission in a laser-plasma accelerator

We show that the control and the mapping of the x-ray emission reveals unique features of the laser-plasma accelerator physics, including strong correlations between electron and x-ray beams, and density-dependence of electron injection position.