Dielectric laser acceleration of electrons in the vicinity of single and double grating structures - theory and simulations

Dielectric laser acceleration of electrons close to a fused-silica grating has recently been observed (Peralta et al., Nature 503, 91 (2013); Breuer, Hommelhoff, PRL 111, 134803 (2013)). Here we present the theoretical description of the near-fields close to such a grating that can be utilized to accelerate non-relativistic electrons. We also show simulation results of electrons interacting with such fields in a single and double grating structure geometry and discuss dephasing effects that have to be taken into account when designing a photonic-structure-based accelerator for non-relativistic electrons. We further model the space charge effect using the paraxial ray equation and discuss the resulting expected peak currents for various parameter sets.Comment: 14 pages, 7 figure

Transverse and longitudinal characterization of electron beams using interaction with optical near-fields

We demonstrate an experimental technique for both transverse and longitudinal characterization of bunched femtosecond free electron beams. The operation principle is based on monitoring of the current of electrons that obtained an energy gain during the interaction with the synchronized optical near-field wave excited by femtosecond laser pulses. The synchronous accelerating/decelerating fields confined to the surface of a silicon nanostructure are characterized using a highly focused sub-relativistic electron beam. Here the transverse spatial resolution of 450 nm and femtosecond temporal resolution achievable by this technique are demonstrated

Enabling ultra-high dose rate electron beams at a clinical linear accelerator for isocentric treatments

BACKGROUND AND PURPOSE Radiotherapy delivery with ultra-high dose rates (UHDR) has consistently produced normal tissue sparing while maintaining efficacy for tumour control in preclinical studies, known as the FLASH effect. Modified clinical electron linacs have been used for pre-clinical studies at reduced source-surface distance (SSD) and novel intra-operative devices are becoming available. In this context, we modified a clinical linac to deliver 16 MeV UHDR electron beams with an isocentric setup. MATERIALS AND METHODS The first Varian TrueBeam (SN 1001) was clinically operative between 2009-2022, it was then decommissioned and converted into a research platform. The 18 MeV electron beam was converted into the experimental 16 MeV UHDR. Modifications were performed by Varian and included a software patch, thinner scattering foil and beam tuning. The dose rate, beam characteristics and reproducibility were measured with electron applicators at SSD = 100 cm. RESULTS The dose per pulse at isocenter was up to 1.28 Gy/pulse, corresponding to average and instantaneous dose rates up to 256 Gy/s and 3⋅10$^{5}$ Gy/s, respectively. Beam characteristics were equivalent between 16 MeV UHDR and conventional for field sizes up to 10x10cm$^{2}$ and an overall beam reproducibility within ± 2.5% was measured. CONCLUSIONS We report on the first technical conversion of a Varian TrueBeam to produce 16 MeV UHDR electron beams. This research platform will allow isocenter experiments and deliveries with conventional setups up to field sizes of 10x10 cm$^{2}$ within a hospital environment, reducing the gap between preclinical and clinical electron FLASH investigations

Initial experience with an electron FLASH research extension (FLEX) for the Clinac system

Purpose: Radiotherapy delivered at ultra-high-dose-rates (≥40 Gy/s), that is, FLASH, has the potential to effectively widen the therapeutic window and considerably improve the care of cancer patients. The underlying mechanism of the FLASH effect is not well understood, and commercial systems capable of delivering such dose rates are scarce. The purpose of this study was to perform the initial acceptance and commissioning tests of an electron FLASH research product for preclinical studies. Methods: A linear accelerator (Clinac 23EX) was modified to include a nonclinical FLASH research extension (the Clinac-FLEX system) by Varian, a Siemens Healthineers company (Palo Alto, CA) capable of delivering a 16 MeV electron beam with FLASH and conventional dose rates. The acceptance, commissioning, and dosimetric characterization of the FLEX system was performed using radiochromic film, optically stimulated luminescent dosimeters, and a plane-parallel ionization chamber. A radiation survey was conducted for which the shielding of the pre-existing vault was deemed sufficient. Results: The Clinac-FLEX system is capable of delivering a 16 MeV electron FLASH beam of approximately 1 Gy/pulse at isocenter and reached amaximum dose rate \u3e3.8 Gy/pulse near the upper accessory mount on the linac gantry. The percent depth dose curves of the 16 MeV FLASH and conventional modes for the 10 × 10 cm2 applicator agreed within 0.5 mm at a range of 50% of the maximum dose. Their respective profiles agreed well in terms of flatness but deviated for field sizes \u3e10 × 10 cm2. The output stability of the FLASH system exhibited a dose deviation of \u3c1%.Preliminary cell studies showed that the FLASH dose rate (180 Gy/s) had much less impact on the cell morphology of 76N breast normal cells compared to the non-FLASH dose rate (18 Gy/s), which induced large-size cells. Conclusion: Our studies characterized the non-clinical Clinac-FLEX system as a viable solution to conduct FLASH research that could substantially increase access to ultra-high-dose-rate capabilities for scientists

The Micro-Accelerator Platform

Multiple applications in attosecond science, standoff nuclear detection, oil-well logging, and medicine require the use of compact high-gradient accelerators. The microstructure-based field of Dielectric Laser Accelerators (DLA's) lever- ages high-power optical lasers and well-established nanofabrication techniques to accelerate electrons with GV/m electromagnetic fields over mm-scale distances, thus filling providing compact high-gradient acceleration. The Micro-Accelerator Platform (MAP) is a micron-scale slab-symmetric resonant-cavity DLA that ac- celerates electrons with a potential acceleration gradient approaching 1 GeV/m. In principle, electrons are synchronously accelerated as they traverse the stand- ing wave resonance in the MAP's vacuum cavity, excited by a side-coupled Ti:Sapphire laser and confined by Distributed Bragg Reflectors above and be- low the vacuum cavity. Extensive analysis, simulations, and a proof-of-principle experiment show that the MAP is a viable candidate for compact high-gradient acceleration.A simplified model of the MAP is used to develop analytic expressions of the resonant fields and associated forces in the vacuum cavity of the MAP. These resonant fields are shown to be capable of accelerating electrons with GeV/m acceleration gradients with no transverse defocusing.To examine the dependence of the quality and frequency of the MAP's reso- nance on it's geometry and component materials, simulations in the frequency- domain EM solver HFSS and the time-domain EM solver and PIC code VORPAL are utilized. After a set of design parameters and materials that are practical to fabricate has been detailed, the quality of the MAP's resonance, spectral char- acteristics of the MAP, error tolerances of the design, ability of the resonance to accelerate electrons, and transverse dynamics of electrons traversing the MAP are presented via simulation results.Optical lithography and sputtering deposition techniques are used to fabricate the final design of the MAP. After characterization of the fabricated sample is described, the testing of the MAP at the Next Linear Collider Test Accelerator is discussed. A 60 MeV electron beam traverses the MAP as it is illuminated by a Ti:Sapphire laser. The energy spectra of the beam after having passed through the illuminated MAP is then compared to the energy spectra of the beam after having passed through the MAP without laser illumination in order to deduce whether acceleration has occurred. The strength of acceleration versus the relative timing of the laser and electron beam is examined. It is deduced that for a subset of the data collected, the MAP accelerated electrons with a 50.6 MeV/m accelerating gradient. The implications of this finding and potential ways to increase the accelerating gradient are discussed

The Micro-Accelerator Platform

Multiple applications in attosecond science, standoff nuclear detection, oil-well logging, and medicine require the use of compact high-gradient accelerators. The microstructure-based field of Dielectric Laser Accelerators (DLA's) lever- ages high-power optical lasers and well-established nanofabrication techniques to accelerate electrons with GV/m electromagnetic fields over mm-scale distances, thus filling providing compact high-gradient acceleration. The Micro-Accelerator Platform (MAP) is a micron-scale slab-symmetric resonant-cavity DLA that ac- celerates electrons with a potential acceleration gradient approaching 1 GeV/m. In principle, electrons are synchronously accelerated as they traverse the stand- ing wave resonance in the MAP's vacuum cavity, excited by a side-coupled Ti:Sapphire laser and confined by Distributed Bragg Reflectors above and be- low the vacuum cavity. Extensive analysis, simulations, and a proof-of-principle experiment show that the MAP is a viable candidate for compact high-gradient acceleration.A simplified model of the MAP is used to develop analytic expressions of the resonant fields and associated forces in the vacuum cavity of the MAP. These resonant fields are shown to be capable of accelerating electrons with GeV/m acceleration gradients with no transverse defocusing.To examine the dependence of the quality and frequency of the MAP's reso- nance on it's geometry and component materials, simulations in the frequency- domain EM solver HFSS and the time-domain EM solver and PIC code VORPAL are utilized. After a set of design parameters and materials that are practical to fabricate has been detailed, the quality of the MAP's resonance, spectral char- acteristics of the MAP, error tolerances of the design, ability of the resonance to accelerate electrons, and transverse dynamics of electrons traversing the MAP are presented via simulation results.Optical lithography and sputtering deposition techniques are used to fabricate the final design of the MAP. After characterization of the fabricated sample is described, the testing of the MAP at the Next Linear Collider Test Accelerator is discussed. A 60 MeV electron beam traverses the MAP as it is illuminated by a Ti:Sapphire laser. The energy spectra of the beam after having passed through the illuminated MAP is then compared to the energy spectra of the beam after having passed through the MAP without laser illumination in order to deduce whether acceleration has occurred. The strength of acceleration versus the relative timing of the laser and electron beam is examined. It is deduced that for a subset of the data collected, the MAP accelerated electrons with a 50.6 MeV/m accelerating gradient. The implications of this finding and potential ways to increase the accelerating gradient are discussed