C. McGuinness

Demonstration of electron acceleration in a laser-driven dielectric microstructure

The enormous size and cost of current state-of-the-art accelerators based on conventional radio-frequency technology has spawned great interest in the development of new acceleration concepts that are more compact and economical. Micro-fabricated dielectric laser accelerators (DLAs) are an attractive approach, because such dielectric microstructures can support accelerating fields one to two orders of magnitude higher than can radio-frequency cavity-based accelerators. DLAs use commercial lasers as a power source, which are smaller and less expensive than the radio-frequency klystrons that power today’s accelerators. In addition, DLAs are fabricated via low-cost, lithographic techniques that can be used for mass production. However, despite several DLA structures having been proposed recently, no successful demonstration of acceleration in these structures has so far been shown. Here we report high-gradient (beyond 250 MeV m−1) acceleration of electrons in a DLA. Relativistic (60-MeV) electrons are energy-modulated over 563 ± 104 optical periods of a fused silica grating structure, powered by a 800-nm-wavelength mode-locked Ti:sapphire laser. The observed results are in agreement with analytical models and electrodynamic simulations. By comparison, conventional modern linear accelerators operate at gradients of 10–30 MeV m−1, and the first linear radio-frequency cavity accelerator was ten radio-frequency periods (one metre) long with a gradient of approximately 1.6 MeV m−1 (ref. 5). Our results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems. This would enable compact table-top accelerators on the MeV–GeV (106–109 eV) scale for security scanners and medical therapy, university-scale X-ray light sources for biological and materials research, and portable medical imaging devices, and would substantially reduce the size and cost of a future collider on the multi-TeV (1012 eV) scale.

Four‐dimensional cone‐beam computed tomography using an on‐board imager

On-board cone-beam computed tomography (CBCT) has recently become available to provide volumetric information of a patient in the treatment position, and holds promises for improved target localization and irradiation dose verification. The design of currently available on-board CBCT, however, is far from optimal. Its quality is adversely influenced by many factors, such as scatter, beam hardening, and intra-scanning organ motion. In this work we quantitatively study the influence of organ motion on CBCT imaging and investigate a strategy to acquire high quality phase-resolved [four-dimensional (4D)] CBCT images based on phase binning of the CBCT projection data. An efficient and robust method for binning CBCT data according to the patients respiratory phase derived in the projection space was developed. The phase-binned projections were reconstructed using the conventional Feldkamp algorithm to yield 4D CBCT images. Both phantom and patient studies were carried out to validate the technique and to optimize the 4D CBCT data acquisition protocol. Several factors that are important to the clinical implementation of the technique, such as the image quality, scanning time, number of projections, and radiation dose, were analyzed for various scanning schemes. The general references drawn from this study are: (i) reliable phase binning of CBCT projections is accomplishable with the aid of external or internal marker and simple analysis of its trace in the projection space, and (ii) artifact-free 4D CBCT images can be obtained without increasing the patient radiation dose as compared to the current 3D CBCT scan.

Dielectric laser accelerators

We describe recent advances in the study of particle acceleration using dielectric near-field structures driven by infrared lasers, which we refer to as Dielectric Laser Accelerators. Implications for high energy physics and other applications are discussed.

Waveguides in three-dimensional photonic bandgap materials for particle-accelerator on a chip architectures

A promising concept for making high-energy particle accelerators less expensive and more compact is to make use of axially polarized optical modes supported by line defect waveguides in three-dimensional photonic bandgap materials. Following a theoretical proposal by B. M. Cowan we here present the first experimental realization of corresponding pilot samples for three-dimensional photonic bandgap particle-accelerator segments. The samples have been fabricated using a combination of direct laser writing and an improved silicon-double-inversion procedure. Regarding optical characterization we have performed transmittance measurements providing unambiguous evidence of a waveguide mode with axial polarization. These results represent an important first step towards actually putting into practice future “particle-accelerator on a chip” architectures.

Design, fabrication, and testing of a fused-silica dual-layer grating structure for direct laser acceleration of electrons

A proof of principle fused-silica grating structure has been designed and fabricated for the purpose of direct laser acceleration of electrons. The optimal structure geometry was determined via 2D-FDTD and 3D-FEFD simulations to maximize the available acceleration gradient. The structure was fabricated with standard nanofabrication techniques, including optical lithography, reactive ion etching, and wafer bonding. Beam tests have been performed with the 60MeV beam at the Next Linear Collider Test Accelerator at SLAC, with successful demonstration of electron transmission through the micron-scale apertures.

Experiment to Detect Accelerating Modes in a Photonic Bandgap Fiber

An experimental effort is currently underway at the E‐163 test beamline at Stanford Linear Accelerator Center to use a hollow‐core photonic bandgap (PBG) fiber as a high‐gradient laser‐based accelerating structure for electron bunches. For the initial stage of this experiment, a 50 pC, 60 MeV electron beam will be coupled into the fiber core and the excited modes will be detected using a spectrograph to resolve their frequency signatures in the wakefield radiation generated by the beam. We will describe the experimental plan and recent simulation studies of candidate fibers.

Woodpile Structure Fabrication for Photonic Crystal Laser Acceleration

We describe initial steps at fabricating a dielectric photonic bandgap accelerator structure designed to operate at near IR frequencies. Such a structure operating at these frequencies requires extremely small, sub‐micron sized features, forcing one to use lithographic means for fabrication. A process based upon lithographic equipment at the Stanford Nanofabrication Facility has been developed and a four layer test structure has been fabricated. Unexpected problems with the final etch step, and corresponding modifications to the process flow addressing these problems, are described. Spectroscopic measurements of the structure have been taken and are compared to simulations.

Inverse-transition radiation laser acceleration experiments at slac

We present a series of laser-driven particle acceleration experiments that are aimed at studying laser-particle acceleration as an inverse-radiation process. To this end we employ a semi-open vacuum setup with a thin planar boundary that interacts with the laser and the electromagnetic field of the electron beam. Particle acceleration from different types of boundaries will be studied and compared to the theoretical expectations from the Inverse-radiation picture and the field path integral method. We plan to measure the particle acceleration effect from transparent, reflective, black, and rough surface boundaries. While the agreement between the two acceleration pictures is straightforward to prove analytically for the transparent and reflective boundaries the equivalence is not clear-cut for the absorbing and rough-surface boundaries. Experimental observation may provide the evidence to distinguish between the models.

Beam dynamics measurements for the slac laser acceleration experiment

The NLC test accelerator (NLCTA) was built to address beam dynamics issues for the Next Linear Collider and beyond. An S-Band RF gun, diagnostics and low energy spectrometer (LES) at 6 MeV together with a large-angle extraction line at 60 MeV have now been built and commissioned for the laser acceleration experiment,E163. Following a four quad matching section after the NLCTA chicane, the extraction section is followed by another matching section, final focus and buncher. The laser-electron interaction point (IP) is followed by a broad range, high resolving power spectrometer (HES) for electron bunch analysis. Optical symmetries in the design of the 25.5deg extraction line provide 1:1 phase space transfer without sextupoles for a large, 6D phase space volume and range of input conditions. Spot sizes down to a few microns at the IP (HES object) allow testing microscale structures with high resolving power at the HES image. Tolerances, tuning sensitivities, diagnostics and the latest commissioning results are discussed and compared to design expectations.

Correlating pulses from two spitfire, 800nm lasers

The E163 laser acceleration experiments conducted at SLAC have stringent requirements on the temporal properties of two regeneratively amplified, 800 nm,Spitfire laser systems. To determine the magnitude and cause of timing instabilities between the two Ti:sapphire amplifiers, we pass the two beams through a cross-correlator and focus the combined beam onto a Hamamatsu G1117 photodiode. The photodiode has a bandgap such that single photon processes are suppressed and only the second order, two-photon process produces an observable response. The response is proportional to the square of the intensity. The diode is also useful as a diagnostic to determine the optimal configuration of the compression cavity.