W. D. Kimura

STELLA experiment: Design and model predictions

The STaged ELectron Laser Acceleration (STELLA) experiment will be one of the first to examine the critical issue of staging the laser acceleration process. The BNL inverse free electron laser (EEL) will serve as a prebuncher to generate {approx} 1 {micro}m long microbunches. These microbunches will be accelerated by an inverse Cerenkov acceleration (ICA) stage. A comprehensive model of the STELLA experiment is described. This model includes the EEL prebunching, drift and focusing of the microbunches into the ICA stage, and their subsequent acceleration. The model predictions will be presented including the results of a system error study to determine the sensitivity to uncertainties in various system parameters.

Inverse Cerenkov acceleration and inverse free-electron laser experimental results for staged electron laser acceleration

The goal of the staged electron laser acceleration (STELLA) experiment is to demonstrate staging of the laser acceleration process whereby an inverse free electron laser (IFEL) will be used to prebunch the electrons, which are then accelerated in an inverse Cerenkov accelerator (ICA). As preparation for this experiment, a new permanent magnet wiggler for the IFEL was constructed and the ICA system was modified. Both systems have been tested on a new beamline specifically built for STELLA. The improved electron beam (e-beam) with its very low emittance (0.8 mm-mrad normalized) enabled focusing the e-beam to an average radius (1/spl sigma/) of 65 /spl mu/m, within the ICA interaction region. This small e-beam focus greatly enhanced the ICA process and resulted in electron energy spectra that have demonstrated the best agreement to date in both overall shape and magnitude with the model predictions. The electron energy spectrum using the new wiggler in the IFEL was also measured. These results will be described as well as future improvements to the STELLA experiment.

Ultrashort microbunch electron source

A new type of electron beam source is proposed that combines a DC field and AC field from a laser beam in between the anode-cathode gap. Prior researchers have demonstrated the ability for this configuration to produce ultrashort electron microbunches from field emission when the AC field adds constructively with the DC field. Our new source improves upon this basic scheme by creating mono-energetic electrons with low emittance that are suitable for feeding into dielectric and vacuum laser accelerators, photonic bandgap accelerators, and dielectric wakefield accelerators. The microbunches will have a length that is a fraction of the laser wavelength, corresponding to fs pulse durations. They are also produced as a train of microbunches separated by the period of the laser length and lasting for the duration of the laser pulse.

CO2 laser technology for advanced particle accelerators

Short-pulse, high-power CO2 lasers open new prospects for development of high-gradient laser-driven electron accelerators. The advantages of λ=10 μm CO2 laser radiation over the more widely exploited solid state lasers with λ≈1 μm are based on a λ2-proportional ponderomotive potential, λ-proportional phase slippage distance, and λ-proportional scaling of the laser accelerator structures. We show how a picosecond terawatt CO2 laser that is under construction at the Brookhaven Accelerator Test Facility may benefit the ATF’s experimental program of testing far-field, near-field, and plasma accelerator schemes.

Plasma-based advanced accelerators at the Brookhaven Accelerator Test Facility

The Accelerator Test Facility at Brookhaven National Laboratory (BNL ATF) offers to its users a unique combination of research tools that include a high-brightness 70-MeV electron beam, a mid-infrared (λ = 10 μm) CO2 laser of terawatt power, and a capillary discharge as a plasma source. These cutting-edge technologies have enabled us to launch a new R&D program at the forefronts of advanced accelerators and radiation sources. The main subjects that we are researching are innovative methods of producing wakes in a linear regime using plasma resonance with the electron microbunch train periodic to the laser’s wavelength and so-called “seeded” laser wakefield acceleration (LWFA) that is driven and probed by a combination of electron and laser beams. We describe the present status of the ATF experimental program, including simulations and preliminary experiments; in addition, we review previous ATF experiments that were the precursors to the present program. They encompass our demonstration of longitudinal-and transverse-field phasing inside the plasma wave, plasma channeling of intense CO2 laser beams, and the generation of e-beam microbunch trains by the inverse FEL technique.

A Multibunch Plasma Wakefield Accelerator

We investigate a plasma wakefield acceleration scheme where a train of electron microbunches feeds into a high density plasma. When the microbunch train enters such a plasma that has a corresponding plasma wavelength equal to the microbunch separation distance, a strong wakefield is expected to be resonantly driven to an amplitude that is at least one order of magnitude higher than that using an unbunched beam. PIC simulations have been performed using the beamline parameters of the Brookhaven National Laboratory Accelerator Test Facility operating in the configuration of the STELLA inverse free electron laser (IFEL) experiment. A 65 MeV electron beam is modulated by a 10.6 μm CO2laser beam via an IFEL interaction. This produces a train of ∼ 90 microbunches separated by the laser wavelength. In this paper, we present both a simple theoretical treatment and simulation results that demonstrate promising results for the multibunch technique as a plasma-based accelerator.

Pseudoresonant laser Wakefield acceleration driven by 10.6-/spl mu/m laser light

This work describes an experiment to demonstrate, for the first time, laser wakefield acceleration (LWFA), driven by 10.6-/spl mu/m light from a CO/sub 2/ laser. This experiment is also noteworthy because it will operate in a pseudoresonant LWFA regime, in which the laser-pulse-length is too long for resonant LWFA, but too short for self-modulated LWFA. Nonetheless, high acceleration gradients are still possible. This experiment builds upon an earlier experiment called staged electron laser acceleration (STELLA), where efficient trapping and monoenergetic laser acceleration of electrons were demonstrated using inverse free electron lasers. The aim is to apply the STELLA approach of laser-driven microbunch formation followed by laser-driven trapping and acceleration to LWFA. These capabilities are important for a practical electron linear accelerator based upon LWFA.

Plasma Density Measurements in Hydrogen‐Filled and Ablative Discharge Capillaries Based on Stark Broadening of Atomic Hydrogen Spectral Lines

Results of plasma density measurements in ablative and hydrogen‐filled discharge capillaries are presented. The method of plasma density measurement is based on Stark broadening of atomic hydrogen spectral lines in the plasma due to interaction of the hydrogen atoms with free charges. To ensure the measured plasma density corresponds to the internal portion of the discharge volume, we also examine a possibility to collect the plasma light emission with an optical fiber inserted inside the capillary channel. We studied the time dependence of the plasma density relative to the beginning of the discharge with a temporal resolution of 150 ns. The plasma density was found to vary over a range of 1017–1015 cm−3. The dependence of the plasma density upon discharge voltage and hydrogen pressure in the hydrogen‐filled capillary was also studied. The possibility of designing a hybrid ablative hydrogen‐filled capillary that allows us to simplify the high voltage generator scheme and reach high plasma densities is di...

Resonant Plasma Wakefield Experiment: Plasma Simulations and Multibunched Electron Beam Diagnostics

In the multibunch plasma wakefield acceleration experiment at the Brookhaven National Lab’s Accelerator Test Facility a 45 MeV electron beam is initially modulated through the IFEL interaction with a CO2 laser beam at 10.6 μm into a train of short microbunches, which are spaced at the laser wavelength. It is then fed into a high‐density capillary plasma with a density resonant at this spacing (1.0 × 1019cm−3). The microbunched beam can resonantly excite a plasma wakefield much larger than the wakefield excited from the non‐bunched beam. Here we present plasma simulations that confirm the wakefield enhancement and the results of a series of CTR measurements performed of the multibunched electron beam.

A Proposed High‐Energy, Compact, Optical Electron Accelerator Based on Particle Acceleration by Stimulated Emission of Radiation

A new type of advanced accelerator is proposed that is capable of producing high‐energy electron beams in a compact geometry, but does not require a high‐power laser beam driver nor a linear accelerator. Moreover, it is capable of generating its own microbunched electron beam source and, thus, an electron gun is also not needed. Hence, this has the potential to enable development of compact, high‐energy accelerators that are less complex and less costly.