P. Schoessow

Observation of the Askaryan Effect: Coherent Microwave Cherenkov Emission from Charge Asymmetry in High-Energy Particle Cascades

We present the first direct experimental evidence for the charge excess in high-energy particle showers and corresponding radio emission predicted nearly 40 years ago by Askaryan. We directed picosecond pulses of GeV bremsstrahlung photons at the SLAC Final Focus Test Beam into a 3.5 ton silica sand target, producing electromagnetic showers several meters long. A series of antennas spanning 0.3 to 6 GHz detected strong, subnanosecond radio-frequency pulses produced by the showers. Measurements of the polarization, coherence, timing, field strength vs shower depth, and field strength vs frequency are completely consistent with predictions. These measurements thus provide strong support for experiments designed to detect high-energy cosmic rays such as neutrinos via coherent radio emission from their cascades.

Radio frequency measurements of coherent transition and Cherenkov radiation: implications for high energy neutrino detection

We report on measurements of (11-18)-cm wavelength radio emission from interactions of 15.2 MeV pulsed electron bunches at the Argonne Wakefield Accelerator. The electrons were observed both in a configuration where they produced primarily transition radiation from an aluminum foil, and in a configuration designed for the electrons to produce Cherenkov radiation in a silica sand target. Our aim was to emulate the large electron excess expected to develop during an electromagnetic cascade initiated by an ultrahigh-energy particle. Such charge asymmetries are predicted to produce strong coherent radio pulses, which are the basis for several experiments to detect high-energy neutrinos from the showers they induce in Antarctic ice and in the lunar regolith. We detected coherent emission which we attribute both to transition and possibly Cherenkov radiation at different levels depending on the experimental conditions. We discuss implications for experiments relying on radio emission for detection of electromagnetic cascades produced by ultrahigh-energy neutrinos.

Demonstration of electron beam self‐focusing in plasma wake fields

In this paper, the direct observation of wake‐field self‐focusing of an electron beam in plasma is reported. The dynamics of beam self‐pinching and the fast collisionless evolution of a Bennett‐like, near‐equilibrium profile are examined theoretically and computationally. The experimental results are compared to predictions of the analysis and discussed in the context of application to the plasma lens and the plasma wake‐field accelerator.

Measurement of deflection-mode damping in an accelerating structure

We have directly measured the damping of wake‐field deflection modes in a slow‐wave accelerating structure consisting of a dielectric‐lined waveguide with segmented conducting boundaries wrapped with rf absorbing material. Such damping of deflection modes is desired to prevent beam breakup instabilities. Attenuation e‐folding times of 246 ps were recorded for deflection modes at the Advanced Accelerator Test Facility while the quality of the desired accelerating mode remained unaffected.

Experimental demonstration of energy-chirp compensation by a tunable dielectric-based structure.

A tunable energy-chirp compensator was used to remove a correlated energy chirp from the 60-MeV beam at the Brookhaven National Laboratory Accelerator Test Facility. The compensator operates through the interaction of the wakefield of the electron bunch with itself and consists of a planar structure comprised of two alumina bars with copper-plated backs separated by an adjustable beam aperture. By changing the gap size, the correlated energy chirp of the electron bunch was completely removed. Calculations show that this device, properly scaled to account for the electron bunch charge and length, can be used to remove residual correlated energy spread at the end of the linacs used for free-electron lasers. The experimental results are shown to be in good agreement with numerical simulations. Application of this technique can significantly simplify linac design and improve free-electron lasers performance.

High power radio frequency generation by relativistic beams in dielectric structures

We have studied the interaction of a high current electron beam with dielectric loaded waveguides as a source of electromagnetic radiation. A unique high current photoinjector-based electron linac was used to generate the drive beam for these experiments, and the fields generated were diagnosed using a trailing probe (witness) beam from a second photocathode gun. Traveling wave dielectric structures with luminal (vphase=c) frequencies of 15 and 20 GHz were used. The radio frequency power levels generated in these initial experiments were very large—up to 11 MW.

The Argonne Wakefield Accelerator-overview and status

The Argonne Wakefield Accelerator (AWA) is a new facility for advanced accelerator research, with a particular emphasis on studies of high gradient (/spl sim/100 MeV/m) Wakefield acceleration. A novel high current short pulse L-Band photocathode gun and preaccelerator will provide 100 nC electron bunches at 20 MeV to be used as a drive beam, while a second high brightness gun will be used to generate a 5 MeV witness beam for Wakefield measurements. We present an overview of the various AWA systems, the status of construction, and initial commissioning results.<<ETX>>

Transformer Ratio Enhancement for Structure-Based Wakefield Acceleration

A limiting factor in the efficiency of wakefield accelerators is the fact that the transformer ratio R, the parameter that characterizes the energy transfer efficiency from the accelerating structure to the accelerated electron beam, is less than 2 for most technologically realizable beam‐structure configurations. We are planning an experiment to study transformer ratio enhancement in a 13.625 GHz dielectric wakefield structure driven by a ramped bunch train. In this paper we present an experimental program for the demonstration of this Enhanced Transformer Ratio Dielectric Wakefield Accelerator (ETR‐DWA).

Design and simulation of a high frequency high power RF extraction device using a dielectric-loaded waveguide.

Abstract We consider the use of a dielectric-loaded structure to extract RF energy from a high-current electron drive beam as a power source for a high-energy two-beam accelerator. This represents an alternative technique which we show to have some significant advantages over the use of the currently proposed corrugated metal structures as power extraction devices. We discuss a particular design that will extract high-power RF (0.1–1 GW) from a high-current drive beam. RF generation and transport in this class of devices have already been demonstrated at lower frequencies. We discuss the design parameters for 15 and 30 GHz dielectric transfer structures and some possible experiments.

The Argonne Wakefield Accelerator high current photocathode gun and drive linac

The Argonne Wakefield Accelerator (AWA) is a new facility for advanced accelerator research. A major component of the AWA is its drive linac, consisting of a unique high current short pulse L-band photocathode based gun and special standing wave preaccelerator designed to produce 100 nC, 30 ps electron bunches at 20 MeV. Commissioning on the drive linac is now underway. We report on our initial operating experience with this novel machine, including bunch length and emittance measurements.