R. Konecny

Observations of microwave continuum emission from air shower plasmas

We investigate a possible new technique for microwave detection of cosmic-ray extensive air showers which relies on detection of expected continuum radiation in the microwave range, caused by free-electron collisions with neutrals in the tenuous plasma left after the passage of the shower. We performed an initial experiment at the Argonne Wakefield Accelerator laboratory in 2003 and measured broadband microwave emission from air ionized via high-energy electrons and photons. A follow-up experiment at the Stanford Linear Accelerator Center in the summer of 2004 confirmed the major features of the previous Argonne Wakefield Accelerator observations with better precision. Prompted by these results we built a prototype detector using satellite television technology and have made measurements suggestive of the detection of cosmic-ray extensive air showers. The method, if confirmed by experiments now in progress, could provide a high-duty cycle complement to current nitrogen fluorescence observations.

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.

Observation of wakefield generation in left-handed band of metamaterial-loaded waveguide

We report on a design of a TM-mode based metamaterial-loaded waveguide. Network analyzer measurements demonstrated a left-handed propagation region for the TM11 mode at around 10 GHz. A beamline experiment was performed with the metamaterial-loaded waveguide. In this experiment, a 6 MeV electron beam passes through the waveguide and generates a wakefield via the Cherenkov radiation mechanism. We detected a signal in the left-handed frequency band at 10 GHz. This is an indirect demonstration of reverse Cherenkov radiation as predicted in the work of Veselago [Sov. Phys. Usp. 10, 509 (1968)] and discussed in the works of Lu et al. [Opt. Express 11, 723 (2003)], Averkov and Yakovenko [Phys. Rev. B 72, 205110 (2005)], and Tyukhtin et al. [IEEE, Proceedings of the PAC, 2007 (unpublished), pp. 4156–4158]. Cherenkov radiation in artificially constructed materials [metamaterials (MTMs)] can provide unusual engineered features that can be advantageous for particle detector design.

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.

High-power RF tests on X-band dielectric-loaded accelerating structures

A joint Argonne National Laboratory (ANL)/Naval Research Laboratory (NRL) program is under way to investigate X-band dielectric-loaded accelerating (DLA) structures, using high-power 11.424-GHz radiation from the NRL Magnicon Facility. DLA structures offer the potential of a simple, inexpensive alternative to copper disk-loaded structures for use in high-gradient radio-frequency (RF) linear accelerators. A central purpose of our high-power test program is to find the RF breakdown limits of these structures. In this paper, we summarize the most recent tests results for two DLA structures loaded with different ceramics: alumina and Mg/sub x/Ca/sub 1-x/TiO/sub 3/ (MCT). No RF breakdown has been observed up to 5 MW of drive power (equivalent to 8 MV/m accelerating gradient), but multipactor was observed to absorb a large fraction of the incident microwave power. The latest experimental results on suppression of multipactor using a TiN coating on the inner surface of the dielectric are reported. Although we did not observe dielectric breakdown in the structure, breakdown did occur at the ceramic joint, where the electric field is greatly enhanced. Lastly, the MCT structure showed significantly less multipactor for the same level RF field.

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.

Multi-Nanosecond High Power Pulse Generation at 7.8 GHz With a Dielectric-Loaded Power Extractor

Power extraction from charged particle beams is a prospective way to develop future high power radio frequency (RF) sources. We have designed and tested a 7.8 GHz power extractor based on a dielectric-loaded waveguide. Building upon earlier work on single electron bunch tests, 10 ns and 22 ns megawatt-level RF pulses have been generated with trains consisting of 16 electron bunches each, by using a laser splitting-recombination scheme. In addition, 44 MW of peak power has been generated with a train consisting 4 electron bunches. Behaviors of higher-order-modes are also explored.

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>>

The Argonne Wakefield Accelerator Facility capabilities and experiments.

A description of the Argonne Wakefield Accelerator is presented, pointing out the unique capabilities of the facility. A photocathode RF gun produces electron bunches with tens of nanocoulombs of charge, which are used to excite wakefields. A second photocathode RF gun generates electron bunches that are used to probe these wakefields. An overview of the experimental program carried out at the facility is also presented.