SLAC FFTB Research Articles

PRELIMINARY RESULTS FROM THE UCLA/SLAC ULTRA-HIGH GRADIENT CERENKOV WAKEFIELD ACCELERATOR EXPERIMENT

The first phase of an experiment to study the performance of dielectric Cerenkov wakefield accelerating structures at extremely high gradients in the GV/m range has been completed. This experiment takes advantage of the unique SLAC FFTB electron beam and its demonstrated ultra-short pulse lengths and high currents (e.g., σz = 20 μ m at Q = 3 nC ). The FFTB electron beam has been successfully focused down and sent through varying lengths of fused silica capillary tubing with two different sizes: ID = 200 μ m / OD = 325 μ m and ID = 100 μ m / OD = 325 μ m . The pulse length of the electron beam was varied in the range 20 μ m < σz < 100 μ m which produced a range of electric fields between 2 and 20 GV/m at the inner surface of the dielectric tubes. We observed a sharp increase in optical emissions from the capillaries in the middle part of this surface field range which we believe indicates the transition between sustainable field levels and breakdown. If this initial interpretation is correct, the surfaced fields that were sustained equate to on axis accelerating field of several GV/m. In future experiments we plan to collect and measure coherent Cerenkov radiation emitted from the capillary tube to gain more information about the strength of the accelerating fields.

PROSPECTS OF HIGH ENERGY LABORATORY ASTROPHYSICS

Ultra high energy cosmic rays (UHECR) have been observed but their sources and production mechanisms are yet to be understood. We envision a laboratory astrophysics program that will contribute to the understanding of cosmic accelerators with efforts to: 1) test and calibrate UHECR observational techniques, and 2) elucidate the underlying physics of cosmic acceleration through laboratory experiments and computer simulations. Innovative experiments belonging to the first category have already been done at the SLAC FFTB. Results on air fluorescence yields from the FLASH experiment are reviewed. Proposed future accelerator facilities can provided unprecedented high-energy-densities in a regime relevant to cosmic acceleration studies and accessible in a terrestrial environment for the first time. We review recent simulation studies of non-linear plasma dynamics that could give rise to cosmic acceleration, and discuss prospects for experimental investigation of the underlying mechanisms.

Coherent radiation recoil effect for the optical diffraction radiation beam size monitor at SLAC FFTB

A short electron bunch with length σ passing through a slit in the diffraction radiation (DR) target generates radiation with a broad spectrum. Optical part of the spectrum (incoherent radiation) may be used for beam size measurements, but in the wavelength range λ ⩾ σ radiation becomes coherent. The coherent DR spectrum per each electron in a bunch is equal to single electron spectrum times by the number of electrons in a bunch N e and bunch form factor. For SLAC FFTB conditions ( N e ∼ 10 10, σ = 0.7 mm, outer target size R ∼ 10 mm, slit width h ∼ 0.1 mm) we approximated coherent DR (CDR) spectrum by coherent transition radiation (TR) one because in the wavelength region λ ∼ σ ≫ h TR and DR spectra coincide with high accuracy. Changing the DR target by a TR target with projection on the plane perpendicular to electron beam as a circle with radius R ⩽ 20 mm we calculated CDR spectra using simple model. Knowing the CDR spectrum we estimated the energy CDR emitting by each electron in the perpendicular direction (due to target inclination angle 45°). It means an electron receives the radiation recoil in this direction. In other words, electron has a transverse kick about 1 μrad that may be considered as permissible.