We report the first experimental test of the physics of plasma wake-field acceleration performed

We have developed a new method for measuring the electromagnetic fields (wake fields) induced in high-frequency accelerating devices by intense relativistic electron bunches. These fields are probed by a second collinear electron bunch which follows the primary bunch at a variable delay. Initial results for several iris-loaded rf structure geometries are presented. This technique is expected to be important for the study of new acceleration methods such as the plasma wake-field accelerator. Another possible application is the measurement of parasitic wakes in conventional accelerator components.PACS numbers: 41.80. Ee, 07.77.+p, The need for higher accelerating gradients to decrease the size, cost, and complexity of future linear colliders for particle-physics research has motivated the development of new ideas for charged-particle acceleration. In particular, many so-called wake-field schemes have been proposed 1 " 3 in which an intense, low-energy beam excites electric fields in a cavity or medium which are then used to accelerate a second, less intense beam to high energies. In addition, even for accelerators not based on the wake-field principle, it is important to understand the effects of transverse wake fields induced in beam-line components as contributing to beam instabilities and emittance growth. 4 To understand the physical mechanism of the wake field, consider the cavity shown in Fig. 1. The relevant parameters are the dimensions of the individual cavity cells a, b, and g, the number of cells TV, the rms bunch length o D and charge Q D of the driver bunch, and the distance the test particle lags behind the driver, z =cr, where r is the relative delay and the particle velocity -c. The beam excites both longitudinal (E z ) and transverse fields (E r and 7/ 0 ). The longitudinal wake potential, which is of particular interest, can be defined as the energy change of the test particle as it passes through the structure. 5 This can be expanded as the sum of cavity modes. Define J 'NgA E z {z\{z' + z)lc)dz' 0 = T

Experimental studies of stimulated Raman back and side scattering, two-plasmon decay, and the generation of high-energy electrons in 0.35 μm laser plasma interaction are presented. To isolate the various phenomena occurring at different densities, we have attempted to control the maximum plasma density by varying the thickness of the foil targets. The scattered light frequency is used as a diagnostic to measure the peak plasma density. Time integrated and time resolved scattered spectra for variable plasma densities are discussed. Effects of self-generated magnetic fields and plasma temperature on SRS and 2ωp decay, respectively, are examined as possible mechanisms responsible for splitting of the backscattered spectrum at ω0/2. A discussion of the effects of a parabolic density profile on the SRS threshold is also included. Finally, the measured energy and angular distribution of the high-energy electrons are discussed. Two-plasmon decay is suggested as the probable mechanism generating the hot electrons.

Experimental studies of the forward scattered spectrum and stimulated Raman backscattering from foam targets are presented. An attempt has been made to isolate the effects of the presence of the quarter critical density on the Raman spectrum by creating plasmas with various peak plasma densities. The plasmas created had a length larger than 600 μm and a variable peak density between 0.11nc and slightly higher than 0.25nc. The total Raman reflectivity in the backward direction is of 0.3%, with 80% of its energy being scattered in a range of frequencies between 470 nm and 500 nm for all the targets used. The scattered intensity near the half-harmonic region shows a weak dependence on the presence of the nc/4 density layer. The forward scattered spectrum obtained from targets with average densities between 0.11nc and 0.22nc shows a broadband of frequencies similar to the backscattered spectrum indicating that the forward spectrum is probably being seeded by the beating of the plasma wave from stimulated Raman backscattering (SRS-B) with the ion acoustic wave from stimulated Brillouin scattering (SBS).

Experimental observation of pulsed radiation ranging from ∼20 GHz to above 100 GHz during the hollow cathode discharge phase of operation of a back-lighted thyratron is reported. The discharge is operated with 120 mTorr Ar gas at 20 kV initial voltage. Pulsed radiation was observed for ∼50 ns, and an electron beam with energy of ∼20 keV was also observed. The observations are correlated with plasma processes predicted in recent computer simulations. The sudden turn-off of the radiation is believed to be a result of plasma expansion and sheath contraction inside the hollow cathode region. A method for varying the pulse length is discussed.