R. Chehab

Positron source using channeling in a tungsten crystal

Abstract A non-conventional positron source using 1.5–20 GeV electron beam impinging on a tungsten crystal oriented along its 〈111〉 axis is described. Computer simulation taking into account photon creation in the crystal, positron generation in a subsequent amorphous target and also positron capture and transport, allowed extensive investigation of this novel type of source. The possible performances are discussed hereafter. Positron yield has been calculated for different operating conditions. Comparisons with classical sources are presented.

High quantum yield from photofield emitters

Abstract Nd-YAG lasers, operated in the ultraviolet, visible and infrared regions of the electromagnetic radiation spectrum at ps and ns pulse widths, have been used to produce high photoemission electron currents from single and array emitters. Photoemitted current densities of 10 12 and 10 10 A/cm 2 were obtained, respectively, in the ps and ns regimes. Quantum yields of 0.75 were obtained for IR light and exceeding unity for green and ultraviolet light in both regimes. Explanation of such high quantum yields is strongly tied to the field effect and to other physical properties of the microemitters.

Pulsed photocurrents from lanthanum hexaboride cathodes in the ns regime

Abstract Photocathode emission investigation represents an essential part in developing new rf sources and new bunched electron guns. In order to determine the photoemissivity of different materials with work functions which can be easily lowered, we studied lanthanum hexaboride (LaB 6 )as a first step. The results of measurements with a nonosecond NdYAG laser, using wavelengths from 1064 to 355 nm, show that pulsed photocurrent densities can reach 7 kA/cm 2 . Quantum yields evolved from 10 −7 to 10 −3 with the lowering of the work function when cleaning the surface under laser exposition. The results also show that with the use of high density photon beams of ≈ 45 J/cm 2 , lanthanum desorption occurs at the surface. Suggestions are given for a cure of this desorption problem.

Short pulse photoemission from a dispenser cathode

Abstract Pulsed photoemission in the picosecond regime has been obtained from a standard thermionic dispenser cathode (WBaCa) at temperatures below the measurable thermoemission threshold. A picosecond Nd : YAG mode locked laser has been used at both green and UV light. Micropulse charges up to 0.5 nC have been measured on a wideband coaxial pickup located behind the anode. They correspond to an electron saturation limit from an approximately 20 mm 2 illuminated cathode area with a surface field of 3 MV/m. The effective cathode efficiency at small laser energies, defined as the number of electrons impinging on the coaxial pickup divided by the number of photons impinging on the cathode, is about 2 × 10 −5 .

Angular Collection Using Solenoids

Pion collection after the target is considered using solenoids. Two collection systems, involving solenoids, the quarter wave transformer and the Adiabatic lens, widely used for the positron collection in linear accelerators, are described. Their main features, as the acceptance volume in transverse phase space, the maximum source radius, the maximum transverse momentum accepted, are provided. The results are presented for the phase space associated to the canonical variables as for the trace space concerning the geometrical variables. The energy acceptances for both systems are presented and compared. Some special properties of the solenoids are then outlined.

Photofield emission in the picosecond regime

Abstract Some results of pulsed photofield emission in the picosecond regime using UV and green lights with microphotocathodes are given below.

Two-Beam Behaviour with DCI

Experimental data on the beam-beam effect with DCI in the e+ e− configuration are presented as a function of current, energy and operating point. The results are quite similar to those obtained before on ACO, as far as stability regions are concerned. However an increase of the beam transverse cross-section and a corresponding saturation of the parameter ξ are observed. Data are presented also in the e+e+ configuration. Beam-beam limit is looked at, as a function of the operating point, in view of the limitation by neighbouring non-linear resonances.