J. Le Duff

Current and current density limitations in existing electron storage rings

Abstract Storage rings dedicated for synchrotron light sources must show good beam performances in order to obtain high flux and brilliance. In particular bunch length, energy spread, beam intensity and transverse emittances have to satisfy special requirements. We intend here to briefly overview twenty years of experience obtained with small and medium size storage rings which were initially built as high energy physics facilities. The limiting effects will be extensively discussed by comparing theoretical models with systematic experimental studies. The possible means to remedy the inconveniences are also mentioned.

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.

VERSATILE DBA AND TBA LATTICES FOR A TAU-CHARM FACTORY WITH AND WITHOUT BEAM MONOCHROMATIZATION

Optics for a high luminosity e+e− collider at a center of mass (CM) energy of 4 GeV, with and without beam monochromatization, using achromatic cells in the arcs have been carefully studied. Two kinds of achromatic cells (double bend achromat and triple bend achromat) have been considered. Chromaticities, dynamic apertures and beam lifetime have also been studied and compared for the two cases.

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 .

Permanent Magnet Storage Rings for Microlithography and FEL Sources

Permanent magnet technology is rapidly improving and it seems possible now to design and build dedicated electron storage rings for peculiar applications such as microlithography and free electron laser. A low capital cost as well as a low operational cost (no power supplies) may lead to industrial solutions. In this paper we describe two possible design studies for machines at energies of 240 and 500 MeV with some indications on an efficient injector.

3D investigations of the 35 MV tandem Vivitron

Abstract Trying to understand the sparking on the metal bars which connect the discrete electrodes on the same radial position, the 3D program Antigone [1] has been used to model the Vivitron structure (which includes terminal electrode, tank, discrete electrodes (DE), and, of course, metal bars) and to find out the electric field distributions on the surface of the electrodes. 3D calculation shows that the maximum electric field happens on the metal bars instead of the edges of the discrete electrodes so in disagreement with the initial machine design expectation. According to the numerical simulations, 20 MV appears as the limit for the terminal voltage with the present machine structure. Improvements on the metal bars seem necessary if nominal voltage is still envisaged.

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.

Transverse Instabilities in the LIL

In view of a Beam Break Up (BBU) study, we have calculated and identified series of transverse deflecting modes in the S band sections of the LEP Injector Linacs (LIL). In these TW quasi-constant gradient structures with various iris diameters from 18 to 26 mm, only it-modes of the C band of the HEM11, having negative group velocities and phase velocities around c will lead to cumulative BBU. These deflecting ¿-modes occur at frequencies in the range from 4558 to 4290 MHz for diameters from 18 to 26 mm. Frequency variation with iris diameter is -15 MHz for each added mm for calculated and measured 0-modes and about twice that value for ¿-modes. Levels for starting transverse instabilities have been determined for various accelerated Beam Currents with different pulse widthes.

Beam Dynamics on ACO

After a shut down of several months during which some improvements were made on ACO such as a computer control of the power supplies, and a new detector for high energy experiments was installed, an extensive beam experiment program was started. This mainly concerned the beam-beam effect with and without small beam separation at the crossing point, the cure of transverse instabilities with new sextupolar coils placed inside the quadrupoles, the check of power supplies stabilizations by means of the synchrotron frequencies, and the development of a new ACO structure with low ss. During the same period new measurements of the neutral modes of the ?o were started on ACO. Finally a beam of synchrotron light extending up to 10 A will soon be put in operation for molecular, atomic, and solid state physics. The typical luminosity at 510 MeV is now about 3.4.1032 cm-2.h-1 with 35 mA in each beam, and this new improvement seems to be related to a better power supplies stabilization.