Christian Travier

Contribution to the Workshop on Electron Gun for Linear Colliders, Fermilab, March 8-9, 1993SIGLEAvailable at INIST (FR), Document Supply Service, under shelf-number : RP 11747 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

CANDELA photo-injector is made of a 2-cell S-band RF gun, using a dispenser cathode illuminated by a Ti:sapphire laser. This electron source provides a single bunch (at 12.5 Hz), with a charge of 1 nC and an energy of 2 MeV. This paper presents the measurement of the bunch length which is done 1.8 m downstream of the gun exit. The measurement system includes a 0.3 mm thick sapphire plate used to produce Cerenkov radiation, a 27 m long optical beamline and a streak camera. Bunch lengths of less than 10 ps were measured. These measurements are the first experimental proof of the fast response of dispenser photocathodes. Introduction Many applications (including high energy linear colliders, free electron lasers, and X-ray radiation sources) need electron sources that can produce intense, bright and short electron pulses. The photo-injector being very attractive with these respects, it is studied in many laboratories around the world [1]. The CANDELA photo-injector is part of this worl...

Un canon hf est constitue d'une cavite hf dans laquelle on loge une cathode, qui peut-etre une photocathode illuminee par un laser, ce qui permet de produire des impulsions electroniques tres breves (quelques picosecondes), tres intenses, et de faible emittance. Cette these decrit le travail effectue durant la periode 1990-1995 sur le projet candela (canon declenche par laser), qui est un photo-injecteur experimental etudie au laboratoire de l'accelerateur lineaire. Apres avoir presente les avantages d'un photo-injecteur par rapport a un injecteur conventionnel, et rappele les modeles de la dynamique de faisceau dans un tel canon, des simulations numeriques sont donnees pour caracteriser le fonctionnement de candela dans les regimes d'impulsions longues (quelques dizaines de picosecondes) et ultra-courtes (quelques picosecondes). La realisation pratique de la cavite est ensuite presentee. Finalement, les resultats experimentaux concernant la cavite (champ accelerateur crete de cent-six megavolts par metre et vide limite de dix puissance moins dix millibars), le courant parasite qu'elle emet, et la charge photo-emise. Celle-ci qui vaut deux nanocoulomb, est obtenue a partir d'une cathode impregnee dont le rendement quantique vaut un pour mille et la duree de vie une douzaine d'heures. Le projet candela decrit dans cette these, constitue le seul photo-injecteur declenche par un laser capable de produire des impulsions sub-picoseconde. Les premiers resultats experimentaux obtenus qui sont conformes aux attentes, demontrent le bon fonctionnement de cette source. Les mesures plus detaillees a venir doivent etre confrontees aux simulations et aux resultats experimentaux des autres machines en fonctionnement dans le monde

Most of present and future electron accelerators require bright sources. This review of electron guns therefore focuses on t,he brightest sources presently available, the photo-injectors. After briefly recalling their principle, t,he most advanced projects are reviewed. Photocathodes, lasers and photo-injector beam dynamics are then dis-cussed.

Free Electron Lasers and future high energy linear colliders require very bright electron beams. Since conven-tional injectors made of DC guns and RF bunchers have intrinsic limitations, an important R&D endeavour has been undertaken worldwide to develop a new type of bright electron source: the microwave or RF gun. After describing briefly the conventional injector and its limitations, the basic principles of RF gun are pre-sented showing that there exist three types of gun. The technology state of the art is then shortly addressed. The thirty currently on-going RF gun projects are reviewed. Their specific characteristics are outlined and their nominal performances are given. Finally, the basic ideas of RF gun design are exposed.

As part of the R&D effort of the TTF collaboration, an in- put power coupler development programme has been un- dertaken by a CEA/DSM and CNRS/IN2P3 collaboration (1). The power test stand which includes a cryostat is described and the cryogenic performances of the cryostat are presented. The results of the test of a tapered coax- ial waveguide are given. The ceramic window is de- scribed and TiN coating characterisation is presented.

As part of an R&D program to develop an input power coupler for the TESLA project, a 1.3 GHz travelling wave window was designed, fabricated and tested. The main advantages of this window are a reduced electric field at the brazing location, low dielectric losses and protection of the ceramic from the direct view of the electrons coming from the cavity. Two versions of this window were fabricated (one with titanium as the outer conductor, and one with a copper outer conductor). The titanium version was tested up to 1 MW (1 ms), both at room temperature and at 100 K. The design of this window, and the results of the high power tests are presented in this paper.

A two-cavity RF gun basic design is presented. The elec- tric field itist :,i?,utinn along the beam axis is chosen to minitnize mtittance growtl~. Iudcpenttmt phasing of the cavities allows to Iniriilrlize rwr~rgy spww 1 Simulation!: with the codes PARMELA (ij a.~ld t'RIA,hl (2) :LI~ prevented. According to this desigi~, a tow-level tno~l?l cavity >vas 1aullrl:rtl. tiolllilmlt In 01lr CHSP, brcausc of re!ativrty lonp, hunches, wc t:sed ttir second crlterlir t.0 d~~~i~~~ ~III~

CANDELA photo-injector is a two cell S-band photo- injector. The copper cathode is illuminated by a 200 fs Tisapphire laser. This paper presents the preliminary ex- perimental results. Measurements of the beam charge as a function of the laser/RF phase, the laser pulse energy, and the accelerating field are presented.

Since the beginning of 1990. the "Laboratoire de I'AccClCmtew Linhire" (LAL) at Orsay, has started to develop a high-brightness photoinjector (1,21. The project, now named CANDELA (CANon DEclenchC par LAser), consists of a two cell 3 GHz microwave electron gun. The photocathode is illuminated by a Ti:Sapphire picosecond laser designed by the "Institut d'optique Thhrique et Appliqu" (IOTA) also located at Orsay. The gun and laser designs are briefly described and the project StiltU!j is reviewed.

High intensity proton linear accelerator R&D program is in progress in France. The high energy part of such a linac uses superconducting accelerating cavities with two differ- ent beta sections, 0.47 and 0.65. Each cavity is fed with 704 MHz radio frequency power through one coupler. The goal is to transmit power up to at least 300 kW with a very high duty cycle (up to CW operation) and with a very high reli- ability. Under these conditions, RF windows are subject to thermo-mechanical stresses and potentially to multipacting discharges. The coupler design should minimize the risk of window failure and, should the window break, avoid fatal consequences for the cavity. This paper presents general design considerations, and the preliminary design of some coupler parts.

ABSTRACT CANDELA is an S-band photo-injector built at Orsay as a collaboration between the “Laboratoire de l'Accélérateur Linéaire” and the “Institut d'Optique Théorique et Appliquée”. The first experimental results concerning vacuum and high-power RF conditioning are presented.

ABSTRACT CANDELA photo-injector is made of a 2-cell S-band RF gun, using a dispenser cathode illuminated by a Ti : sapphire laser. This electron source provides a single bunch (at 12.5 Hz), with a charge of 1 nC and an energy of 2 MeV. After recalling the experimental set-up, this paper presents some results concerning mainly energy and bunch length measurements, and also comparisons with simulations done with the PARMELA code. Measured pulse durations of less than 10 ps show for the first time that dispenser photocathodes are “fast response” cathodes.