P. Le Taillandier

Design and initial operation of LELIA induction accelerator

Abstract The objective of the LELIA program is to produce a high-brightness and high-average-power electron beam for FEL applications. Therefore a linear induction accelerator is under development at CESTA. As a first step, a prototype induction cell has been constructed and tested in order to check its mechanical design feasibility and experimental electrical response, gap geometry and vacuum technology. Moreover, by the end of 1990, an injector will operate with the following parameters: 1.5 MeV beam energy; 1/5 kA beam current; 60 ns flat-top pulse with a few Hz repetition rate capability. An osmium dispenser cathode will produce the high-current-density electron beam. Numerical simulations have also been processed in order to study the beam cavity coupling, minimize the beam breakup (BBU) instability, and optimize the injector electrode configuration. To drive the induction cells a high-power pulse generator has been developed. It consists of two parts: (i) a command resonant charging system (CRCS), (ii) a pulse-forming and magnetic compression device (MAG). Initially the CRCS was tested with a resistive load and the MAG with a spark-gap driver. These preliminary experiments have been successful and the high-voltage flat top has been further improved by charging the MAG-forming line in the middle. A new generator including a cooling system, designed for high-repetition operation, is now under construction.

First FEL experiment in the millimeter range at CESTA The ONDINE experiment

Abstract We present the differents steps of our FEL program called ONDINE at CESTA. The goal is to produce multi-megawatt pulses at 35 GHz. The first step uses the pulse line machine EUPHROSYNE which delivers an electron beam of 1.3 kA at 2.5 MeV from a thermoionic cathode. Beam transport studies are under way. The second step will use the injector of the induction linac LELIA (1.5 MeV, 3 kA, 1 Hz repetition rate). Finally, we will utilize the induction linac LELIA (3 MeV, 3 kA, 5 kHz) with a permanent magnet wiggler.

Design and progress of the AIRIX induction accelerator

A new electron accelerator is now being studied and designed for flash radiography application. It consists of a pulsed injector (4 MeV-3,5 kA-60 ns) and an induction accelerator increasing the energy of the electrons up to 20 MeV. We briefly describe the Injector built by PSI and similar to the DARHT injector at LANL. We present studies and experimental tests carried out in order to design and build new induction cells and high voltage generators suitable for this application. Information is given on the PIVAIR milestone planned for a beforehand validation of the whole AIRIX machine. Cavity is separated from the last output line by a segmented radial insulator interface with a water resister allowing the resistance matching of the diode.<<ETX>>

Progress in free electron maser experiment at CESTA

Abstract At CESTA, the 35-GHz FEL experiment ONDINE I is in progress. The work done this year focused on beam creation and propagation using the Euphrosyne generator with both an oxide cathode and a cold cathode: typically, 1 kA at 2.5 MeV is propagated. Many calculations were done for these problems and we have also developed simulation codes for FEM interaction in both Compton and Raman regimes, taking into account the 3D wiggler field, guiding field, waveguide modes, electron beam injection to obtain trajectories, gain and efficiency. Good agreement with some previous experiments and codes was obtained. The Ondine II expirement will start at 1 Hz with the 1.5-MeV LELIA injector (see companion paper) using the same wiggler technology to gain a better understanding of the FEL physics.

High power induction linac for FEL applications at CESTA

The purpose of the LELIA program developed at CESTA is to acquire the knowledge on induction accelerator technology for high peak power FEL applications. In a first step we study basic technology: (1) A high voltage pulse generator (150 kV, 60 ns, 2 ω) has been designed to drive the induction injector and the accelerating cells. It is able to work at high repetition rate (typically 1 kHz) by the use of magnetic switches. A flat top of 130 kV with ΔVV = ±0.8% has been obtained for about 50 ns. (2) An induction cell prototype has been built in order to check technological choices (vacuum, mechanics, magnetic guiding, voltage supply, etc.) for injector and accelerating modules. (3) The injector geometry is being studied using Euphrosyne (a classical intense relativistic electron beam device) which consists of a concave thermo-ionic oxide cathode, an intermediate electrode and a hollow anode with a magnetic guiding channel. This diode delivers an electron beam between 1 and 3 MV, about 1 kA and a flat top pulse during 20 ns. We will carry on the injector studies with our induction injector LELIA I (1.5 MV, 1.5 kA, 1 kHz) which will be available at the end of 1990. An accelerating module is also being designed, taking into account guiding and stability problems. All these points are described in more detail in a poster paper [J. Bardy et al., these Proceedings (12th Int. FEL Conf., Paris, France, 1990) Nucl. Instr. and Meth. A304 (1991) 311]. The main goal is to build a 10 MV, 1–3 kA, 1 kHz induction accelerator and to have it running at CESTA in 1993. On the other hand, we want to use the electron beam provided by Euphrosyne then in LELIA I to perform FEL experiments at 35 GHz using a bifilar helical wiggler [H. Bottollier-Curtet et al., these Proceedings, p. 197].

First operation of the LELIA induction accelerator at CESTA

An induction linac has been studied and built in order to acquire induction technology for free electron laser (FEL) and other applications. It comprises a 1.5 MeV injector with a thermionic cathode which can deliver a high current electron beam (1 to 2 kA) and a 12 cells accelerating block which raises the electron energy up to 3 MeV. The injector has been tested and fully characterized and the accelerator is now being completed. We describe the induction cells and the high voltage generator designed to feed the cells with 150 kV pulses at 1 kHz repetition rate.<<ETX>>

Microwave FEL experiments at CESTA

Abstract At CESTA the EUPHROSYNE pulsed generator was used for Ondine I, our first FEL experiment at 35 GHz. The work was focused on beam creation and propogation first with an oxide cathode and then with a cold cathode. Typically 1 kA at 2.5 MeV was propagated in single shot operation. A lot of calculations have been done on these problems and we have also developed simulation codes on FEL interaction in both Compton and Raman regimes, taking into account 3D wiggler field, guiding field, waveguide modes, and electron beam injection, to obtain trajectories, gain and efficiency. Good agreement with some previous experiments and codes has been found. In the frame of a preliminary superradiant experiment using a bifilar helical wiggler, we obtained an amplification of the spontaneous emission up to 1 MW in the Kα band and 0.2 MW in the 33–36 GHz band. A microwave apparatus has been completed to work in an amplifier configuration: injection magnetron tube (100 kW), amplification line and diagnostics. All these components are compatible of the new Ondine II experiment designed for operation with the LELIA induction linac. This experiment is to start at 0.1 Hz with the induction injector (1.3 MeV, 1 kA, 50 ns) using the same wiggler technology as Ondine 1. In the future we plan to carry out the 35 GHz experiment at 3 MeV by adding an accelerating module to the LELIA injector. An hybrid planar wiggler with parabolic pole faces is being studied for further experiments.

Progress in free-electron maser experiment at CESTA

At C.E.S.T.A., the 35 GHz FEM ONDINE I experiment is in progress. The work done this year focused on beam creation and propagation on the Euphrosyne generator with both an oxide cathode and a cold cathode: typically, 1 kA at 2.5 MeV is propagated. A lot of calculations have been done on these problems and we have also developed simulation codes on FEM interaction in both Compton and Raman regimes, taking into account 3D wiggler field, guiding field, waveguide modes, electron beam injection to obtain trajectories, gain and efficiency. Good agreement with some previous experiments and codes has been found. A microwave apparatus has already been completed to work in an amplifier configuration: injection magnetron line (100 kW), amplification line and diagnostics (wavenumber analysis for full mode analysis, filters, detectors, etc.). All these components are compatible with the future high repetition rate ONDINE II experiment using the LELIA induction linac. The FEM experiment is now starting at 0.1 Hz with the 1.5 MeV LELIA injector. Beam creation from a thermionic cathode and propagation have been fully studied theoretically and experimentally. An electron beam of 1.3 kA at 1.3 MeV with an emittance of around 200 (pi) -mm-mrad is likely to be compatible with an efficient FEM operation.