S.S. Yu

Simulation of superradiant free electron lasers

Abstract It has been suggested that slippage effects in a single-pass free electron laser can generate an output signal near the electron beam tail with a peak amplitude many times greater than the expected saturated value [1]. The growth of such “superradiant” pulses in a helical-wiggler FEL is characterized here using 1-D and 2-D numerical simulations. The 1-D calculations show the effects of detuning, energy spread, and the axial current distribution on superradiant pulses, while preliminary 2-D simulations illustrate the effects of betatron motion and the radial structure of the electron beam.

Use of a FEL as a buncher for a TBA scheme

Abstract The CERN linear collider (CLIC) [W. Schnell, Proc. Workshop on Physics of Linear Colliders, Capri, 1988, eds. l. Palumbo, S. Tazzari and V.G. Vaccaro (1989) p. 345] is a two-beam-accelerator (TBA) scheme in which the driving beam consists of an intense 3 to 5 GeV electron beam bunched at 30 GHz. One possible way to produce this drive beam is to start with a low-energy (or order 10 MeV), high-current (about 5 kA) beam from an induction linac. In passing through a wiggler, this beam is bunched at 30 GHz into micropulses, each with about 10 12 electrons. To construct the format required for the CLIC drive beam, the bunched beam is subsequently chopped at 350 MHz. It is then accelerated to 3 to 5 GeV in an rf linac driven by conventional, low-frequency klystrons. Rf power is extracted at the bunch frequency of 30 GHz and fed into high-gradient structures to accelerate electron or positron beams to TeV energies. The drive beam is repeatedly reaccelerated in 350 MHz superconducting cavities. This study examines the design trade-offs of the proposed “front end” of the CLIC TBA, the linear induction accelerator and FEL. We examine the relevant figure of merit, the efficiency of bunching, as a function of beam energy, current, and emittance, and we consider the effects of wiggler errors, energy spread and slew, and beam misalignment.

A 17.1 GHz free-electron laser as a microwave source for TeV colliders

Abstract We describe the proposed developement of a 17.1 GHz microwave source based on an induction-linac-driven free-electron laser that is expected to produce peak power levels at the several-GW level. This system is being designed to serve as the power source for a TeV linear collider or for other high-gradient accelerators. We will discuss the power-source requirements for TeV colliders and present the initial design of an FEL that will produce the desired power. In addition, we shall discuss efficiency enhancement schemes such as a relativistic klystron “afterburner”, the requirements for beam reacceleration, and concepts for extraction of the microwave power while preserving key properties of the beam such as emittance and phase.

Waveguide suppression of the free electron laser sideband instability

A theoretical analysis is given of an FEL operating in the millimeter range and, hence, operating in a waveguide. For study of the situation in which the average longitudinal velocity of electrons v/sub parallel/ is close to the electromagnetic pulse group velocity v/sub g/, it is shown that the usual paraxial approximation is invalid. The complete analysis is then specialized to a single particle model which, for operation in a steady state, yields a dispersion relation. The resulting dispersion relation is analyzed analytically and numerically. It is shown that when v/sub parallel/ greater than or equal to v/sub g/ there is no sideband instability, and an analytic expression is given for the stability boundary. 10 refs., 7 figs.

Envelope model of beam transport in ILSE

CIRCE is an efficient beam dynamics code developed to facilitate the design and analysis of heavy‐ion accelerators. The code combines an envelope description of the beam transverse dynamics with a fluid‐like treatment of longitudinal dynamics, and terms are included to account for the effects of space charge, emittance, and image forces, CIRCE is currently being adapted to model the Induction Linac Systems Experiments (ILSE) facility, a proposed heavy‐ion accelerator designed to test aspects of an inertial‐fusion driver. The numerical model in the code is discussed, and changes needed for modeling ILSE are outlined. Preliminary work is presented on beam matching along the ISLE lattice and on transport around the ILSE achromatic bend.

Prospects for a soft X-ray FEL powered by a Relativistic-Klystron High-Gradient Accelerator (RK-HGA)

Abstract We present here a conceptual design for an X-ray FEL operating at 45 A and producing 0.1–1.0 GW of power. Its architecture is a single-pass amplifier with growth from spontaneous noise. It uses an electron beam of about 1.5 GeV and with a normalized emittance of about 20 mm mrad, a beam which may be accelerated in a relatively short length of a high-gradient accelerator driven by an X-band relativistic klystron. Examination of sensitivity of the FEL performance to energy spread, wiggler field errors, jitter in the positioning of the electron beam, and finite steering accuracy indicates that the requisite technologies for this design are imminently available.

Phase and amplitude stabilization of short-pulsed, high-power microwave amplifiers

The authors address the problem of phase and amplitude stability of the RF pulse and present a technique for improving it in high-power, short-pulsed free-electron lasers and relativistic klystrons to a level that is acceptable for accelerator applications. They summarize the results of bench tests and computer simulations, and discuss a proposed high-power klystron experiment aimed at establishing the feasibility of the overall concept and the workability of the stabilization circuits.<<ETX>>

Validity of 1D free electron laser sideband models

Analytic 1D models have been used to estimate sideband growth in a free electron laser (FEL) with a constant-amplitude primary wave [1,2]. A simple 2D sideband model is used here to show that the 1D approximation is valid only when the beam radius rb is large compared with [2k8min(ω0, dφ0dz)]−12, where ks and φ0 are the wave number and phase of the primary wave and ω0 is the maximum synchrotron wave number of trapped electrons. For smaller rb values, sideband growth rates are reduced below 1D predictions, and both the growth rate scaling with beam density and the primary-wave amplitude are altered.

The stability of free electron lasers against filamentation

Abstract We estimate the growth rate of the filamentation instability in a relativistic electron beam travelling parallel to an intense laser beam. We further estimate how the altered index of refraction in a high power FEL will affect the growth rate. We find that due to the finite emittance, FELs will be stable against filamentation.

Physics issues in the design of a recirculating induction accelerator for heavy ion fusion

It is pointed out that recirculating induction accelerators offer the potential of providing a driver for a heavy-ion fusion reactor at a lower cost than a linear induction accelerator. The authors discuss some of the design issues facing a recirculator and compare induction recirculators with induction linacs, RF-linac/storage rings, and synchrotron/storage rings. They briefly discuss two critical physics issues, in common with the RF-linac approach, emittance growth and beam interactions with the residual gas.<<ETX>>