J. M. Peterson

Issues in storage-ring design for operation of high-gain FEL

Abstract A high-gain free electron laser (FEL), placed in a special by-pass of a storage ring, can provide tens of megawatts of coherent power at wavelengths shorter than 1000 A. The requirements on beam quality (few hundred amperes of peak current, emittance of the order of 10−8 m·rad, and relative energy spread less than 0.002), are demanding but lie within the limits of modern storage ring technology. In this paper, we study various issues in storage ring design and FEL physics for operation of a high-gain FEL. Topics included are the requirements on beam parameters for FEL operation, coherent instabilities and intrabeam scattering effects in the storage ring, lattice design, and FEL performance computed by 2-D simulations.

Reduction of Beam Emittance by a Tapered-Foil Technique

The use of a tapered energy-loss foil plus dispersion and optical matching in the succeeding beam transport system can reduce the emittance of a particle beam in one transverse phase plane at the expense of increasing it in the longitudinal plane. Multiple Coulomb scattering in the foil increases the emittance in both transverse planes but can be acceptable provided that the incident beam is sufficiently mono-energetic and is tightly focussed at the foil. This technique can be applied to advantage in typical non-relativistic proton beams from RF linacs and to high-emittance, high-intensity, low-energyspread electron beams from induction linacs, but multiple scattering effects make unprofitable for typical electron beams from RF linacs, unless the energy spread can be appreciably reduced. Claud Bovet6 has previously emittance-reducing properties of arrangement. His report includes sults of this paper.

Synchrotron Radiofrequency System

A description of the radiofrequency system of the Berkeley synchrotron is given.A description of the radiofrequency system of the Berkeley synchrotron is given.

Selection of Injector Synchrotron Parameters to Minimize Cost of the 200-BeV Accelerator

In choosing the type of injector system and in optimizing the parameters of any given type of system, we must consider relative cost advantages as well as relative technical advantages eventually entering into the arguments. In this paper we shall discuss a method of determining the cost of accelerator systems and give some examples that arose in the selection of an injector system for the proposed 200-GeV accelerator.1 The cost of the injector system cannot be considered apart from the total accelerator cost because the aperture of the main ring depends upon the energy and emittance of the beam transferred from the injector. Since the main ring is relatively much larger and more expensive than the injector system, a change which lowers injector cost can cause a much larger increase in main ring cost.

A storage ring FEL for the VUV

Abstract A free electron laser for the VUV operating in a storage ring requires an electron beam of high density and low energy spread and a short wavelength, narrow-gap undulator. These conditions tend to produce longitudinal and transverse beam instabilities, excessive beam growth through multiple intra-beam scattering and a short gas-scattering lifetime. Passing the beam only occasionally through the undulator in a by-pass straight section, as proposed by Murphy and Pellegrini [1], allows operation in a high-gain, single-pass mode and a long gas-scattering lifetime. Several storage ring designs have been considered to see how best to satisfy the several requirements. Each features a by-pass, a low-emittance lattice and built-in wigglers for enhanced damping to counteract the intra-beam scattering.

Storage Ring Parameters for High Gain FEL

We use one-dimensional free electron laser (FEL) theory to find criteria for choosing electron beam and undulator parameters for operation of a high gain FEL at 400 A. The criteria are moderate electron beam energy (<1 GeV), high peak current (several hundred amperes), small emittance ( about10/sup -8/ m-rad), small relative momentum spread ( about0.001), and narrow undulator gap ( about3 mm). Results of two-dimensional simulations on FEL performance are also presented.

A Method of Analyzing the Momentum and Betatron Amplitude Distributions in a Circulating Beam

We have developed a two-probe method of analyzing the momentum and radial-betatron-amplitude distributions in a circulating particle beam. It has been used successfully in an electron-ring research program to help diagnose beam instabilities and injection techniques.

Recent Experimnts on Forming Electron Rings at Berkeley

In the initial electron-ring experiments with a new high-current, 2 MeV electron injector an efficient method of trapping of the injected beam was developed, and a suitable compression cycle was found in which betatron amplitude growth on single-particle resonances is negligible. The ring behavior is satisfactory during the compression cycle at low intensity (Ne 1012 electrons a longitudinal (negative mass) instability causes particle loss and radial spreading of the electron rings. Increasing the energy spread in the beam has raised the threshold for several phenomena observed in connection with this instability. The effects of changing the electromagnetic environment of the electron rings to suppress this instability are being investigated.

Design Features and Operational Characteristics of the PEP Beam-Transport and Injection System

The PEP beam-transport system was designed to transmit 4-to-15 GeV electron and positron beams from the SLAC linac within a ± 0.8 percent momentum band, to have flexible tuning of the betatron and offmomentum functions for matching into the PEP storage ring, and to have convenient operating characteristics. The transport lines were brought into operation quickly and have operated well. Electron injection has been consistent and efficient and relatively easy to accomplish. Positron injection also has been satisfactory but is variable and more sensitive to ring conditions.

A Superconducting Proton Storage Ring for PEP

In order to provide for electron-proton collisions in the PEP system, plans for a high-energy superconducting proton storage ring are being explored. The energy is constrained to 300 GeV by the radius of the PEP tunnel (350 meters) and the field strength (7 tesla) expected to be available in practical superconducting magnets. A new configuration has been developed in which the proton ring vertically crosses the horizontal electron ring in 4 of the 6 straight-sections. Synchronization of the two beams is provided by means of bypasses in the two non-crossing straight sections in the electron ring. The proton injector is a 5 GeV/c synchrotron 1/18 as large as the main ring.