M. Cornacchia and B. McSwain

SSRL made substantial progress in the design study that will form the basis of a formal proposal for the development of a 4th generation light source, the Linac Coherent Light Source (LCLS). The LCLS is an X-ray Free Electron Laser (FEL) operating on the Self-Amplified-Spontaneous Emission (SASE) principle. Its operation principles and general layout were described in the last two issues of this Newsletter. The continuing study is a collaboration that involves several divisions of SLAC and a number of other scientific institutions. Progress was made in all areas of the Design Study; a few of the highlights include:

Simulation

Progress in the analysis of FEL related questions through the use of sophisticated simulation tools continued during 1997. A variety of simulation codes such as Ginger and Fred3D, (both developed at LLNL) and TDA3D (originally developed at MIT) have been used to characterize the physics of FELs. A detailed list of parameters has been developed; parameter sensitivities and tolerances have been studied.

The Photoinjector

The LCLS will utilize a high performance photoinjector employing a low-emittance rf electron gun. Work continues on the development of a high-gradient 1.6-cell S-band normal conducting rf gun with a copper photocathode illuminated by intense optical pulses at 260 nm provided by a system comprised of a Nd:YAG-pumped Ti:sapphire (with tripled fundamental frequency). This gun has evolved from a collaboration that includes SLAC, Brookhaven National Laboratory, and the University of California at Los Angeles. Initial measurements on prototypes of the gun have been very encouraging. The Team is attacking a number of challenging design issues that directly affect the system performance, including emittance compensation and temporal pulse shaping.

The Accelerator

The LCLS requires a very high electron peak current. The Design Team has developed and refined an LCLS bunch compression and acceleration system comprised of a series of linac sections and magnetic chicanes arranged so that processes that degrade the beam quality are partially canceled. Simulations indicate that the accelerator section will produce an electron beam that will meet the stringent requirements of the undulator.

The Undulator

Using simulations and semi-analytic models, a planar undulator approximately 100 meters long has been designed. As the design effort progressed during the last six months, it was determined that the undulator could be designed to be segmented rather than continuous. This has allowed the adaptation of a "lumped focusing strategy" and the relaxation of some of the more demanding tolerances. A great deal of attention has been given to undulator alignment; four very promising approaches have been examined in detail.

The Experimental Area

Scientists using the LCLS will work in a hutch housing two experimental stations. To cover the full spectral range of the system, both specular (for wavelengths longer than 4.5 Å) and crystal (for wavelengths shorter that 4.5 Å) optics will be employed. The effect of radiation of this extraordinary intensity on optical materials is still, to some extent, unknown. The experimental area will include attenuation systems for controlling the extremely intense photon beam and optical components for directing it to experimental setups.

There were many programmatic advances during the year. Memoranda of Understanding were put in place with LANL and LLNL for joint R & D projects. Technical reviews were held for the undulator, the photoinjector, and the linac; a cost review was held for the preliminary cost package. The reports from all of the reviews supported the view that the LCLS concept, design and planning were on the on track for meeting project goals.

As pointed out in articles in past volumes of this Newsletter, our Design Team is inspired by the fact that the remarkable characteristics of LCLS radiation have the potential for opening up a number of important new scientific frontiers. This source would create a photon beam of unprecedented brightness, coherence and peak power, far surpassing anything available in 3rd generation sources today. Design parameters are a peak brightness of 5 x 10³³ photons /(s-mm² -mrad²-.01% bandwidth) (13 orders of magnitude greater than any existing radiation sources) and a peak power of 10 GW in the wavelength region 1.5-15 Å. Furthermore, the accompanying spontaneous radiation will be 4 orders of magnitude greater than radiation from existing 3rd generation sources. The Design Team is in the process of producing a design report, with a construction schedule and a cost estimate by late 1997.