FREE-ELECTRON LASERS

We can now produce intense, coherent light at wavelengths where no conventional lasers exist. The recent successes of devices known as free-electron lasers mark a striking confluence of two conceptual developments that themselves are only a few decades old. The first of these, the laser, is a product of the fifties and sixties whose essential characteristics have made it a staple resource in almost every field of science and technology. In a practical sense, what defines a laser is its emission of monochromatic, coherent light (that is, light of a single wavelength, with its waves locked in step) at a wavelength in the infrared, visible, or ultraviolet region of the electromagnetic spectrum. A second kind of light, called synchrotron radiation, is a by-product of the age of particle accelerators and was first observed in the laboratory in 1947. As the energies of accelerators grew in the 1960s and 70s, intense, incoherent beams of ultraviolet radiation and x--rays became available at machines built for high-energy physics research. Today, several facilities operate solely as sources of synchrotron light. Unlike the well-collimated monochromatic light emitted by lasers, however, this incoherent radiation is like a sweeping searchlight--more accurately, like the headlight of a train on a circular track--whose wavelengths encompass a wide spectral band. Now, in several laboratories around the world, researchers have exploited the physics of these two light sources and have combined the virtues of both in a single contrivance, the free-electron laser, or FEL (1). The emitted light is laserlike in its narrow, sharply peaked spectral distribution and in its phase coherence, yet it can be of a wavelength unavailable with ordinary lasers. Furthermore, like synchrotron radiation, but unlike the output of most conventional lasers, the radiation emitted by free-electron lasers can be tuned, that is, its wavelength can be easily varied across a wide range. The promise of this new technology extends from the fields of solid-state physics, gas- and liquid-phase photochemistry, and surface catalysis to futuristic schemes for ultrahigh-energy linear accelerators

FREE ELECTRON LASERS

The free electron laser (FEL) uses a high quality relativistic beam of electrons passing through a periodic magnetic field to amplify a copropagating optical wave (1-4). In an oscillator configuration, the light is stored between the mirrors of an open optical resonator as shown in Figure 1. In an amplifier configuration, the optical wave and an intense electron beam pass through the undulator field to achieve high gain. In either case, the electrons must overlap the optical mode for good coupling. Typically, the peak electron beam current varies from several amperes to many hundreds of amperes and the electron energy ranges from a few MeV to a few GeV. The electrons are the power source in an FEL, and provide from a megawatt to more than a gigawatt flowing through the resonator or amplifier system. The undulator resonantly couples the electrons to the transverse electrical field of the optical wave in vacuum. The basic mechanism of the coherent energy exchange is the bunching of the electrons at optical wavelengths. Since the power source is large, even small coupling can result in a powerful laser. Energy extraction of 5% of the electron beam energy has already been demonstrated. The electron beam quality is crucial in maintaining the coupling over a significant interaction distance and of central importance to all FEL systems is the magnetic undulator. The peak undulator field strength is usually several kG and can be constructed from coil windings or permanent magnets. In the top part of Figure 2, the Halbach undulator design is shown for one period. The field can be achieved, to a good approximation, using permanent magnets made out of rare earth compounds; a technique developed by K. Halbach (5), and now employed in most undulators. The undulator wavelength is in the range of a few centimeters and the undulator length extends for a few meters, so that there are several hundred periods for the interaction (6-8). The polarization of the undulator can be either linear or circular or a combination (9). The optical wave has the same polarization as the undulator driving it. This is an illustration of the FELs most important attribute, the flexibility of its design characteristics

A POSSIBLE PHASE TRANSITION IN LIQUID He3

A possible phase transition in liquid He{sup 3} has been investigated theoretically by generalizing the Bardeen, Cooper, and Schrieffer equations for the transition temperature in the manner suggested by Cooper, Mills, and Sessler. The equations are transformed into a form suitable for numerical solution and an expression is given for the transition temperature at which liquid He{sup 3} will change to highly correlated phase. Following a suggestion of Hottelson, it is shown that the phase transition is a consequence of the interaction of particles in relative D-states. The predicted value of the transition temperature depends on the assumed form of the effective single-particle potential and the interaction between He{sup 3} atoms. The most important aspects of the single-particle potential are related to the thermodynamic properties of the liquid just above the transition temperature. Two choices of the two-particle interaction, oonstituent with experiments, yield a second-order transition at a temperature between approximately 0.01 K and 0.1 K. The highly correlated phase should exhibit enhanced fluidity

THE EQUILIBRIUM LENGTH OP HIGH-CURRENT BUNCHES IN ELECTRON STORAGE RINGS

An equilibrium theory of the length of intense electron bunches circulating in a storage ring is presented. The consequence of electrical interaction with various resonant structures is expressed in terms of quadratures over the impedance of the structures, and impedance functions for a variety of elements are evaluated. It is shown that elements having resonances at high frequency can, above transition, cause bunches to increase in length with increasing current. The parametric dependence of the bunch lengthening is found to be in good agreement with observations, and numerical estimates, which are in substantial agreement with experiment, are presented

PHASE AND AMPLITUDE CONTROL OF THE RADIO FREQUENCY WAVE IN THE TWO-BEAM ACCELERATOR

The sensitivity of the radio frequency (rf) wave generated by the free electron laser portion of a Two-Beam Accelerator (TBA) is analyzed, both analytically and numerically in a 'resonant particle' approximation. It is shown that the phase of the rf wave is strongly dependent upon errors in the wiggler strength and wavelength and upon the electron beam characteristics of energy and current. The resulting phase error is shown to be unacceptable for a TBA, given reasonable errors in various components. A feedback system is proposed which will keep the rf wave phase within acceptable bounds. However, the feedback system is, at best, cumbersome and a simpler system would be desirable

HEAVY ION INERTIAL FUSION

Inertial fusion has not yet been as well explored as magnetic fusion but can offer certain advantages as an alternative source of electric energy for the future. Present experiments use high-power beams from lasers and light-ion diodes to compress the deuterium-tritium (D-T) pellets but these will probably be unsuitable for a power plant. A more promising method is to use intense heavy-ion beams from accelerator systems similar to those used for nuclear and high-energy physics; the present paper addresses itself to this alternative. As will be demonstrated the very high beam power needed poses new design questions, from the ion source through the accelerating system, the beam transport system, to the final focus. These problems will require extensive study, both theoretically and experimentally, over the next several years before an optimum design for an inertial fusion driver can be arrived at

THE TWO-BEAM ACCELERATOR

A Two-Beam Accelerator, in which one of the beams is an intense low energy beam made to undergo free electron lasing and the other beam is a compact bunch of high energy electrons, is shown to be an interesting possibility for a linear collider

A Plasma Channel Beam Conditioner for a Free Electron Laser

By "conditioning" an electron beam, through establishing acorrelation between transverse action and energy within the beam, theperformance of free electron lasers (FELs) can be dramatically improved.Under certain conditions, the FEL can perform as if the transverseemittances of the beam were substantially lower than the actual values.After a brief review of the benefits of beam conditioning, we present amethod to generate this correlation through the use of a plasma channel.The strong transverse focusing produced by a plasma channel (chosen tohave density 1016/cm3) allows the optimal correlation to be achieved in areasonable length channel, of order 1 m. This appears to be a convenientand practical method for achieving conditioned beams, in comparison withother methods which require either a long beamline or multiple passesthrough some type of ring

Phase stability of a standing-wave free-electron laser

The standing-wave free-electron laser (FEL) differs from a conventional linear-wiggler microwave FEL in using irises along the wiggler to form a series of standing-wave cavities and in reaccelerating the beam between cavities to maintain the average energy. The device has been proposed for use in a two-beam accelerator because microwave power can be extracted more effectively than from a traveling-wave FEL. A simplified numerical simulation indicates that, with appropriate prebunching, the standing-wave FEL can produce an output signal that is effectively the same in all cavities. However, changes in the beam energy of less than 1% are found to introduce unacceptably large fluctuations of signal phase along the device. Analytic calculations and single-particle simulations are used here to show that the phase fluctuations result from beam synchrotron motion in the initial signal field, and an approximate analytic expression for the signal phase is derived. Numerical simulations are used to illustrate the dependence of phase fluctuations on the beam prebunching, the beam-current axial profile, and the initial signal amplitude