L. De Salvo

Collective atomic recoil laser (CARL) optical gain without inversion by collective atomic recoil and self-bunching of two-level atoms

Abstract We suggest a novel tunable laser concept, the Collective Atomic Recoil Laser (CARL) which unifies the physics of the FEL and of the atomic lasers. We demonstrate that a cold beam of two-level particles driven coherently by a counter-propagating resonant wave can amplify exponentially a co-propagating optical probe up to a saturation value through an instability very similar to that of a high gain FEL. In addition, the two level atoms undergo collective recoil and exponential self-bunching in space and form a longitudinal grating on the scale of the wavelength of the amplified optical signal.

The SLAC soft X-ray high power FEL

We discuss the design and performance of a 2 to 4 nm FEL operating in Self-Amplified Spontaneous Emission (SASE), using a photoinjector to produce the electron beam, and the SLAC linac to accelerate it to an energy of about 7 GeV. Longitudinal bunch compression is used to increase the peak current to 2.5 kA, while reducing the bunch length to about 40 μm. The FEL field gain length is about 6 m, and the saturation length is about 60 m. The saturated output power is about 10 GW, corresponding to about 1014 photons in a single pulse in a bandwidth of about 0.1%, with a pulse duration of 0.16 ps. Length compression, emittance control, phase stability, FEL design criteria, and parameter tolerances are discussed.

A study of linewidth, noise and fluctuations in a FEL operating in SASE

We study the evolution of the FEL radiation intensity and spectrum starting from noise in the electron beam longitudinal distribution. Due to the slippage both the time and frequency structure of the emitted radiation pulse show a very different behavior when the bunch length is much longer than or of the order of the cooperation length lc. The occurrence of superradiant spikes is discussed. We present analytical and numerical results.

Collective resonant Compton scattering by two-level particles

Abstract In this paper we provide a simple analytical demonstration that an unbunched beam of two-level particles coherently driven by a quasi-resonant counterpropagating field can give rise to coherent Compton back scattering due to self bunching which occurs via an instability very similar to that of a high gain Free Electron Laser. Since the initially weak scattering light is exponentially amplified and is Doppler upshifted in frequency, this effect constitutes the basic physical principle of a tunable source of coherent radiation: the Collective Atomic Recoil Laser (CARL). CARL extends the FEL bunching concept to any system of particles having two internal energy levels.

A millimeter wave FEL driven by a photocathode rf linac

Abstract We present the design of a millimeter wave FEL based on the UCLA photocathode rf linac. The linac energy can be varied between 5 and 18 MeV. The electron pulse duration is 2 ps FWHM, with a peak current exceeding 150 A. The FEL is designed to operate in the high gain Compton regime, controlling the slippage with the propagating radiation in a waveguide. The design will permit the exploration of the basic FEL physics in this regime, including the exploration of saturation and lethargy in the superradiant and steady state regime.

An rf-linac, FEL buncher

Abstract We describe a means of producing a train of 40 kA pulses of 3 ps duration as the drive beam for CLIC using an rf linac driven free electron laser (FEL) buncher. Potential debunching effects are discussed. Finally we describe a low energy test experiment.

Transverse laser cooling of two-level particle beams

Abstract We describe a simple method of decreasing the transverse emittance of particle beams with two internal energy levels by laser cooling of the transverse velocity. A specific example of ions in a storage ring will be discussed.

Limitations on plasma acceleration due to synchrotron losses

Abstract In this letter we consider the effect of synchrotron radiation losses due to the betatron motion of the electron beam in its self-induced magnetic field in a plasma accelerator taking into account the charge neutralization factor. The most favorable case is where the plasma density is smaller than the beam density. The contrary regime is strongly disfavored by the synchrotron radiation loss for beams with characteristics for TeV energies. In both cases we find that upon increasing the plasma density the synchrotron losses kill the acceleration process, so that there are limitations on the maximum allowable plasma density.

Propagation effects in a collective atomic recoil laser

Abstract We present a theoretical investigation of propagation effects in a collective atomic recoil laser (CARL) operating in the FEL limit. We consider the cases where the system evolves while in free space and while enclosed in a ring cavity. In the case where no cavity is present, we show that the scattered radiation consists of soliton-like superfluorescent pulses. In the case of a ‘good’ cavity we arrive analytically at a condition to neglect propagation effects. This condition implies that in order to use the so-called mean field approximation, the condition (Δ L ) l 3 2 c →0, T → 0 must be satisfied with (Tl c (Δ L ) 2 3 finite where lc is the cooperation length of the system, T is the transmission coefficient of the mirrors, L and Λ are the sample length and cavity length respectively. We confirm the validity of this condition using a numerical analysis and provide a simple physical interpretation. In the mean field limit, we show that if the cavity linewidth is greater than the spectral width of the pulse emitted by the sample, the emission remains superfluorescent and is not sensitive to the presence of the cavity. We also show that in the opposite case the emission is sensitive to the cavity parameters and no longer superfluorescent.

RELATIVISTIC THEORY OF THE COLLECTIVE ATOMIC RECOIL LASER

Abstract We propose a new method for producing coherent, tunable radiation at short wavelength using the collective Compton backscattering of a counter-propagating radiation field from a relativistic two-level particle beam (generically called atoms). Under proper conditions one can have exponential growth of the co-propagating, back-scattered radiation at the Doppler up-shifted frequency at the expense of the recoil energy of the particle beam. This growth arises from self-bunching of the particle beam which generates the coherent back scattering of the incident field. We show that the gain and efficiency of the system, for a given density, scale as the inverse cube root of the beam energy instead of the inverse of the energy as in the free electron laser (FEL). Examples of an ion beam and of electric spins are discussed.