Relativistic Bunches Research Articles

Free-electron lasing in the wake field of an elliptical pill-box cavity

It is shown using the photon concept that free-electron lasing (or net stimulated bremsstrahlung) is unrelated to the electron phase with respect to the laser wave, while the net acceleration (or net two-photon absorption) in an RF acceleration cavity depends on the electron phase with respect to the RF wave. The gain formula for the free-electron laser using a magnetic wiggler (MFEL) derived using the recently developed quantum-augmented classical theory in which the electron phase is ignored is in excellent agreement with that obtained quantum-mechanically. It is found by means of this theory that if an electric wiggler is added to a MFEL, the synchronization between the transverse velocity and the laser wave, which is required for coherence of the laser light, is not affected, while the laser gain is enhanced owing to the increase in the amplitude of the energy modulation by the electric wiggler. As a configuration of this turbo-MFEL, a two-beam elliptical wake-field cavity is proposed. An electron beam injected in the antiparallel direction along the lasing-beam path in this cavity lases through transverse wiggling by the transverse wake field and energy modulation by the longitudinal wake produced by relativistic drivingbeam bunches. This laser (WFEL) becomes of greater advantage compared with the MFEL as the laser wavelength is made shorter. It is also shown that the amplification of the WFEL is much greater than that of the present MFEL if we can produce a wake field whose longitudinal component has field strength greater than 1 MV m–1.

Basic features of coherent radiation generated by relativistic charge bunches

Abstract Radiation generated by relativistic charges can be analyzed and described in exquisite detail. One reason that such detailed analysis is possible is because the phases of radiated photons often are determined completely by the initial conditions of the relativistic charges and the radiating system. The phase relationships between the initial charges and the radiated photons represent coherence in the emitted radiation. A previous paper decribed how this coherence could affect the spatial and spectral distributions of radiation generated by a single charge in a periodic radiator. The present paper discusses a complementary issue; namely, how the temporal shape of a relativistic charge bunch can emphasize specific features of the radiation generated at a single interaction site.

On the amplification mechanism of the ion-channel laser

The amplification mechanism of the ion-channel laser (ICL) in the low-gain regime is studied. In this concept, a relativistic electron beam is injected into a plasma whose density is comparable to or lower than the beam's density. The head of the electron beam pushes out the plasma electrons, leaving an ion channel. The ion-focusing force causes the electrons to oscillate (betatron oscillations) about the axis and plays a role similar to the magnetic field in a cyclotron autoresonance maser (CARM). Radiation can be produced with wave frequencies from microwaves to X-rays depending on the beam energy and plasma density: omega approximately 2 gamma /sup 3/2/ omega /sub pe/, where gamma is the Lorentz factor of the beam and omega /sub pe/ is the plasma frequency. Transverse (relativistic) bunching and axial (conventional) bunching are the amplification mechanisms in ICLs; only the latter effect operates in free-electron lasers. The competition of these two bunching mechanisms depends on beam velocity nu /sub 0z/; their dependences on nu /sub 0z/ cancel for the cyclotron autoresonance masers. A linear theory is developed to study the physical mechanisms, and a PIC (particle-in-cell) simulation code is used to verify the theory. The mechanism is examined as a possible explanation for experimentally observed millimeter radiation from relativistic electron beams interacting with plasmas. >

Threshold Behavior for Longitudinal Stability of Induction LINAC Bunches

The induction linac bunches of heavy ion fusion scenarios are strongly influenced by the longitudinal space charge impedance. This is in distinct contrast to relativistic bunches in storage rings where most of the data on stability have been obtained. Simulation results reveal that when space charge effects are large, the stability requirement of small growth rate relative to the synchrotron frequency for relativistic bunches is replaced by the relaxed condition of small growth rate relative to the frequency spacing of the space charge wave modes on the bunch. Dispersive effects from finite pipe size tend to make the lower frequencies less susceptible to instability than higher frequencies. Since induction modules have a high resistive component only for the lowest bunch modes, stability is better than would occur for a broadband impedance of comparable magnitude. These results indicate that long term longitudinal bunch stability is realizable for induction linac drivers for heavy ion fusion.

Bench Measurements of the Energy Loss of a Stored Beam to Vacuum Components

Since 1975 measurements of higher order mode losses have been carried out at DESY. The method in use had been proposed by J. Rees and M. Sands): Short current pulses (2 σ = 100 psec) on a thin coaxial wire simulate a relativistic particle bunch. Changes in the bunch shape allow the determination of energy losses due to higher mode excitation. Measurements on simple cavity structures agree well with calculations. Results for a large variety of vacuum structures in the PETRA storage ring are given including structures, the losses of which can no longer be easily calculated. For these structures the results of the measurements described here are the only source of information for improvements.

Relativistic effect in electron cyclotron transverse wave devices

This note presents a simple physical argument which demonstrates the importance of relativistic electron bunching interactions between rotating electrons and an RF electric field in the plane of rotation, normal to a dc magnetic field. A magnitude comparison between the relativistic angular bunching and the well-known longitudinal bunching due to the transverse RF magnetic field shows that the close-to-cutoff operation of a number of recent cyclotron-frequency transverse-wave experiments is due solely to the relativistic effect.

Effect of the coherent radiation on the phase distribution of a relativistic bunch

The particles of a bunch circulating in an accelerator are submitted to a force originating from the coherent radiation. It is of interest to determine the effect of this force on the particle synchrotron motion. We do this from the point of view of the phase distribution function. We show that the coherent radiation gives rise to an increase of the longitudinal dimension of the bunch.