A. T. Lin

Application of electromagnetic particle simulation to the generation of electromagnetic radiation

A three velocity and one‐space dimensional nonrelativistic electromagnetic particle simulation code employing the fast Fourier transform algorithm is described and used to simulate the amplification of electromagnetic radiation by an electron beam passed over a rippled static magnetic field. In the beam frame the rippled magnetic field looks like an intense electromagnetic pump and thus a parametric instability can be produced. In one case, it was observed that 30% of the beam energy was converted to electromagnetic radiation.

Particle simulations of a finite‐length free electron laser

One‐dimensional particle simulations of a finite‐length free electron laser show that the convectively‐unstable, short‐wavelength mode is the dominant instability initially for systems long compared with the growth length of the mode. Also, a generalized one‐dimensional linear theory of a finite length system has been derived with all kinetic effects included. Comparison between the unstable modes seen in the simulation and those predicted by this theory are in good agreement. However, the simulations show a significantly higher efficiency than that predicted by the theory when trapping by a single mode is the saturation mechanism. This may be caused by a local dc electric field (space charge fields) that arises self‐consistently as the beam slows down.

Stimulated Compton scattering of electromagnetic waves in plasma

The so−called stimulated Compton effect is investigated on a one−and−two−halves−dimensional electromagnetic plasma model. This effect results in a parametric instability in which an electromagnetic wave is backscattered off bunches of particles rather than off coherent plasma waves. Numerical simulations were carried out with pump frequency ranging from 1.8 ωpe to 5 ωpe. The momentum imparted by the pump wave to the plasma produces an averaged plasma drift motion in the direction of the pump wave and with velocity comparable to the electron thermal velocity (V0 ≃ VT ≃ 0.1 c). In some cases a multiple backscatter process is believed to be the saturation mechanism of the instability. Stimulated Compton scattering has been observed even for frequencies below 2 ωpe where the Raman instability cannot occur. However, in this region, a sufficiently strong pump can overcome the frequency mismatch and drive the system hydrodynamically unstable. In this situiation strong acceleration of some electrons occurs produc...

Gyrophase‐coherent electron cyclotron maser

The basic physics of an electron cyclotron maser driven by a coherently gyrophased, helical electron beam in a uniform magnetic field is analyzed. Theoretical predictions of gain enhancement and reduction in different regimes of the cyclotron instability, compared to a gyrotropic cyclotron maser, are verified with computer simulations. A physical explanation is presented for these findings. Beam‐to‐radiation energy conversion efficiency in the autoresonance regime is studied with computer simulations and compared with that for a gyrotropic beam.

Nonlinear saturation and thermal effects on the free electron laser using an electromagnetic pump

The production of visible laser radiation by a low energy relativistic electron beam (MeV) is most readily accomplished through the use of a high frequency (far infrared) electromagnetic pump (λr=λ0/4γ02); such radiation can be generated by reflecting the radiation the relativistic electron beam generates when it interacts with a rippled static magnetic field. It is important to investigate what limits the efficiency in this situation. It is found that with a weak pump field, the pump energy can be amplified by a factor of 4γ20 and the pump intensity by 16γ40 with the instability being terminated by pump depletion. With a strong pump field, particle trapping in the longitudial potential wave stops the growth of the instability and the initial momentum spread (temperature) limits the saturation amplitude.

Plasma heating from upper‐hybrid mode conversion in an inhomogeneous magnetic field

Large‐amplitude short‐wavelength electrostatic fields can be generated at the upper‐hybrid resonance layer due to the mode conversion process when an incident wave of appropriate frequency falls on a plasma. As a consequence, a host of nonlinear and kinetic phenomena can take place. Among them, the generation of energetic electrons due to the breaking of adiabatic electron motion in the wave field, the enhancement of electron second harmonic cyclotron heating at the double resonance (ωuh = 2ωce), and parametric instabilities in which the upper‐hybrid mode decays into another upper‐hybrid mode and a low‐frequency mode. Investigations of these phenomena through the use of computer simulations are presented. The parametric process is observed to produce a substantial number of energetic ions localized at the resonance layer.

Enhancement of free electron laser radiation by applying a direct current electric field

The intensity of the electromagnetic radiation produced by a free electron laser can be substantially increased by applying a dc electric field at the time of saturation. By using a proper dc electric field strength, a substantial number of trapped electrons can become clamped in the decelerating phase and continuously transfer the dc electric field energy into the locked high frequency radiation. A modulational instability is observed when the radiation intensity exceeds some threshold, and ultimately terminates the conversion process.

Cross-field electron transport due to thermal electromagnetic fluctuations

The cross‐field electron particle and energy transport for a high β thermal plasma is investigated using a magnetostatic particle code. The diffusion process due to magnetic fluctutations alone is observed to have a Bohm‐like scaling. However, when both electrostatic convective cells and magnetic fluctuations are allowed, there is interference between the two diffusive processes; the diffusion takes on the character of strong turbulence for the interaction between these static modes. The resulting diffusion is smaller than would be the case when assuming that these modes are independent.

Linear theory of an electron cyclotron autoresonance maser with a ‘‘phase filter’’

The linear growth rate of an electron cyclotron autoresonance maser (CARM) with a phase filter, constructed by imposing a transverse magnetostatic wiggler, is derived from relativistic kinetic theory. The theoretical results confirm previous findings from computer simulation showing substantial enhancement of the growth rate near the autoresonance condition. This theory provides an additional, more readily accessible tool for designing a novel phase filter‐assisted CARM. The mathematical formalism developed to treat the complex electron motion in this device is introduced.

Nonlinear saturation of free electron lasers around gyroresonance

A mechanism is proposed to interpret the experimental results which show a reduction in the short wavelength output power when gyroresonance is reached. The presence of an axial magnetic field in a free electron laser around gyroresonance introduces a strong long wavelength Raman backscattering instability which itself gives rise to electron trapping and causes the desired short wavelength radiation to saturate earlier than unmagnetized free electron lasers.