Avner Amir

Observation of power instability and multimode behavior in a far‐infrared free‐electron laser

Measurements of the time dependence and the frequency spectrum of the output power in the far‐infrared free‐electron laser at the University of California at Santa Barbara are reported. In typical light pulses of 20–50 μs we have observed unexpected oscillations of the laser output with a characteristic period of 5 μs. At the same time, the laser frequency swept over a discrete set of frequency modes, separated by 1.3 GHz. We also present measurements of the gain and loss of the optical mode and discuss the problem of the accelerator terminal voltage drop in a single electron beam pulse with relation to the light spectrum.

Three-dimensional free electron laser gain and evolution of optical modes

Abstract A three-dimensional FEL theory in the small signal regime is presented. We derive a semi-analytical energy theorem which relates the gain and the loss of an optical beam of arbitrary shape, driven by an electron of arbitrary profile, and apply it in several cases. We also derive an equation which describes self-consistently the field evolution near the axis by using a set of generalized “Gaussian” parameters for the optical beam.

Spectral characteristics of the UCSB FEL 400 μm experiment

Abstract Time resolved spectral measurements of the UCSB Free Electron Laser operated at 400 μm are reported. In typical light pulses of 20–50 μs we have observed unexpected oscillations of the laser output with a characteristic period of 5 μs. We found that at the same time, the laser frequency swept over a discrete set of frequency modes, separated by regular intervals of 1.3 GHz.

Injection locking experiment at the UCSB FEL

Abstract The free electron laser at the University of California at Santa Barbara (UCSB FEL) is driven by an electrostatic accelerator with an electron beam recovery system. Because of the pulse to pulse terminal voltage fluctuations and the stochastic nature of the laser startup process the lasing frequency varies randomly by about 0.1%. To eliminate this effect, an experiment is carried out where power from an external coherent source is injected into the laser cavity and is expected to lock the laser to a fixed frequency. Several sources such as continuous and pulsed molecular far-infrared (FIR) lasers as well as backward-wave oscillators are considered on the basis of power, bandwidth and tunability. In all cases the power/bandwidth is many times higher than the FEL spontaneous emission. Preliminary results, using a pulsed FIR laser driven by a TEA CO 2 laser show an early startup of the FEL pulse increase in magnitude and achievement of saturation dictated by the characteristic spectral sweep of the UCSB FEL gain curve.

Bandwidth narrowing of the UCSB FEL by injection seeding with a cw laser

Abstract Narrow bandwidth operation of a free electron laser (FEL) at the University of California at Santa Barbara (UCSB) by seed injection from an external continuous wave molecular laser has been demonstrated. The pulse-to-pulse frequency bandwidth was reduced by one order of magnitude, from 1.6 to 0.16 GHz and an order of magnitude reduction of the sideband mode beating was observed in a single pulse together with a 30% reduction in laser startup time. Numerical simulations of the multimode FEL show that in normal operation of the UCSB FEL sideband mode suppression is rather slow underlining the importance of seeding to obtain single-mode operation for the available pulse length.

Transverse optical mode trapping in a free electron laser

Abstract Transverse optical mode trapping (“active guiding”) in the free electron laser (FEL) is discussed via a variational approach based on classical field theory. The variational principle for the field leads to a set of simple evolution equations for the mode parameters which generalizes earlier discussion where a parabolic expansion of the interaction term was used. The method can be applied to calculate the evolution of the optical beam parameters under the interaction with an arbitrary electron beam profile. Electron beam inhomogeneities are included by used a distribution function. In the linear regime, the stationary condition of these equations results in a simple algebraic equation for the trapped mode parameters. The values obtained in this method are compared with the results of several numerical simulations. The equations of motion for the optical beam parameters are also derived for the full nonlinear FEL problem using the same principles.