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In this paper we have analyzed the wake field amplitude in two types of double slab structures, one with dielectric slabs and the another with both dielectric and plasma liner. It is observed that the plasma liner enhances the dielectric wake field amplitude, thus reduces the requirement of large amplitude drive current.

This paper details our study of a laser-beat-wave photoinjector that is capable of generating periodically bunched MeV electrons with a bunching factor larger than 0.1 at THz frequencies, for a total amount of 1 nC charges in a 10 ps time duration. Also described is a driver laser technology that can tune the electron bunch frequency with ease, as well as help the growth of high harmonics in the bunching spectrum of accelerated electrons.

A simple laser technology is proposed to generate a laser pulse sequence with a pulse rate of 2 THz frequency. The laser pulse sequence is used as the driving laser for a photocathode radio frequency (RF) gun. The RF gun generates the pre-bunched electron beams into an undulator for immediate superradiation. Terahertz (THz) radiation, based on a free electron laser (FEL) at a megawatt (MW) level, could be achieved from a compact FEL facility. We describe the laser handling technology for a 16-pulse laser. The detailed calculation of the bunching factor due to the injection phase, beam charge, and bunch number, as well as the differences in the time spacing and energy spread among the laser pulses, are also presented.

A physics experiment for laser-driven, electron acceleration in a structure-loaded vacuum is being carried out at Stanford University. The experiment is to demonstrate the linear dependence of the electron energy gain on the laser field strength. The accelerator structure, made of dielectric, is semi-open, with dimensions a few thousand times the laser wavelength. The electrons traverse the axis of two crossed laser beams to obtain acceleration within a coherence distance. We predict that the demonstration experiment will produce a single-stage, electron energy gain of 300 keV over a 2.5 mm distance. Ultimately, acceleration gradients of 1 GeV m−1 should be possible.

For a hole-coupled resonant cavity without FEL gain, it is possible to have amplitude cancellation of the optical Gaussian modes at the hole location and, as a result, a null is generated to give very low cavity loss due to the presence of the hole. However, in a warm cavity, i.e. when the electron beam is present and the optical power starts to build up, the FEL gain tends to amplify the modes unequally, and the unbalanced gain for different optical modes results in significant power radiated through the hole. In this paper we present a guideline, based upon variation of the Rayleigh length, for mode selection that maximizes net gain.

The performance of a staggered-array wiggler for the far infrared free-electron laser (FIRFEL) project at Stanford has been characterized analytically and experimentally. A 10.8 kG peak wiggler field was measured for a 2.0 mm gap and a 1.0 cm wiggler period at a 7.0 kG solenoid field. The wiggler field uniformity was investigated by the pulsed-wire technique, and measurements showed a 1.2% rms field variation over a 50 cm wiggler section. With fairly simple design schemes, studies indicate the velocity drift in a staggered wiggler can be eliminated.

A possible approach to high efficiency FEL operation is to combine a microwave linear accelerator and magnetic wiggler into a single structure. As the electrons lose energy to the radiation at the FEL oscillation wavelength (e.g. 10 μm), energy is replaced by the microwave linac. The electron beam acts as a catalyst for the conversion of microwave power to infrared power. Several advantages to the accelerator/wiggler are: it is possible to obtain high conversion efficiency in a short length; small-signal gain reduction can be avoided; power extraction may be increased by increasing length; there is little detrapping; and electron beam energy out of the wiggler is relatively monochromatic, permitting efficient energy recovery. A six period, full scale model of the accelerator/wiggler has been fabricated to check the computer simulations for both microwave and magnetic properties. Microwave measurements on this model indicate that the predicted Q is accurate, that there is an approximate 1.5% error in the predicted resonant frequency and that critical coupling to the structure in the presence of the electron beam can be achieved with a direct, rectangular waveguide feed.

A 10 μm infrared FEL is being assembled to provide high efficiency operation by means of combining a linear accelerator and magnetic wiggler into a single structure. As energy is removed from the electron beam to amplify the infrared radiation, an equal amount of energy is injected into the beam from a muwave source. Thus, synchronism is maintained, and the electrons act as a catalyst for the conversion of muwave to infrared power. Comparisons will be made between the performance of an accelerator/wiggler and a tapered structure. One distinction is that the muwave power will be delayed in time so that there is maximum gain at low infrared signal levels. Another feature is that the device can be lengthened almost indefinitely to obtain more power, whereas the taper is limited to the initial value of the wiggler parameter. The system parameters will be discussed relative to the expected performance. Additional topics to be considered include beam breakup modes in the accelerator, wakefield effects, and wavelength scaling.

A compact size far-infrared free-electron laser (FIR FEL) is currently being built at Stanford. A microwave gun products 1–3.3 MeV electrons, which are sent into a 50 cm long wiggler of 1 cm period through a hole on the upstream mirror to generate radiation at a wavelength of 100 to 1000 μm. A superconducting solenoid along with an array of permeable material is used to generate a 9.6 kG rms wiggler field with a 2.0 mm gap. The electron beam consists of 10 ps micropulses with 10 A peak current, 1% energy spread and unnormalized emittance for 90% of the particles of 2π mm mrad. A 10 dB small signal gain has been calculated with the parameters mentioned above. An overview of the design details as well as a discussion on the uniqueness of our wiggler are presented.

If the free-electron laser is to move from the category of “national facility” to the designation of a “laboratory instrument” it must meet several conditions, including a reduction in cost. For the far infrared, an FEL can be constructed for a component cost of approximately $300 000 including the accelerator. Such a device has been assembled, using a 1 1/2 cell RF cavity gun for the accelerator and a staggered-array wiggler consisting of permeable pole pieces in the field of a superconducting solenoid. Spontaneous emission measurements have been performed, and laser gain has been observed. Measurements have been in good agreement with theory.