We report the design of a laser wakefield accelerator (LWA) with external injection by a rf photogun and acceleration by a linear wakefield in a capillary discharge channel. The design process is complex due to the large number of intricately coupled free parameters. To alleviate this problem, we performed front-to-end simulations of the complete system. The tool we used was the general particle-tracking code, extended with a module representing the linear wakefield by a two-dimensional traveling wave with appropriate wavelength and amplitude. Given the limitations of existing technology for the longest discharge plasma wavelength (∼50μm) and shortest electron bunch length (∼100μm), we studied the regime in which the wakefield acts as slicer and buncher, while rejecting a large fraction of the injected bunch. The optimized parameters for the injected bunch are 10pC, 300fs at 6.7MeV, to be injected into a 70mm long channel at a plasma density of 7×1023m−3. A linear wakefield is generated by a 2TW laser focused to 30μm. The simulations predict an accelerated output of 0.6pC, 10fs bunches at 90MeV, with energy spread below 10%. The design is currently being implemented. The design process also led to an important conclusion: output specifications directly comparable to those reported recently from “laser-into-gas jet” experiments are feasible, provided the performance of the rf photogun is considerably enhanced. The paper outlines a photogun design providing such a performance level.

A parameter study for laser wakefield acceleration is presented, in which externally injected electrons are accelerated in low amplitude plasma waves, represented by an analytical two-dimensional description. Results have been obtained for plasma densities up to 2.6 ϫ 10 24 m −3 , plasma lengths up to 300 mm, laser intensities up to 3.5ϫ 10 21 W / m 2 , and injection of Gaussian model bunches at energies up to 12 MeV. For the range of parameters studied, effects of laser depletion and the influence of the electron bunch on the plasma can be ignored. In the parameter space, a region is identified where final energies of over 100 MeV are reached, at an energy spread of less than 5% and a rms emittance of a few micrometers.

The effects of experimental variations in the synchronization, laser power, and plasma density on the final beam parameters of externally injected electrons accelerated in a plasma wave are studied using a hybrid model. This model combines a relativistic fluid description of the plasma wave generated by the laser pulse with particle tracking of the accelerated electrons. For cases in which the effects of beam loading and laser depletion can be neglected, the two parts can be separated, allowing a significant reduction in computational power needed compared to particle in cell codes. Two different approaches to externally injecting electrons into plasma waves are studied: In the first case, the electrons are injected behind a laser pulse with a 0 ¼ 0:32. In the second case, electrons are injected in front of the laser pulse in three different laser regimes a 0 ¼ 0:32, a 0 ¼ 0:56, and a 0 ¼ 1:02, ranging from linear to nonlinear. For these four cases, the effects of expected experimental variations in synchronization (AE 500 fs), laser power (AE 10%), and plasma density (AE 30%) are studied. From these simulations, it becomes clear that in some cases, even a small variation in one of these parameters can create a large change in the final energy, energy spread, and trapped charge. For lower laser intensities, the method of injecting behind the laser pulse is the least sensitive to fluctuations while injection in front of the laser pulse becomes less sensitive at higher intensities.

The conditions of the existence and transformations of surface electromagnetic waves (SEWs) on metals (surface plasmons) and dielectrics (phonon-polaritons) are discussed. Interferometric SEW experiments provide the possibility for the direct determination of the real and imaginary parts of the dielectric constants at the frequencies in the tuning range of a free electron laser (FEL) without any preliminary models. The important role of the outstanding facilities of FEL—namely, the broad tuning range, high power, narrow bandwidth of emission, and well-collimated beam—in SEW experiments is outlined. It is demonstrated by the examples of the infrared absorption spectra of a Langmuir-Blodgett film on metal, of a metal oxide film, and of polymeric films of nanometer thicknesses on metals. Free surfaces of single-crystal CaF2 and the thin polymeric film were studied by an interferometric experiment for the optical constant determination. Nonlinear spectroscopic applications of the SEW-FEL technique to studies of a second harmonic generation (SHG)—the frequency dependence of efficiency, the angle dependence of SHG, and the influence of a thin-film deposition on a quartz surface—are described.

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