Advanced high-brightness beam applications such as inverse-Compton scattering (ICS) depend on achieving of ultrasmall spot sizes in high current beams. Modern injectors and compressors enable the production of high-brightness beams having needed short bunch lengths and small emittances. Along with these beam properties comes the need to produce tighter foci, using stronger, shorter focal length optics. An approach to creating such strong focusing systems using high-field, small-bore permanent-magnet quadrupoles (PMQs) is reported here. A final-focus system employing three PMQs, each composed of 16 neodymium iron boride sectors in a Halbach geometry has been installed in the PLEIADES ICS experiment. The field gradient in these PMQs is 560 T=m, the highest ever reported in a magnetic optics system. As the magnets are of a fixed field strength, the focusing system is tuned by adjusting the position of the three magnets along the beam line axis, in analogy to familiar camera optics. This paper discusses the details of the focusing system, simulation, design, fabrication, and experimental procedure in creating ultrasmall beams at PLEIADES.

No monochromatic (! x =! x < 1%), high peak brightness [ > 10 20 photons=mm 2 mrad 2 s 0:1% bandwidth], tunable light sources currently exist above 100 keV. Important applications that would benefit from such new hard x-ray and-ray sources include the following: nuclear resonance fluorescence spectroscopy and isotopic imaging, time-resolved positron annihilation spectroscopy, and MeV flash radiography. In this paper, the peak brightness of Compton scattering light sources is derived for head-on collisions and found to scale quadratically with the normalized energy, ; inversely with the electron beam duration, , and the square of its normalized emittance, "; and linearly with the bunch charge, eN e , and the number of photons in the laser pulse, N :B x / 2 N e N =" 2. This 2 scaling shows that for low normalized emittance electron beams (1 nC, 1 mm mrad, <1 ps, >100 MeV), and tabletop laser systems (1-10 J, 5 ps) the x-ray peak brightness can exceed 10 23 photons=mm 2 mrad 2 s 0:1% bandwidth near "! x 1 MeV; this is confirmed by threedimensional codes that have been benchmarked against Compton scattering experiments performed at Lawrence Livermore National Laboratory. The interaction geometry under consideration is head-on collisions, where the x-ray flash duration is shown to be equal to that of the electron bunch, and which produce the highest peak brightness for compressed electron beams. Important nonlinear effects, including spectral broadening, are also taken into account in our analysis; they show that there is an optimum laser pulse duration in this geometry, of the order of a few picoseconds, in sharp contrast with the initial approach to laser-driven Compton scattering sources where femtosecond laser systems were thought to be mandatory. The analytical expression for the peak on-axis brightness derived here is a powerful tool to efficiently explore the 12-dimensional parameter space corresponding to the phase spaces of both the electron and incident laser beams and to determine optimum conditions for producing highbrightness x rays.

A high peak brilliance, laser-based Compton-scattering-ray source, capable of producing quasimonoenergetic photons with energies ranging from 0.1 to 0.9 MeV has been recently developed and used to perform nuclear resonance fluorescence (NRF) experiments. Techniques for characterization of-ray beam parameters are presented. The key source parameters are the size (0:01 mm 2), horizontal and vertical divergence (6 Â 10 mrad 2), duration (16 ps), and spectrum and intensity (10 5 photons=shot). These parameters are summarized by the peak brilliance, 1:5 Â 10 15 photons=mm 2 =mrad 2 =s=0:1% bandwidth, measured at 478 keV. Additional measurements of the flux as a function of the timing difference between the drive laser pulse and the relativistic photoelectron bunch,-ray beam profile, and background evaluations are presented. These results are systematically compared to theoretical models and computer simulations. NRF measurements performed on 7 Li in LiH demonstrate the potential of Compton-scattering photon sources to accurately detect isotopes in situ.

The measurement of emittance in space-charge dominated, high brightness beam systems is investigated from conceptual, computational, and experimental viewpoints. As the self-field-induced collective motion in the low energy, high brightness beams emitted from photoinjector rf guns are more important in determining the macroscopic beam evolution than thermal spreads in transverse velocity; traditional methods for phase space diagnosis fail in these systems. We discuss the role of space charge forces in a traditional measurement of transverse emittance, the quadrupole scan. The mitigation of these effects by use of multislit-or pepper-pot-based techniques is explained. The results of a direct experimental comparison between quadrupole scanning and slit-based determination of the emittance of a 5 MeV high brightness electron beam are presented. These data are interpreted with the aid of both envelope and multiparticle simulation codes. It is shown that the ratio of the beam's b function to its transverse plasma wavelength plays a central role in the quadrupole scan results. Methods of determining the presence of systematic errors in quadrupole scan data are discussed.

We report the measurement of electron-beam microbunching at the exit of a self-amplified spontaneous-emission free-electron laser (SASE FEL), by observation of coherent transition radiation (CTR). The CTR was found to have an angular spectrum much narrower than spontaneous transition radiation and a narrow-band frequency spectrum. The central frequency of the fundamental CTR spectrum is found to differ slightly from that of the SASE, a finding in disagreement with previously invoked CTR theory. The CTR measurement establishes the uniformity of microbunching in the transverse dimension, indicating the SASE FEL operates in a dominant transverse mode.[S0031-9007(98)08027-2]