incoherent broadband and line emission, and monochromators or filters are needed to produce a clean, well-characterized source of radiation. X-ray production from solid targets also produces a considerable amount of material debris that would have to be filtered out. The low duty cycle of these high-peak-power laser sources does not interface well with most x-ray applications. Currently most of the experiments with these sources have been done mainly by those who built the laser.

Soft x-ray lasers pumped with very high intensity lasers have also been demonstrated. The low repetition rate and the fact that the x-ray emission is at fixed wavelengths may prevent these lasers from becoming general-purpose x-ray sources.

Compton Backscattering

Tunable radiation in the x-ray region can also be produced by the interaction of relativistic electrons with intense laser fields via Compton backscattering. In this scheme for producing x-rays, the periodic magnetic field of an FEL is replaced by an electromagnetic wave propagating opposite the motion of the electrons. Like synchrotron radiation, the radiation generated by this technique will have limited coherence. At present, however, this method of generating x-rays is not competitive with synchrotron sources. Additional research must be done before we will know if this idea can be turned into a practical, sufficiently intense source of x-rays.

Similarly, channel radiation (x-ray radiation emitted by relativistic electrons propagating through the interatomic space in a crystal) and plasma-wave undulators (electric fields generated by plasma oscillations replacing a magnetic undulator) for a high-energy electron beam are still in a highly speculative, exploratory stage of development and cannot be counted on as a viable source of x-rays for users.

WHAT IS ENVISIONED

Proposals have been developed at SLAC for x-ray FELs at 40 Å, 4.5 Å and eventually 1.5 Å. The 40-Å FEL is estimated to have 10-GW peak power in 160-fs pulses at 120 Hz. The spectral brightness and transverse coherence are estimated to be several orders of magnitude greater than those of either the ALS or the APS. The construction of a hard x-ray FEL would require a huge step in the development of FELs. For comparison the shortest-wavelength FEL operated to date is at 2400 Å at the Novosibirsk storage ring.

In order to achieve coherent emission in the x-ray range, extremely low emittance, high-energy electron beams have to be developed. The high-quality SLAC linear accelerator beam, developed for the linear collider, could provide the starting point for this project, but the existing SLAC electron beam would have to be upgraded.

A number of other technical challenges have to be solved. These include (1) the development of a reliable, high-intensity electron gun such as a laser-excited



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