R.L. Swent

Radiation from channeled leptons

Abstract Radiation from planar- and axial-channeled positrons (56 MeV) and electrons (28 and 56 MeV) in Si has been observed. In axial channeling a large, almost featureless, low-energy enhancement is observed, but in planar channeling relatively sharp features occur. For planar-channeled positrons the energies of the spectral peaks (ranging from 35 to 50 keV) agree well with a theory based on transitions between eigenstates in an almost-harmonic potential. A Monte-Carlo simulation of the positron motion coupled with a classical calculation of the emitted radiation gives agreement with the results and also gives the classical equivalent of coherent bremsstrahlung (CB). It further predicts strong coupling between planar oscillation frequencies and CB frequencies with the formation of strong sidebands. For planar-channeled electrons sets of spectral peaks are observed in the range from 30 to 130 keV. An analysis based on a V 0 exp[− k | x |] potential yields energies in close agreement with experiment.

Comparison of channeling radiation from diamonds with and without platelets

Channeling‐radiation spectra produced by planar‐channeled relativistic positrons and electrons in Type‐Ia and Type‐IIa diamonds have been measured. Because of the presence of platelets in the Type‐Ia diamond, some of the spectra measured for this crystal differ markedly from their counterparts for the Type‐IIa diamond. These striking differences illustrate the potential applications of channeling radiation as a diagnostic tool for studies of impurities or defects in crystals.

Electron and positron channeling radiation from type-Ia and type-IIa diamonds☆

Abstract Planar channeling radiation spectra have been obtained from type-IIa and type-Ia diamonds with 54.5 and 30.5 MeV electrons and with 54.4 MeV positrons. Type-IIa diamonds are relatively free of impurities while Type-Ia diamonds contain “platelet defects” [N atoms bonded in the (100) plane]. For platelet free (type-IIa) diamond, theoretical calculations are found to fit experimental results quite well for electron channeling radiation. However, significant discrepancies (~ 5%) exist between the theoretical predictions and the positron data. Measurements on a type-Ia diamond show large differences in the planar channeling radiation spectra which are anisotropic and which depend upon whether electrons or positrons are used as a probe.

Observation of radiation from Δn=3 transitions for planar-channeled electrons

Abstract Planar-channeled electrons make spontaneous electric-dipole transitions between states of opposite parity. Measured values for the relative intensities of radiation from Δn = 3 and Δn = 1 transitions are shown to be in reasonable agreement with calculated values for 54 MeV electrons channeled along {110} planes in silicon.

The effects of temperature and crystal defects on electron channeling radiation

Abstract Channeling radiation from relativistic electrons has been studied by several groups in the past few years, and the basic mechanism is understood. There are, however, a number of areas remaining to be explored. Bound-state population lengths, which are determined by non-radiative transitions, are much longer than coherence lengths, resulting in radiation intensities appreciably greater than initially expected. Experimental results have been obtained which demonstrate this effect. Also, the temperature dependence of channeling radiation from Si has been measured recently. In addition, experiments have been performed on Si and diamond crystals with defects, to determine the effect of crystalline imperfections upon channeling radiation.

Status of the SCA-FEL☆

Abstract The SCA-FEL can now provide FEL beams to users in a wavelength range extending from longer than 3.5 to shorter than 0.5 μm. Harmonic generation can extend the short-wavelength range. As an example of the operational characteristics, during a 3 week run in May–June 1989, the SCA-FEL provide 18 days of FEL beam at 16–18 hours/day. The FEL operated at 3.5 μm, and in a 20% band centered at 1.54 μm. 30 W of optical power was extracted in 3 ms macropulses with a 0.08% line width and a 3 ps micropulse.

Positron and electron channeling radiation from germanium

Abstract Channeling-radiation spectra have been obtained from a germanium crystal with ≈54.3 MeV positrons and electrons and with 16.9 MeV electrons. Theoretical calculations were performed and are found to agree reasonably well with the experimental results. When the emission spectra for (110) planar channeling in diamond, silicon, and germanium are compared, the corresponding lines for both positrons and electrons are found to be lowest in energy for silicon.

Electron and positron planar channeling radiation from diamond

Abstract Channeling-radiation spectra have been obtained from a diamond with ≈ 54.5 MeV electrons and positrons. Theoretical calculations are found to fit the experimental results well for electrons, but significant discrepancies exist between theoretical predictions and the positron data.

The Physics experiment for a laser driven electron accelerator

Abstract 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 300xa0keV over a 2.5xa0mm distance. Ultimately, acceleration gradients of 1xa0GeVxa0m−1 should be possible.

FEL center user diagnostics and control

Abstract In the past year, the Stanford Picosecond FEL Center has produced more than two thousand hours of beam time dedicated to user experiments. To assure reliable beam delivery and to maximize productivity of our users we have developed a sophisticated system of diagnostics and control. An integrated display is now available in all experimental areas which provides continuously updated measurements of beam spectrum, micropulse duration, power, position, and pointing — all of which may be saved to document beam conditions during an experiment. The beam is actively wavelength and amplitude stabilized to better than 0.01% and 2%, respectively. Direct wavelength control is available to users in every experimental area, allowing changes of wavelength as large as a few percent. Larger wavelength shifts, and adjustments in macropulse or micropulse width or timing, are readily available with operator assistance.