The spiking behavior of free-electron lasers, or sideband instability, has been observed for the first time in the time domain using an autocorrelator on the MKIII free-electron laser at Stanford University. The measurement is compared with simulations and qualitative theory.PACS numbers: 42.55.Tb, 42.60.Jf, It is important to understand the spiking behavior or sideband instability in order to maximize the performance of the free-electron laser (FEL). This is especially true for applications that require high power and high optical-beam quality. The theory of the sideband instability has been thoroughly discussed. Both qualitative theory and computer simulations have been presented, 1 " 5 and the effects of tapering 3 ' 5 and energy spread 3 on the sideband instability have been studied. The change in extraction efficiency 1 ' 2 is perhaps the most important aspect of the sideband instability. Other items of importance for FEL operation are transverse-and longitudinal-mode quality, and overall stability of the laser power.In this paper we present the results of autocorrelation measurements done on the MKIII FEL at Stanford University. These measurements show clearly the existence of power spikes within the laser pulses brought about by the sideband instability. Although evidence for the sideband instability has previously been obtained from measurements of the optical power spectrum, 6 ' 7 the spectral data obtained in those measurements were not sufficient to determine the time dependence of the variations in intensity caused by this instability. The autocorrelation measurements reported here provide the first direct experimental evidence for the existence of the intensity spikes predicted by theory.The equations of motion which describe the behavior of the FEL 8 are given bywhere £(/) -(k+ko)z(t) -cot is the relative phase between an electron and the laser field, z(t) is the longitudinal position of the electron averaged over one wiggler oscillation, and derivatives are with respect to normalized time T-ct/L. The normalized magnetic field is K-eB/komc 2 , ^-^2/4(l+K 2 ) is the planar wiggler normalized strength, and !H(t;) -Jo($) "~J\(0 is the planar wiggler interaction strength. Here a^lnN *e3i(£,)LE(t)lylmc 2 is the normalized laser field envelope, where a-\a \e l * and E(t) is the complex electric field envelope. The normalized current density is given by r-4n 2 Ne 2 3{ 2 (4)L 2 po/Y$mc 2 , where po is the average current density. The brackets () Vo mean an average over all electrons within an optical wavelength. FEL sideband theory has been discussed previously at length. 1 ' 2 Equations (la) and (lb) are the familiar self-consistent pendulum equations. Figure 1 shows the graphical representation in the (£,v) phase space. 8 ' 9 Saturation of the laser is achieved when most of the electrons are contained within the separatrix. The electrons close to the centers of the separatrix oscillate around the center with frequency n ™{c/L) \ a | 1/2 . This is the synchrotron frequency which leads to the development of...
It was found in these experiments that wavelengths in the mid-IR could be selected for optimal ablative properties. Using tunable, high-peak-power pulsed lasers, it will be possible to produce well-defined photoablative lesions that conform to small, irregularly shaped neurosurgical targets.
The design and application of an uncoated sapphire plate that acts as both the beam splitter and the output coupler of an interferometric laser resonator are described. Output coupling is provided at one of the surfaces by p-polarized (TM) reflection near the Brewster angle, and axial-mode selection is enforced at the other surface by s-polarized (TE) reflection at the same angle of incidence. The design is discussed in the context of the phase-locked, rf linac free-electron laser, in which the coupling of adjacent optical pulses at the beam splitter induces temporal phase coherence among all the pulses in the output beam; this coherence is manifested in the frequency domain as a reduction in the number of axial modes per rf frequency interval. The Michelson and Fox-Smith resonator designs are compared, and applications to cavity dumping are discussed.
Successful experiments on the isotope-selective infrared multiphoton dissociation (IR MPD) of nitromethane molecules in the region of stretching vibrations of the NO 2 group have been performed for the first time under IR free electron laser (FEL) irradiation in a mixture with the natural content of the 15N isotope of 0.4%. The content of the ISN isotope in the products of NO dissociation varies within 0.1--1.6% as a function of the laser radiation frequency.Key words: multiphoton dissociation, isotopic selectivity, free electron laser.We have previously reported I on the first successful attempt at using IR free electron laser (FEL) radiation for isotope-selective multiphoton dissociation (MPD) of formic acid. Varying the wavelength of FEL radiation in the region of stretching vibrations of C=O, we were able to increase the contents of ~3C and Jso from 1.I to 22% and from 0.2 to 1%, respectively, in the CO formed due to the dissociation. In this work, we performed nitrogen isotope-selective IR MPD of nitromethane molecules by resonance excitation of the stretching vibrations of the NO 2 group. The absorption bands of these vibrations appear in the region of 6.3 ~tm and have high intensities and sufficiently great spectral shifts for nitrogen isotopes 14N and lSN (-25 cm-l). ExperimentalExperiments were carried out on a Marc llI free electron laser (Duke University, N. C., USA). The radiation was varied within 6.0--7.1 ~tm and consisted of macropulses with frequency I0 Hz, energy 4--8 mJ, and duration 1--1.5 ~s. Each rnacropulse was a sequence of micropulses with duration 1--2 ps separated by an interval of 350 ps. The radiation was focused by a BaF 2 lens with a 90-era focus distance to the center of a glass cylindrical cell with diameter 1.2 cm and length 12 em. The cell was directly attached to a quadrupole mass spectrometer for analysis of the dissociation products. The MeNO~ pressure in the cell was varied from 0.25 to 6 Torr in different experiments. The irradiation time was varied from 10 to 30 rnin, and conversion was within 2--6%. The compo-
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