We report numerical simulations (using the TDA code) and analytic verification of the generation of 64 {Angstrom} high power soft X- rays from an exponential regime single pass seeded FEL. The seed is generated in the FEL using the High Gain Harmonic Generation (HGHG) technique combined with the `Fresh bunch` technique. A seed pulse at 2944 {Angstrom} is generated by conventional laser techniques. The seed pulse produces an intense energy modulation of the rear part of a I GeV, 1245 {Angstrom} electron beam in a `modulator` wiggler. In the `radiator` wiggler, (resonant to 64 {Angstrom}), the energy modulation creates beam density modulation followed by radiation of the 46{sup th} harmonic of the seed. We use a magnetic delay to position the 64 A{Angstrom} radiation at the undisturbed front of the bunch to serve as a seed for a single pass, exponential growth FEL. After a 9 m long exponential section followed by a 7 m long tapered section the radiation power reaches 3.3 GW.

/pdf/high-gain-harmonic-generation-of-soft-x-rays-with-the-fresh-4f6jm08k7s.pdf

High-gain harmonic generation of soft X-rays with the `fresh bunch` technique

Pulsed discharge plasmas are considered to be important candidate sources of extreme ultraviolet (EUV) radiation for application in future lithography tools for the high-volume manufacturing of computer chips. Two specific types of such plasmas have been the subject of research in this work: (1) a hollow-cathode triggered source, developed by Philips EUV in Aachen, Germany; and (2) a laser-ignited discharge in tin vapor, from the Institute of Spectroscopy (ISAN) in Troitsk, Russia. In the evolution of the discharge pulses of these and similar types of EUV sources, generally four different phases can be distinguished. After a device-dependent ignition phase, a strong current starts to flow ("prepinch phase"). A Lorentz force, associated with the electric current, causes a strong radial compression of the plasma ("pinch phase"). After this, in the decay phase, the plasma cools down, expands, and finally dies out. For the further development and optimization of discharge plasma EUV sources, a better understanding of the plasma properties and dynamics is needed; to obtain such understanding, time-resolved measurements of the plasma properties are indispensable. Optical diagnostics are strongly preferred because they provide a lot of information about the plasma while in general they cause little or no disturbance of the plasma itself. In this work, a number of different optical diagnostic techniques have been applied to both types of discharges; their results are summarized below. First of all, time-resolved imaging of the plasma, both in the EUV and in the visible light ranges of the spectrum, serves mainly to obtain basic, qualitative understanding of the evolution of the discharge pulse. The visible light images help to visualize the parts of the plasma that are not hot enough to emit EUV radiation. Plasma imaging has, for instance, helped to identify a preferential direction of expansion of the plasma along the axis of symmetry in both types of discharges, with supersonic velocities of roughly 4×104 m s-1. Also, time and space resolved spectra have been recorded for the EUV wavelength range. These have shown, together with EUV plasma imaging, that high ionization stages already exist in a ring-shaped plasma in the prepinch phase, before the onset of compression. The recorded spectra have further been compared to EUV spectra produced by a computer model. This comparison gives information about which plasma processes play a prominent role in the studied part of the discharge; detailed consideration of, among others, radiative deexcitation and the influence of both Doppler and Stark broadening on the opacity of the radiation, proved to be important to produce good reproductions of the experimentally obtained spectra. Furthermore, the "lagging" of the ionization stage populations compared to the instantaneous electron temperature, had to be taken into account in the form of an effective net ionization rate—hence, the ionization stage population was shown to be essentially non-stationary. Finally, doubly excited states were shown to play a role in determining the shapes of the EUV spectra of both discharges. Best matches between simulated and experimental spectra were obtained with electron temperatures near 25 eV and pinch electron densities of about 1×1025 m-3 and 3×1025 m-3 for the xenon and tin plasmas, respectively. These and other plasma parameters agree fairly well with the results of other diagnostic techniques. The Stark broadening of specific spectral lines of tin ions in the visible wavelength range has been measured as a function of time during the evolution of the tin vapor discharge. A cross-calibration has led to the determination of four new Stark broadening parameters for lines of doubly ionized tin. From the spectral line widths, information about electron densities for various parts of the plasma evolution have been derived. Finally, the Thomson scattering (TS) technique has been applied to the tin vapor discharge to determine space and time resolved electron temperatures and densities simultaneously. First experiments were performed with an existing TS setup. However, to obtain a better ratio between the TS signal and the background radiation, generated by the plasma itself, a new setup for sub-ns Thomson scattering has been designed and built. A laser with a shorter pulse duration, a camera with a shorter gate time, and improved synchronization between the two, together have enabled this signal-to-background ratio to be improved by more than an order of magnitude. This has greatly expanded the applicability of the TS technique to EUV generating discharge plasmas. In application to the tin vapor discharge, electron temperatures and densities of up to about 10 eV and 2×1023 m-3, respectively, have been found for the laser-induced ignition plasma. Once the electron density near the anode reaches a value of around 2×1022 m-3, an electric current can start to flow. In the subsequent high-current phase, both densities and temperatures increase fast, with temperatures reaching to about 30 eV after 100 ns. Especially in the beginning of this phase, the plasma exhibits a hollow radial temperature profile. Compression leads to the pinch phase, in which electron densities on the order of 3×1025 m-3 are achieved. After the end of the pinch phase, additional evaporation of tin from the cathode results in a cooler plasma with relatively slowly decreasing electron densities, from initially above 1024 m-3 to around 1021 m-3 after 4 µs. Nearly all densities, except for those in the pinch phase, have been confirmed independently by the Stark broadening results. The experiments described above have led, first of all, to a more complete understanding of the plasma evolution. Certain phenomena have been confirmed, others have been newly discovered—such as the aforementioned EUV emission from a ring-shaped plasma. Furthermore, extensive information, both temporally and spatially resolved, has been gathered on the electron temperatures and densities in the tin vapor discharge. Such data could serve as a benchmark for future computer simulations of the evolution of discharge plasmas.

https://pure.tue.nl/ws/files/2421546/200512577.pdf

Transient behavior of EUV emitting discharge plasmas : a study by optical methods

We present the calculated prediction and the experimental confirmation that doubly ionized Ag and Sn plasmas can have an index of refraction greater than one for soft x-ray wavelengths. Interferometry experiments conducted using a capillary discharge soft x-ray laser operating at a wavelength of 46.9 nm 26.44 eV confirm that in few times ionized laser-created plasmas of these elements the anomalous dispersion from bound electrons can dominate the free electron contribution, making the index of refraction greater than one. The results confirm that bound electrons can strongly influence the index of refraction of numerous plasmas over a broad range of soft x-ray wavelengths confirming recent observations. The understanding of index of refraction at short wavelengths will become even more essential during the next decade as x-ray free electron lasers will become available to probe a wider variety of plasmas at higher densities and shorter wavelengths.

/pdf/prediction-and-observation-of-tin-and-silver-plasmas-with-18n6p280wj.pdf

Prediction and observation of tin and silver plasmas with index of refraction greater than one in the soft x-ray range.

For many decades optical interferometers have been used to measure the electron density of plasmas. During the last ten years X-ray lasers in the wavelength range 14–47 nm have enabled researchers to use interferometers to probe even higher density plasmas. The data analysis assumes that the index of refraction is due only to the free electrons, which makes the index of refraction less than one and the electron density proportional to the number of fringe shifts. Recent experiments in Al plasmas observed plasmas with an index of refraction greater than one and brought into question the validity of the usual formula for calculating the index of refraction. Recent calculations showed how the anomalous dispersion from the bound electrons can dominate the index of refraction in many types of plasma and make the index greater than one or enhance the index such that one would greatly overestimate the electron density of the plasma using interferometers. In this work we calculate the index of refraction of C, Al, Ti, and Pd plasmas for photon energies from 0 to 100 eV (12.4 nm) using a new average-atom code. The results show large variations from the free electron approximation under many different plasma conditions. We validate the average-atom code against the more detailed OPAL code for carbon and aluminum plasmas. During the next decade X-ray free electron lasers and other sources will be available to probe a wider variety of plasmas at higher densities and shorter wavelengths so understanding the index of refraction in plasmas will be even more essential.

/pdf/impact-of-anomalous-dispersion-on-the-interferometer-xhw2zybaec.pdf

Impact of anomalous dispersion on the interferometer measurements of plasmas

Studies of harmonic generation in free electron lasers

The BESSY soft X-ray FEL is planned as a linac-based single pass FEL multi user facility covering the VUV to soft X-ray spectral range (0.02 keV≤hω≤ 1 keV) [ 1 , 2 ]. A Photoinjector and a superconducting CW linac [ 3 ] will feed three independent SASE-FEL-lines consisting of APPLE II type undulators providing variable polarized radiation [ 2 , 4 ]. The performance of the SASE-FEL can be enhanced using a coherent seed which dominates the spontaneous emission at the beginning of the process. We present simulation studies for three BESSY-SASE-FELs as well as for possible multistage high-gain harmonic generation (HGHG) FELs and discuss their performance in terms of pulse power, pulse duration, pulse shape and signal-to-noise ratio. © 2004 Elsevier B.V. All rights reserved. PACS: 41.60.Cr;07.85.Fv

BESSY soft X-ray FEL

Contrary to storage rings, where the electron bunch properties are damped to equilibrium values, every bunch in a linac driven single pass free electron laser (FEL) will show a characteristic profile from its passage through the injector and the linac. Based on the output of ASTRA and ELEGANT tracking studies, GENESIS calculations have been performed for the BESSY FEL undulators. The effect of timing and charge errors caused by the photo cathode laser, and phase and amplitude errors of the RF fields in the injector and the linac on the FEL radiation were studied. The resulting bunch parameter fluctuations determine the shot-to-shot performance of the FEL. The shot-to-shot parameter variations due to these errors are smaller than or comparable to their variation over the bunch length. The parameter variation along the bunches is imprinted by the typical electron distribution at the end of the gun and the impact of the passed structures. It is independent of the errors. The shot-to-shot variations resulting from the assumed error sources in the FEL output are discussed.

https://epaper.kek.jp/f05/papers/mopp006.pdf

Start-to-end simulations for the bessy low and medium energy fel line including errors

In present FEL designs, the undulators are often optimised for an electron bunch with properties constant along the length of the bunch. The mean energy, emittance and other parameters are assumed not to vary from slice to slice. This paper investigates the impact of more realistic bunch properties, extracted from start-to-end simulations of the BESSY FEL [1]. The energy chirp needed for the bunch compression, and the impact of the passed structure imprint typical parameter profiles on the bunch at the end of the linac. Especially in high gain harmonic generation (HGHG) structures, that use consecutive parts of a long electron bunch, each stage must be adjusted to the expected bunch parameters. Due to this individual adjustment, synchronisation between the bunch and the seeding radiation becomes an issue. Even assuming an on-time seed laser pulse, the changing properties along the bunch in combination with jitter in its arrival time cause varying conditions for the interaction of the electron bunch with the seed laser radiation for every shot. This paper investigates the impact of the expected timing jitter and the realistic bunch profile on the BESSY low energy FEL at = 10nm and outlines counter measures.

/pdf/impact-of-realistic-bunch-profiles-and-timing-jitter-on-the-2iu32bfnnk.pdf

Impact of realistic bunch profiles and timing jitter on the output of the bessy low energy fel line

http://inspirehep.net/record/703649/files/C0508213.pdf

Prospects of the BESSY High-Energy FEL

The BESSY soft X-ray FEL is planned as a High-Gain Harmonic-Generation (HGHG) FEL facility. The associated tunable seed lasers will cover the spectral range of 230 nm to 460 nm. Cascaded HGHG stages will be used to reduce the seed wavelength to the desired wavelength range of 1.24 nm ≤ λ ≤ 51 nm. Using the harmonic con tent of the high-intensity radiator output the number of necessary HGHG stages can be reduced. Moreover the higher harmonic content of the final amplifier extends the spectral range and thus is of high interest to the user community. In this paper, the higher harmonic content of the final output as well as of the radiator ouputs are investigated. The main parameters such as output power, pulse duration and band-width as well as their suitability for seeding are discussed.

Harmonic Content of the BESSY FEL Radiation

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