Free-electron Sources Research Articles

High harmonic generation as a seed source for free electron lasers

The design of a high harmonic generation (HHG) source for seeding the proposed New Light Source free electron laser (FEL) is presented. The seed source is designed to cover the energy range from 50 to 100 eV with a peak power of 400 kW and a 20 fs FWHM pulse length and is tuneable to provide continuous coverage over the full 50–100 eV range. The seed pulses are produced by HHG in rare gases in the loose-focusing regime, driven by a high-power kHz laser system. Continuous tunability is achieved by varying the wavelength of the drive laser over ±25 nm from the 800 nm centre wavelength. Future prospects for increasing the HHG yield and short wavelength cut-off by using multi-wavelength generation schemes and longer drive wavelengths are considered, as is the potential for FEL seeding at higher repetition rates.

Auger Electron Angular Distribution of Double Core-Hole States in the Molecular Reference Frame

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×10(11) photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.

Double Core-Hole Production inN2: Beating the Auger Clock

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.

Colossal Magnetoresistive Manganite Based Fast Bolometric X-ray Sensors for Total Energy Measurements of Free Electron Lasers

Bolometric detectors based on epitaxial thin films of rare earth perovskite manganites have been proposed as total energy monitors for X-ray pulses at the Linac Coherent Light Source free electron laser. We demonstrate such a detector scheme based on epitaxial thin films of the perovskite manganese oxide material Nd{sub 0.67}Sr{sub x0.33}MnO{sub 3}, grown by pulsed laser deposition on buffered silicon substrates. The substrate and sensor materials are chosen to meet the conflicting requirements of radiation hardness, sensitivity, speed and linearity over a dynamic range of three orders of magnitude. The key challenge in the material development is the integration of the sensor material with Si. Si is required to withstand the free electron laser pulse impact and to achieve a readout speed three orders of magnitude faster than conventional cryoradiometers for compatibility with the Linac Coherent Light Source pulse rate. We discuss sensor material development and the photoresponse of prototype devices. This Linac Coherent Light Source total energy monitor represents the first practical application of manganite materials as bolometric sensors.

Sufficient close-to-necessary conditions for the blowup of solutions to a strongly nonlinear generalized Boussinesq equation

An initial boundary value problem for the generalized Boussinesq equation with allowance for linear dissipation and free electron sources is considered. The strong generalized time-local solvability of the problem is proved. Sufficient conditions are obtained for the blowup of the solution and for time-global solvability. Two-sided estimates of the blowup time are derived.

Diagnostics of the Start-Up Process of an Arc Hollow Cathode

Hall-effect thrusters scaled to power levels below 300 W are of great interest due to their compact size but still require further system optimization. A major component of these thrusters is the free electron source. The majority of the current systems utilize heated element hollow cathodes, but in the event of heater failure, the overall system becomes inoperable. For this reason, a simplistic alternate system such as an arc hollow cathode has been examined. The drawback of utilizing the arc hollow cathode is the reduction in the operational lifetime, especially when the cathode experiences multiple start-up cycles. In order to remedy this, we have developed a soft start-up and continuous operation power supply system. Utilizing this system, we were able to minimize the start-up process from the lifetime influences and examine other factors.

Identification of free radicals induced by UV irradiation in collagen water solutions

Electron paramagnetic resonance (EPR) method has shown that hydrogen atoms and acetic acid free radicals appear in surrounding acetic acid–water solution of collagen under ultraviolet (UV) irradiation. These free radicals interact with the collagen molecule; consequently, seven superfine components of EPR spectrum with the split of a H = 11.3 G and g-factor 2.001 appear. It is assumed that this spectrum is related to the free radical occurred on the proline residue in collagen molecule. In order to discover OH hydroxyl radicals even in minor concentration, spin trap 5.5-dimethyl-1-pyrroline N-oxide (DMPO) has been applied. During the irradiation of collagen water solution in the presence of spin trap, EPR spectrum of the DMPO/ OH adduct has not been identified, while the above mentioned spectrum has been observed once the hydrogen peroxide H 2O 2 and FeSO 4 were added to the sample. That means that water photolysis does not take place in collagen water-solution due to UV irradiation. It was suggested that occurrence of hydrogen radical is connected with the electron transmission to the hydrogen ion. The possible source of free electrons can be aromatic residues, photo ionization of which takes place in collagen molecule due to UV irradiation.

Development of CMR Manganite Sensors for Energy Measurement of the Linear Coherent Light Source Free Electron Laser Xray Pulses

ABSTRACTWe are developing Colossal Magnetoresistive (CMR) manganite thin film bolometric sensors to be employed as total energy detectors for beam diagnostics of the Linear Coherent Light Source (LCLS) Free Electron Laser (FEL) (at the Stanford Linear Accelerator). LCLS is an ultra bright, ultra short coherent x-ray source whose peak brightness will exceed that of third generation x-ray sources by about ten orders of magnitude and average brightness by three orders of magnitudes. It is expected to produce 1012 x-rays per 200 fs pulse with a repeat frequency of 120 Hz through self-amplified stimulated emission. In characterizing the beam, it will be necessary to measure the total energy of the FEL pulse. The Advanced Detector Group at Lawrence Livermore Laboratory has developed a scheme for FEL total energy measurements based on bolometric detection and are collaborating with Towson University to implement such a detector using CMR manganite thin films. Here we discuss the basic scheme, results of simulations of the thermal response and the materials development efforts towards fabricating the thin film detectors.

The influence of meteoroid rotation on the diffraction characteristics of underdense radio-meteors

We discuss the problem of meteor plasma formation and consequently of radio-wave scattering from an ionized meteor trails produced by rotating meteoroids. The problem is solved under the following approximations: the meteoroid is represented by a cube, its rotation axis is perpendicular to the velocity vector, meteoroid deceleration can be neglected, the only acting ablation mechanism is the evaporation, the deionization effects are negligibly small. In this case the ambipolar diffusion approximation applies, and, supposing that the source of free electrons is known, the distribution of free electrons inside a trail, Ne(r,t), considered as a function of position and time, can be computed. It is shown that the electron distribution along a trail axis oscillates, which is not the case with respect to the radial distribution. The function Ne(r,t) found is then used for the solution of the problem of the scattering of radio-waves on the underdense-type trails produced by rotating particles. The values for amplitude-time as well as phase-time characteristics computed under the above assumptions differ only very slightly from those computed when meteoroid rotation is not taken into consideration.

Effects of photoassisted deposition and/or photoassisted annealing on characteristics of ZnS:Mn thin-film electroluminescent devices

Some essential photoinduced effects on the characteristics of ZnS:Mn electron-beam evaporated films and thin-film electroluminescent (TFEL) devices based on them have been revealed when Hg-lamp irradiation and quartz halogen lamp illumination are used during deposition and postannealing, respectively. The photoassisted deposition with the irradiation of the ZnS:Mn flux or a substrate results in some improvement of the crystal structure of the films, an enhancement of the intensity of photoluminescence and electroluminescence (EL), an increase of the threshold voltage of EL and the luminous efficiency, a lesser change and the quicker stabilization of EL characteristics during short-term forced aging. The use of the photoassisted rapid annealing instead of a usual long-term annealing in a vacuum furnace causes some enlargement of the crystallites and the improvement of performance of the TFEL devices. Both photoassisted processes used lead to a significant reduction of the impurity photocurrent in the photodepolarization spectra. The above results indicate that the following photoinduced changes take place in the ZnS:Mn film: a more densely packed lattice, an increase of the energy depth of the free-electron source, a decrease of the number of pores and defects, particularly, the defects responsible for the radiationless processes and the instability of the EL characteristics in the course of the device operation. Possible mechanisms of the photoinduced effects are discussed.

Multiple ionization of atom clusters by intense soft X-rays from a free-electron laser.

Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.

The influence of erbium on the physical properties of GaN crystals grown from N solution in Ga at high nitrogen pressure

Oxygen contamination is the main source of free electrons in bulk GaN grown at high-pressure. We used erbium as a promising getter/compensation center to reduce the electron content in GaN. Both optical and electrical measurements indicate the reduction of electron concentration from the initial 6 × 1019cm−3 down to 1 × 1019cm−3. A photoluminescence study proves that erbium is incorporated into the GaN host lattice, and emits light at ∼ 1.5μm.

Si in GaN — On the Nature of the Background Donor

AbstractA characterization of the Si impurity in GaN is performed by Raman spectroscopy. Applying hydrostatic pressure up to 25 GPa we study the behavior of the LO phonon‐plasmon mode in a series of high mobility Si doped GaN films. In contrast to earlier results on unintentionally doped bulk GaN crystals no freeze out of the free carriers could be observed in Si doped samples. We find that Si is a shallow hydrogenic donor throughout the pressure range studied. This result positively excludes Si incorporation as a dominant source of free electrons in previously studied bulk GaN samples.

Coherent Radiation Reaction in Free-Electron Sources.

Spontaneous coherent radiation is an important process in existing and proposed experiments. Energy conservation in theoretical simulation of these experiments requires that the corresponding radiation reaction be taken into account. Existing conventional proposals for radiation reaction forces are valid when electrons emit independently, ignoring collective aspects of the radiation. In this paper we present radiation reaction prescriptions in which coherent effects are taken into account.

Recombination and Ionization in a Molecular-Ion-Dominated Helium Afterglow

An intense source of free electrons has been observed to strongly influence the time history of the electron density in a high-pressure (> 15-Torr) helium afterglow. The electronic recombination coefficient $\ensuremath{\alpha}$ of molecular helium ions was measured by utilizing this source and was found to be over five times larger than the effective recombination coefficient obtained by equating the net loss rate of free electrons to the recombination loss rate in the usual manner. If $\ensuremath{\alpha}$ is assumed to be of the form $\ensuremath{\alpha}={\ensuremath{\alpha}}_{0}+{k}_{p}p$, then ${\ensuremath{\alpha}}_{0}=1.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ and ${k}_{p}=4.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{Torr}}^{\ensuremath{-}1}$ over the pressure range from 15 to 56 Torr. A model is presented which indicates that ionizing metastable-metastable collisions are responsible for the source of free electrons.

Effects of nonequilibrium chemistry on electrical properties of solid propellant rocket exhaust plumes

Application of basic chemical kinetic and gasdynamic considerations to the problem of calculating electrical properties of turbulent afterburning exhaust plumes of solid propellant rocket motors is discussed. The sources of free electrons are described, and the importance of incorporating chemical kinetic (rather than thermochemical equilibrium) data is illustrated by reference to several sample calculations. It is shown that for systems in which potassium is the dominant impurity, above-equilibrium electron concentrations are formed, principally viaK+C1→K++C1- followed by C1-+H→HC1+e- The free radical concentrations are produced in above-equilibrium concentrations from the H2-CO-Cl-O2 afterburning reactions. Exhaust plume properties are shown to be sensitive to values selected for rate coefficients of several reactions, and the need for more accurate determinations of these rate coefficients is thus demonstrated. The accuracy of the calculations is further limited by uncertainties in the gasdynamics of the turbulent mixing process. Within the limits of error imposed by these uncertainties, however, the model described does account adequately for experimental observations.

Electronic Recombination Coefficient of Molecular Helium Ions

The effect of metastable-metastable ionizing collisions has been neglected in previous measurements of the electronic recombination coefficient of molecular helium ions. In this study, a source of free electrons was taken into account and the recombination coefficient was observed to be much larger than previously reported. Over the pressure range from 15 to 50 Torr, we have measured this coefficient to range from (1.8 to 4.3) \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ ${\mathrm{cm}}^{3}$ ${\mathrm{sec}}^{\ensuremath{-}1}$.

Display devices based on low-voltage excitation of ZnO in a gas plasma

Using a gas plasma as a source of free electrons, a brightness of 60 fL is obtained from an extended layer of ZnO phosphor with about 5 V applied to it. The phosphor efficiency, >10 lm/W, shows little change during 5000 h of operation. For display purposes devices have been constructed incorporating multiple phosphor-coated anodes whose potentials are controlled by low-voltage semiconductor circuits.

Dielectric Breakdown and Recovery of X-Cut Quartz under Shock-Wave Compression

While a shock wave is traversing a disk of X-cut quartz, a piezoelectric current flows in an external circuit connected across the faces of the disk. In this paper measurements of this current are used to study dielectric breakdown and subsequent recovery which occurs in quartz. Quartz specimen disks were impacted at various stress levels in such a way as to produce shock waves that propagated along the X axis either in the direction of or opposite to that of the pressure-induced polarization. In the latter case, short-circuit current measurements show that breakdown occurs at a threshold stress greater than 10 and less than 13 kbar. Since the impact experiment produced one-dimensional electrical and mechanical conditions in the specimen disk, it was possible to formulate a mathematical model that permitted solutions for internal electrical fields and resistivity in terms of the measured current. Computations with this model show that the field in the stressed portion of the disk at breakdown is about 7.0×105 V/cm, which is an order of magnitude lower than the value observed at atmospheric pressure. Computations with the model also show that recovery from breakdown to essentially infinite values of resistivity occurs during the transit time of the shock wave when the field in the stressed region of the disk is ``quenched'' to a value of about 1.9×105 V/cm. This critical field appears to be the same for all shock stress levels investigated from 13 to 35 kbar. The dependence of the initiation of breakdown on the direction of wave propagation relative to the polarization direction indicates that the shock-wave front furnishes a source of free electrons.

Analysis of radio signal interference effects due to ionized layer around a re-entry vehicle

The theoretical section of this paper discusses the electromagnetic properties of the ionized shock layer about a missile re-entering the atmosphere; a treatment is then given of the problem of predicting the transmission, through the shock layer, of electromagnetic waves originating on the vehicle. The major portion of this section is given to outlining the important quantitative aspects for calculating the extent of the attenuation problem. Theoretical curves are given which allow rapid prediction of the attenuation effects due to air ionization once temperature and density are specified in the shock layer. If other sources of free electrons are significant, e.g. ablation products, separate determination of the density distribution must be made. Specific results are given on certain Polaris configurations and trajectories as computed using this method; these predictions are based upon two specific transmitter frequencies. Although the method utilizes the Lorentz theory and the plane wave-plane homogeneous sheet assumption is made, the resulting estimates are shown to agree satisfactorily with signal strength records of actual re-entry tests. A quantitative discussion is given concerning the approximations inherent with the use of the Lorentz theory of conductivity in view of “strong fields” near a transmitter and the velocity dependence of collision cross-sections. A portion is given to possible solutions of the transmission problem suggested by the parameters available in various ranges of temperature and density; particular attention is given to transmitter frequency and position of the antenna. A method is demonstrated for extrapolating the results of a minimum of predictions for a given combination of radiator and re-entry body to include predictions for a wide range of altitudes and velocities.