Short Pulse Electron Beam Research Articles

Trigger-timing signal distribution system for the KEK electron/positron injector linac

The KEK electron/positron injector linac provides a short-pulse electron beam with an energy of 2.5 GeV for the Photon Factory (PF) storage ring, that with an energy of 3 GeV for the Advanced Ring (PF–AR) for pulse X-rays, and single-bunch electron and positron beams with energies of 8 and 3.5 GeV respectively for the KEK B-Factory (KEKB) storage rings. A trigger-timing signal distribution system of the injector linac was newly developed and extended for stable injection to the KEKB rings with the required injection-timing accuracy highly stabilized to less than 30 ps in root mean square because the old trigger system distributed the trigger-timing signals for the injection to the PF ring and the PF–AR with an injection-timing accuracy of 300 ps . In this report the design and its performance of the new trigger-timing signal distribution system are described in detail.

High energy laser wakefield acceleration

Laser wakefield acceleration (LWFA) experiment using 100TW laser is planned at JAERI-Kansai. High quality short pulse electron beams and the precise electron-laser synchronization are necessary to accelerate the electron beam by the laser. A microtron with a photocathode rf-gun was developed as a high quality electron injector. The quantum efficiency(QE) of the photocathode was measured to be 2 × 10 −5 . Emittance and a pulse width of the electron beams were 6π mm-mrad and 10ps, respectively. In order to produce a short pulse electron beam, and to synchronize between the electron beam and the laser pulse, the inverse free electron laser (IFEL) is planed. A gas jet was set up and a density of the gas was measured by Mach-Zehnder interferometer. In order to overcome a short acceleration length of the LWFA, the Z-pinch plasma waveguide has been developed for 1 GeV laser wakefield acceleration.

Pulsed photoelectric field emission from needle cathodes

Experiments have been carried out to measure the current emitted by tungsten needles with 1-μm tip radius operated up to 50 kV. This corresponds to electric fields in the order of 10 9 to 10 10 V/m. The needles were illuminated with 10-ns laser pulses at 532, 355 and 266 nm. The laser intensity was varied from 10 10 to 10 12 W/m 2, limited by damage to the needle tip. The observed quantum efficiency depends on the wavelength and the electric field, approaching unity at the highest electric fields when illuminated at 266 nm. Peak currents up to 100 mA were observed in nanosecond pulses, corresponding to an estimated brightness of 10 16 A/m 2 sr. Since the current is controlled by the laser intensity, with only a weak voltage dependence, these cathodes can be used for infrared and ultraviolet tabletop free-electron lasers and other applications that demand short electron-beam pulses with high brightness.

Exponential gain and saturation of a self-amplified spontaneous emission free-electron laser.

Self-amplified spontaneous emission in a free-electron laser has been proposed for the generation of very high brightness coherent x-rays. This process involves passing a high-energy, high-charge, short-pulse, low-energy-spread, and low-emittance electron beam through the periodic magnetic field of a long series of high-quality undulator magnets. The radiation produced grows exponentially in intensity until it reaches a saturation point. We report on the demonstration of self-amplified spontaneous emission gain, exponential growth, and saturation at visible (530 nanometers) and ultraviolet (385 nanometers) wavelengths. Good agreement between theory and simulation indicates that scaling to much shorter wavelengths may be possible. These results confirm the physics behind the self-amplified spontaneous emission process and forward the development of an operational x-ray free-electron laser.

Production of focused electron beams in a plasma implantation system

Rapid heating/quenching using plasma extracted electrons is a unique and very cost effective method to with which we can create new non-equilibrium, nanocrystaline, and amorphous materials. We recently demonstrated that short pulse (<2 μs) electron beam irradiation is a viable and inexpensive means of providing rapid heat pulses for surface material processing. North Star Research laboratory experiments using our plasma ion implantation equipment have indicated a very efficient delivery of an energy flux to a target surface (up to 70% using a plasma electron beam). This technique offers extremely low cost per processing watt when compared to other heat pulse generation techniques such as lasers or ion beams. Preliminary experiments conducted with aluminum and plastic substrates proved that our new method produced exposures of 1–2 J/cm 2.

Absolute Beam-Charge Measurement for Single-Bunch Electron Beams

The absolute beam charge of a single-bunch electron beam with a pulse width of 10 ps and that of a short-pulsed electron beam with a pulse width of 1 ns were measured with a Faraday cup in a beam test for the KEK B-Factory (KEKB) injector linac. It is strongly desired to obtain a precise beam-injection rate to the KEKB rings, and to estimate the amount of beam loss. A wall-current monitor was also recalibrated within an error of ±2%. This report describes the new results for an absolute beam-charge measurement for single-bunch and short-pulsed electron beams, and recalibration of the wall-current monitors in detail.

Theory of the undulator superradiance of an electron beam pulse in the group synchronism regime

The properties of the superradiance of a short electron beam pulse moving in an undulator field and exciting a waveguide mode field under group synchronism conditions are analyzed. This regime is shown to be characterized by the maximum growth rate of superradiance instability. The development of such instability results in bunching of the particles and the coherent emission of a short electromagnetic pulse by the beam pulse.

The Neptune photoinjector

The RF photoinjector in the Neptune advanced accelerator laboratory, along with associated beam diagnostics, transport and phase-space manipulation techniques are described. This versatile injector has been designed to produce short-pulse electron beams for a variety of uses: ultra-short bunches for injection into a next-generation plasma beatwave acceleration experiment, 2 C. Clayton et al., these proceedings. 2 space-charge dominated beam physics studies, plasma wake-field acceleration driver, plasma lensing, and free-electron laser microbunching techniques. The component parts of the photoinjector, the RF gun, photocathode drive laser systems, booster linac, RF system, chicane compressor, beam diagnostic systems, and control system, are discussed. The present status of photoinjector commissioning at Neptune is reviewed, and proposed experiments are detailed.

Bias voltage dependence of the signal polarity of superconducting tunnel junction detectors

Abstract Using Low Temperature Scanning Electron Microscopy (LTSEM) we have measured the bias voltage dependence of the charge output of superconducting tunnel junction detectors, which have different energy gaps Δb and Δc for the base and the counter electrode, respectively. The time dependence of the detector response was measured for various bias voltages Vb after perturbing the counter electrode with short electron beam pulses of 10 ns duration and about 50 keV total energy. For Nb/Al/AlOx/Nb tunnel junctions with Δc − Δb ≈ 0.2 meV we observed a negative and positive quasiparticle tunneling current for Vb 0.2 mV, respectively. The observed change of the signal polarity can be explained using standard tunnelling theory taking into account the different bias voltage dependence of electron and hole like quasiparticle current.

Generation of near-infrared light pulses from ZnS:Cr under laser-enhanced cathode-beam excitation

The method of additional heating of a tungsten hair-pin cathode by laser pulses for generating strong and short electron-beam pulses is applied to cathodoluminescence measurements with the aim of examining effects of high excitation densities. A change in the risetime of the Cr2+(d4) luminescence at different current densities is observed in ZnS. The mechanism of this effect is discussed.

Spectroscopic Measurements of Intense Relativistic Electron Beam Driven Turbulence

High frequency strong electric fields originating from interaction of an intense short-pulse relativistic electron beam with an unmagnetized plasma were measured using two optical diagnostic techniques; the Stark shift measurement and the plasma satellite method. Strong electric fields with Gaussian distribution existed in the plasma. The dimensionless electrostatic energy density W ∼1.1. So the plasma was in a strong Langmuir turbulent state. The strong field regions were found to occupy a few percent of the beam volume. The final scale of caviton was determined to be about 22 λ D . The turbulent state lasted about thirty times as long as the IREB duration. The photon intensity decreased exponentially, while the distribution of electric fields remained the same. These aspects are remain to be explored further.

Application of low temperature scanning electron microscopy for the investigation of single-electron tunneling circuits

We present the first experimental results on a spatially-resolved investigation of a SET (single-electron tunneling) circuit using the method of low temperature scanning electron microscopy. The new technique can be operated with short electron beam pulses down to 1 μs and small beam current below 1 pA, which gives the value of the charge per pulse as low as one elementary charge. By recording the circuit voltage response to the modulated electron beam irradiation as a function of the beam coordinates on the circuit, we were able to image the potentials of different parts of the circuit. For a SET-transistor which displayed the Coulomb blockade we found evidence of memory-effects due to charge trapping in the vicinity of one of the junctions. Further possible applications of our method for the spatially resolved study of single-electron circuits are suggested.

Single bunch injection of the storage ring NIJI-IV for FEL

A single bunch injection system for the storage ring NIJI-IV was designed and tested on a test stand. The system generates short pulse electron beams of 2 ns duration. These pulsed beams are synchronized with the revolution frequency (10.1 MHz) of the storage ring. The number of pulses in one beam train is 20. The repetition rate of the beam train is 2 pps. The temporal structures of the pulse beams were measured by an amorphous core monitor. The measured temporal structure was shorter than the longitudinal space of the rf-bucket. So it was confirmed that the system can inject electron beams into only one of the 16 rf-buckets in the ring.

Spectroscopy of short electron beam pulses by superconducting Nb/AlOx/Nb tunnel junctions

We have used superconducting Nb/AlOx/Nb tunnel junctions (STJs) for the spectroscopy of short electron beam pulses, which contain only a few electrons with energy ranging between 6 and 35 keV. The spectroscopy was performed by Low Temperature Scanning Electron Microscopy (LTSEM) at T=2.6 K. The spectra show that the number of beam electrons within each single pulse follows the Poisson statistics and agree well with the theoretically expected spectra. Increasing the average number of beam electrons per pulse shows that the STJ-detector response is linear up to a deposited energy of more than 500 keV. For a large average number of beam electrons the achievable energy resolution of the detector is limited by the statistics of the beam electrons and the influence of the backscattered electrons.

Fluorescence measurements in a Low-pressure Kr/F2 medium

The time-resolved measurement of the sidelight fluorescence of KrF* formed by a short electron-beam pumping pulse (20 ns FWHM, 19 kA, 860 kV) was performed as a function of both the total pressure ranging from 100 Torr to 400 Torr and the partial F2 concentration (0.1–1.0%). We have estimated the fluorescence yields (FY) for various laser-gas conditions experimentally and the results were compared with numerical predictions. This study indicates a superior amplification performance for a single ultrashort pulse in a low-pressure medium because of the higher FY for KrF*.

Subpicosecond pulse amplification in low-pressure KrF laser medium

An experimental investigation was made of subpicosecond KrF pulse (400 fs) amplification in a low-pressure (<400 Torr) Kr/F 2 laser medium pumped by a short-pulse electron beam (20 ns fwhm, 860 keV, 19 kA) in a longitudinal lambda-geometry configuration with a pulsed axial magnetic field of 2.7 kG. The net small-signal gain was observed to be 1%/cm at a total pressure of 200 Torr (Kr/F 2=99.5/0.5 (%)) and an electron current density of 39 A/cm 2. Because of the low small-signal gain length product the amplified spontaneous emission (ASE) fluence in this mode was confirmed to be extremely small (less than μJ/cm 2) compared to the normal pressure mode (> 1 atm) operation.

Melting and conduction during short electron beam pulses

The experimental results of surface melting of metallic specimen, subjected to short and intense electron beam pulses (energy densities are in the range of 5 × 10 6 to 5 × 10 7 J/m 2 and pulse time vary from 5 to 10 ms) can be described by a simplified energy balance for the melting process; melting and heat conduction are the main physical processes involved in the problem. These and other empirical observations (proof of convective motion in the liquid pool) allowed the derivation of a semi-empirical treatment for a concentrated energy pulse and the development of an analytical model in the case of a one-dimensional geometry. This model allows a calculation of the melt penetration and shows a good agreement with experimental results obtained for stainless steel and aluminium.

Kinetic study of a short-pulse electron-beam pumped Kr/F2 laser amplifier medium at very low pressure operation

A numerical investigation has been performed for very low pressure (≤200 Torr) buffer-free KrF laser-amplifier medium pumped by a short pulse (10 ns FWHM) electron beam with low excitation rate operation (200 kW/cm3). The small-signal-gain coefficient (g0) and absorption coefficient (α) have been estimated for this new operational mode. The formation and quenching processes are also discussed kinetically.

Consequences of short electron-beam pulses in the FELIX project

We discuss the consequences of short micropulses on the output of infrared and far-infrared free electron lasers with special reference to the FELIX project which operates with 3 ps long electron pulses.

Rapid thermal desorption of di-atomic molecules: A theoretical study

Rapid thermal desorption of a di-atomic molecule following irradiation of the surface by a short laser or electron beam pulse has been studied. The calculations were performed using a stochastic trajectory method in which the adsorbate motion is described by effective equations of motion. The simulations yield information about various distribution functions of the desorbates such as translational and internal energies, as well as angular and residence time. These distribution functions were computed for various adsorption geometries, molecular force constants and surface corrugation.