Short-pulse Beam Research Articles

Redeposition characteristics of focused ion beam milling for nanofabrication

In this article, a new method is presented to measure the focused ion beam induced sputter distribution, obtained with 30keV Ga+ ions. Small holes with diameters ranging from 400to1750nm have been created in a lamella with a thickness of approximately 300nm. Short ion beam pulses sputter small amounts of material from the bottom of the hole that redeposit at the opposite side. Assuming axial spatial symmetry, imaging the hole before and after redeposition yields a quantitative cross section of the sputter distribution. It has been found that at low ion currents (<6pA) and low ion dose (<10+8) the method produces reliable, reproducible results for 30keV Ga+ ions on silicon. The method is compared with SRIM simulations in Si and with previously published results. The simulated and measured results are in good agreement. At the higher dose∕current regime, small Ga droplets are formed, probably as a result of sputtered Ga dopants in Si. As a result of this, the method becomes less accurate and the simulation is no longer valid due to the fact that the substrate milling has to include the many Ga dopants to a certain depth.

Development of a neutron time-of-flight source at the ELBE accelerator

The radiation source ELBE at Forschungszentrum Dresden-Rossendorf (FZD) has a superconducting linear accelerator with electron energies up to 40 MeV as a central instrument, and it will be used to produce intense neutron beams. The neutron radiator consists of a liquid-lead circuit where bremsstrahlung photons generated from the electron beam produce neutrons in ( γ , n ) reactions. Monte Carlo simulations with MCNP4C3/MCNP5 were performed to characterise neutron and photon intensities as well as time and energy distributions, and to optimise the neutron transport. The short beam pulses (5 ps at FWHM) provide the basis for an excellent time resolution for neutron time-of-flight experiments, allowing an energy resolution of about 1% with a flight path of 4 m when using a fast detector stop signal (e.g., 1 ns for 1.5 MeV neutrons). The neutron beam is shaped by a 2.4 m long collimator made from borated polyethylene and lead, reducing the background of scattered neutrons and of photons at the sample position. About 96% of the neutrons at the experiment site retain their correct energy to time-of-flight correlation. The neutron flux at the sample position is calculated to be 10 7 cm - 2 s - 1 . As ELBE will be able to deliver a full intensity beam with a repetition rate of 0.5 MHz, the usable neutron energy range goes from 50 keV up to 10 MeV. In this energy interval, there is a need for neutron cross-section measurements relevant for the transmutation of minor actinides in nuclear waste, as well as for applications to fission and fusion reactors. A BaF 2 scintillation detector array of 42 crystals is being constructed for neutron-capture γ -rays. The setup covers up to 96% of the total solid angle. Simulations of γ -ray cascades from Fe 56 ( n , γ ) Fe 57 reactions have been combined with detector simulations to characterise the response of the detector setup.

A collimated pulsed supersonic metastable helium atomic beam

The supersonic short-pulse helium atomic beam and its metastables were studied. A pulsed high-density helium atomic beam was ejected from a nozzle, collimated by a skimmer and then flowed through a penning-type discharge device. After discharging, the metastable density of the pulsed helium atomic beam was up to 1.24×1011 cm−3. Excellent symmetry of atomic beam profile was obtained. The deviation of angle of the atomic beam with respect to the axis was less than 0.14°. The mean diffusion angle of the pulsed beam was 4°, and a mean velocity of atomic beam was up to 1350 ms−1. Laser induced fluorescence of metastable atoms has been studied.

A photo-neutron source for time-of-flight measurements at the radiation source ELBE

The radiation source ELBE ( Electron Linear accelerator with high Brilliance and low Emittance) at Forschungszentrum Dresden-Rossendorf uses the high brilliance electron beam from a superconducting LINAC to produce various secondary beams. Electron beam intensities of up to I e - = 1 mA at energies up to E e - = 40 MeV can be delivered with a pulse width of less than 10 ps. With these parameters the electron beam allows to generate sub-ns neutron pulses by stopping the electrons in a heavy (high atomic number) radiator and producing neutrons by bremsstrahlung photons through (γ, n)-reactions. In order to enable measurements of energy resolved neutron cross sections like (n, γ), (n, n′γ), (n, p), (n, α), and (n, f) at a time-of-flight arrangement with a short flight path of only a few meters it is necessary to keep the volume of the radiator for neutron production as small as possible to avoid multiple scattering of the emerging neutrons, which would broaden the neutron pulses. It is the primary physics objective of this neutron source to measure neutron cross sections firstly for construction materials of fusion and fission reactors, for which it is important to select materials with low activation cross sections, and secondly for the handling of waste from such reactors, especially in order to find processes which transmute long-lived radioactive nuclides into short-lived and finally stable ones. Furthermore experiments can be performed which address problems of nuclear astrophysics. The power deposition of the electron beam in the small neutron radiator volume of 1 cm 3 reaches up to 25 kW. This is such a high power density that any solid high Z number material would melt. Therefore, the neutron radiator consists of liquid lead circulated by an electromagnetic pump. The heating power introduced by the electrons is removed through the heat exchanger in the liquid lead circuit. Typical flow velocities of the lead are between 1 m/s and 5 m/s in the radiator section. From the thermal and mechanical point of view, molybdenum turned out to be the most suited target wall material in the region where the electrons impinge on the neutron radiator. To reduce the radiation background at the measurement position, the neutrons are decoupled from the radiator at an angle of about 90° with respect to the impinging electrons. Particle transport calculations using the Monte Carlo codes MCNP and FLUKA predict a neutron source strength in the range of 7.9 × 10 12 n / s to 2.7 × 10 13 n / s for electron energies between E e - = 20 and 40 MeV. At the measuring place 3.9 m away from the radiator, a neutron flux of about 1.5 × 10 7 n / ( cm 2 s ) will be obtained. The short beam pulses allow for a neutron energy resolution of better than 1% for neutron energies between E n = 50 keV and 5 MeV. The usable energies range up to about 10 MeV.

Quantum Emission by Intense Laser Field and Its Application to Materials Dynamics

Quantum emission is high-energy and short pulsed beam of electrons, ions and photons (x-rays) generated by interaction between intense femtosecond laser field and matter. This article review recent research activities on generation of quantum emission and its application to dynamics research in material sciences.

Density Measurements of Shock Compressed Matter Using Short Pulse Laser Diagnostics

In this paper, experimental results on X-ray and proton radiography of shock compressed matter are presented. It has been performed at the Rutherford Appleton Laboratory (RAL) using three long pulse beams to generate a shock wave in a multi-layer foil and a short pulse beam to create either an X-ray or protons source for a transverse radiography. Depending on the probe material (aluminium or carbon foam) a Molybdenum Kα source or a proton beam are used. Density data of the shocked aluminium, in the multimagabar regime are presented.

Installation of electric field electron beam blanker in high-resolution transmission electron microscopy

We have newly installed an electric field electron beam blanker in a transmission electron microscopy, which chops an electron beam very quickly without the effect of hysteresis. The electric field, which is generated by the electron beam blanker, deflects the electron beam, and the electron beam is intercepted by an aperture. The response time of the beam blanker is 50μs. Therefore, a very short pulsed electron beam enables a charge-coupled device camera to directly expose an electron beam spot or diffraction pattern. Moreover, we measured the response of a deflector coil, which is usually used as an electron beam blanker, using our electron beam blanker. Our beam blanker will become a key component in a computer-assisted minimal dose system, which enables us to reduce the electron dose of the sample.

Target diagnostics for commissioning the AWE HELEN Laser Facility 100TW chirped pulse amplification beam

The capability of the HELEN laser at the Atomic Weapons Establishment Aldermaston has been enhanced by the addition of a short-pulse laser beam to augment the twin opposing nanosecond time scale beams. The short-pulse beam utilizes the chirped pulse amplification (CPA) technique and is capable of delivering up to 60J on target in a 500fs pulse, around 100TW, at the fundamental laser wavelength of 1.054μm. During the commissioning phase a number of diagnostic systems have been fielded, these include: x-ray pinhole imaging of the laser heated spot, charged particle time of flight, thermoluminescent dosimeter array, calibrated radiochromic film, and CR39 nuclear track detector. These diagnostic systems have been used to verify the performance of the CPA beam to achieve a focused intensity of around 1019Wcm−2 and to underwrite the facility radiological safety system.

Multidimensional short-pulse space-charge-limited flow

The two-dimensional models of the space-charge-limited (SCL) current density at the short pulse limit for which the electron pulse length is comparable or smaller than the electron transit time across the gap (i.e., XCL⩽1) have been developed. In particular, the scaling laws for short-pulse SCL electron emission in a planar diode with a circular emitting strip and a cylindrical diode with a finite length have been obtained and verified with particle-in-cell simulation. It is found that the enhancement (in terms of the long-pulse SCL current density) is proportional to XCL−1 for small XCL for both planar and cylindrical cases. The enhancement of the cylindrical short-pulse SCL current density is also found to be larger for the convergent flow (cathode outside) than divergent flow (cathode inside). Multidimensional effects are important only for small emitting strips with size comparable to the effective penetration distance (into the gap) of the short-pulse electron beam. Smooth transition between the short-pulse regime and the long pulse (steady-state) regime is demonstrated.

A short-pulsed compact supersonic helium beam source for plasma diagnostics

A short-pulsed (less than 1 ms) compact supersonic helium beam source has been developed for plasma diagnostics purposes. The beam has been characterized experimentally and the results compared with calculations from a simple model that takes into account interaction between the supersonic beam and gas in the region between the nozzle and the skimmer. For a 200 μs driving pulse, a beam of density N > 1 × 1018 m−3 on the axis, a width of D ≈ 1 cm at a distance of 30 cm from the nozzle and a velocity v ≈ 1.8 × 103 m s−1 has been achieved.

Secondary electron emission and surface charging evaluation of alumina ceramics and sapphire

The breakdown of alumina rf windows is mostly caused by multipactor, as well as by material defects and contamination. Since multipator induces localized surface heating, leading to surface melting, it is necessary to observe secondary electron emission (SEE) coefficients of alumina ceramics under high temperature conditions. The SEE coefficients of commercial alumina ceramics and sapphire were measured by a scanning electron microscopy (SEM) with a single short-pulsed electron beam (100 pA, 1 ms) at room temperature and at 650degC. Additive materials used for sintering alumina, such as SiO 2 and MgO, were also investigated. Surface charging evaluations have also become important because the accumulated charges are discharged at the threshold Held, resulting in surface discharge. The surface charging evaluations were carried out by multi-pulse measurements with the injection of successive pulses on the sample. As a result, reductions in the SEE coefficients with temperature were confirmed, except for sapphire. The multi-pulse measurement results indicated that surface charging of the sapphire was higher than that of other samples. This may be one of the factors that causes sapphire not to be durable for rf window applications, compared with alumina ceramics. Although there are few exceptions, it was found that the SEE coefficients of alumina ceramics increased with the purity and the average grain size

Thermo-elastic deformation of a titanium foil by ion irradiation

Experimental results were presented for elastic deformation of a titanium foil by MeV ion irradiation. In situ observations of the deformation have been performed by using a laser displacement meter. In order to study time-evolution of the deformation, the foils were irradiated with short-pulsed ion beams. We found that the deformation occurs by piling up of small deformation generated by one bunch of pulsed beams. We also investigated the magnitude of deformation as a function of beam flux. The result obtained was examined in terms of a model for strain due to beam heating.

Sound wave generated by swift heavy ions

We irradiated samples of polycrystalline metals (Al and Cu) and ceramics (MgO and machinable ceramic) with a short-pulse beam of 3.54-GeV Xe ions and detected sound waves in ultrasonic frequencies with piezoelectric sensors on the surface. On the Al sample, waveforms were observed at different positions relative to the epicenter and were compared with those by 3.8-GeV Ar ion and YAG-laser irradiation. The gross structure of the waveform was similar between the Xe and laser irradiation, where we observed a shear wave which was not found in the Ar-beam irradiation. For the metal samples, the propagation time of the compression wave to the epicenter is shorter for the Xe ion than the laser irradiation. Using the time difference and separately-measured value of speed of sound, we estimated the depth of the sound source in metals and found that it is near the end of the ion range.

High current ion sources and injectors for induction linacs in heavy ion fusion

Heavy ion beam driven inertial fusion requires short ion beam pulses with high current and high brightness. Depending on the beam current and the number of beams in the driver system, the injector can use a large diameter surface ionization source or merge an array of small beamlets from a plasma source. In this paper, we review the scaling laws that govern the injector design and the various ion source options including the contact ionizer, the aluminosilicate source, the multicusp plasma source, and the metal vapor vacuum arc (MEVVA) source.

Hydrodynamic simulations of integrated experiments planned for the OMEGA/OMEGA EP laser systems

Integrated fast-ignition experiments on the combined OMEGA/OMEGA EP laser systems have been simulated with the multidimensional hydrodynamic code DRACO. The OMEGA laser system provides up to 30 kJ of compression energy, and OMEGA EP will provide two short-pulse beams, each with energies up to 2.6 kJ. In the electron transport model included in DRACO, the relativistic electrons are introduced at the pole of a two-dimensional (2D) simulation and transported in a straight line toward the target core. The electron's energy is calculated from the laser irradiance using a semi-empirical formula. An OMEGA cryogenic DT target designed to reach a one-dimensional fuel ρR of 0.5 g cm−2 has been simulated in 2D, with and without non-uniformities, to assess the sensitivity to energy, timing, and irradiance of the Gaussian fast-ignitor beam. For the uniform case, the neutron yield is predicted to be in excess of 1015 (compared to ∼1014 without an ignitor beam) over a synchronization range of ∼80 ps. Implosions with the ignitor beam show little decrease in the neutron yield with increasing inner-ice non-uniformity, in contrast to implosions without the ignitor beam, which show a significant decrease in neutron yield with increasing non-uniformity.

Charge-exchange-producedK-shell x-ray emission fromAr16+in a tokamak plasma with neutral-beam injection

High-resolution spectroscopy of a hot tokamak plasma seeded with argon ions and interacting with an energetic, short-pulse neutral hydrogen beam was used to obtain a high-resolution $K$-shell x-ray spectrum formed solely by charge exchange. The observed $K$-shell emission of ${\mathrm{Ar}}^{16+}$ is dominated by the intercombination and forbidden lines, providing clear signatures of charge exchange. Results from an ab initio atomic cascade model provide excellent agreement, validating a semiclassical approach for calculating charge-exchange cross sections.

Production of short-pulsed beam for ion-beam pulse radiolysis

Toward the study of the free-radicals behavior, we have proposed a production method of a short-pulsed beam, based on a bunch rotation after momentum cooling. The method was experimentally investigated at the HIMAC synchrotron. In the experiment, after the momentum spread was cooled down to around 0.02% at FWHM through electron cooling, an RF-field was non-adiabatically applied. Under an intensity of (3–7)×106 argon-ions and an RF amplitude of 340V, a short-pulsed beam with the duration of 30–35ns at FWHM was obtained after a quarter period of the synchrotron oscillation.

Experimental study of the converging density wave excitation in a cylindrical anode of a high-current diode

Density waves excited in a cylindrical anode irradiated by a short pulsed high-current electron beam in a relativistic vacuum diode have been experimentally studied. The electron trajectories are almost perpendicular to the side surface of a cylindrical anode coaxial with the cathode. As a result of the discharge, the surface layer of the continuous anode exhibits no visible changes, whereas an inner region of the anode is evaporated. The mechanism of the isentropic wave excitation is elucidated by experiments with a coaxial anode comprising a tube tightly fit to the central rod. According to the most probable explanation, the electron beam induces collapse in the subsurface layer of the anode.

Production of a high brightness beam from a large surface source

An experiment was set up on a 500-kV test stand to study the beam optics of an extraction diode using a 10-cm diameter alumino-silicate source. The beam was also sent through an aperture to reduce aberrations, thus enhancing its brightness. In comparing the experimental results with WARP-3D computer simulations, we found excellent agreement in the emittance diagrams and in the beam current rise-time. By benchmarking the computer code, we now have the capability to perform detail optical designs and to create the proper voltage waveform for producing a current pulse with a fast rise-time. A fast rise-time pulse is desirable because many potential near-term HIF experiments will require short beam pulses which have a fast rise-time.

Production of ultrahigh ion current densities at skin-layer subrelativistic laser–plasma interaction

Some applications of fast ions driven by a short (⩽1 ps) laser pulse (e.g. fast ignition of ICF targets, x-ray laser pumping, laboratory astrophysics research or some nuclear physics experiments) require ion beams of picosecond (or shorter) time durations and of very high ion current densities (∼1010 A cm−2 or higher). A possible way of producing ion beams with such extreme parameters is ballistic focusing of fast ions generated by a target normal sheath acceleration (TNSA) mechanism at relativistic laser intensities. In this paper we discuss another method, where the production of short-pulse ion beams of ultrahigh current densities is possible in a planar geometry at subrelativistic laser intensities and at a low energy (⩽1 J) of the laser pulse. This method—referred to as skin-layer ponderomotive acceleration (S-LPA)—uses strong ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a proper preplasma layer in front of a solid target. The basic features of the high-current ion generation by S-LPA were investigated using a simplified theory, numerical hydrodynamic simulations and measurements. The experiments were performed with subjoule 1 ps laser pulses interacting with massive or thin foil targets at intensities of up to 2 × 1017 W cm−2. It was found that both in the backward and forward directions highly collimated high-density ion beams (plasma blocks) with current densities at the ion source (close to the target) approaching 1010 A cm−2 are produced, in accordance with the theory and numerical calculations. These ion current densities were found to be comparable to (or even higher than) those estimated from recent short-pulse TNSA experiments with relativistic laser intensities. Apart from the simpler physics of the laser–plasma interaction, the advantage of the considered method is the low energy of the driving laser pulses allowing the production of ultrahigh-current-density ion beams with a high repetition rate. It opens a prospect for unique tabletop experiments in various fields of physical and technological research.