Relative Energy Spread Research Articles

Time focusing magnetic wedge‐field sectors for time‐resolved ion momentum spectrometry

AbstractThe time‐of‐flight properties of magnetic wedge‐field sectors have been derived in a second‐order approximation. The following quantities were included: source slit width, radial and axial opening angles of the ion beam and relative energy spread. The transfer matrix elements of the time‐of‐flight (longitudinal) row were derived in terms of the initial and final angles and of K‐parameter dependence on the ion momentum and magnetic field. The longitudinal mass dispersion coefficient required in the time resolution formula was derived. The time‐of‐flight ion optical properties of a 60° symmetric, homogeneous magnetic sector mass spectrometer, already employed for time‐resolved ion momentum spectrometry studies, were compared with those of a wedge‐field, 180° deflecting stigmatic arrangement with K=0.7423. The time‐of‐flight aberrations were calculated from transfer matrix multiplications. The advantages of the wedge‐field geometry are: independence, up to second order, of the ion packet length upon the ion source slit width, independence to first order of the ion energy spread and stigmatic focusing in space. These factors ensure both improved sensitivity and time resolution.

Design of a 6.3-m optical klystron for a storage-ring ultraviolet free-electron laser

A 6.3-m optical klystron (OK) to be used for free-electron laser (FEL) experiments with wavelength from visible to ultra-violet is now in the stage of design. The OK will be installed in a 7.25-m straight section of a racetrack-type storage ring NIJI-IV dedicated to FEL. Pure permanent-magnet blocks made of NEOMAX-35H are used for the OK. The number of periods (72 mm) in each normal-undulator section is 42 with 18-mm thickness, and the dispersive section is 216 mm long and 36 mm thick. Each of the sections will be constructed separately, but the actuators will be driven either in synchronism or independently. With the above configuration, the FEL oscillation with wavelength ranging from 600 to 200 nm is expected by changing the electron-beam energy from 200 to 400 MeV. The FEL gain is estimated to be about 4.8 × 10 −3 / mA per bunch at 600 nm, and 22 × 10 −3 / mA per bunch at 350 nm. Here, the degradation due to the filling factor is neglected and the relative energy spread is conservatively assumed to be 7.5 × 10 −4 . The effect of the long OK on the electron beam is also estimated.

Super-reltron theory and experiments

A highly efficient, high-power microwave tube called super-reltron is reported. The authors have achieved operation at >400 MW with approximately 50% efficiency at 1 GHz, and 250 MW with 40% efficiency at 3 GHz. The RF pulse durations are typically a few hundred nanoseconds. These compact lightweight tubes do not require an external magnetic field. The RF output coupling is straightforward and delivers the power directly via the fundamental TE/sub 10/ wave in a rectangular waveguide without a mode converter. The key features of the tube include (i) generation of a well-modulated electron beam by periodic virtual cathode formation, (ii) postacceleration of the modulated beam to reduce the relative electron energy spread, and (iii) a multicavity output section that efficiently extracts power without RF breakdown. Various theoretical aspects of the device are discussed and the experimental results are summarized. >

Ideal and quasi-ideal time focusing of charged particles

Deals with the problem of the determination of scalar and vector potentials of electromagnetic fields providing time focusing of strongly diverging packs of charged particles (specific values of relative energy spread being about 100%, angular spread attaining tens of degrees). The problem is solved within the framework of static non-relativistic corpuscle optics. A theory of ideal time and space-time focusing encompassing and systematizing all known particular cases of such focusing is developed. Necessary and sufficient conditions of isochronous motion and requirements for inner symmetry of motion (the latter increasing with the growth of the quality of focusing) are determined. It is shown that the obtained results make it possible to realize an adequate choice of harmonic fields for constructing the time-of-flight apparatus. They can also be used to determine the accuracy of field sustaining according to the known time-focusing properties.

Low-Voltage Diffraction-Limited Probe Current: Limits Due to e-e Interactions in Intermediate Cross-Over

The minimum diameter of an electron probe is limited by diffraction to the FWHM of the Airy disc, δa≃ λ/2α where the objective aperture semi-angle will be limited by aberrations. For probe voltages Vp less than 10 kV, the relative energy spread ε will cause a to be limited by chromatic aberration to about (λ/Ccε)1/2. The total probe diameter is then δp = Kλ/2α:, K=1. Thus δp = (Ccε;)1/2, which is proportional to V −3/4. Altho the diameter of the diffraction limited probe depends on the voltage, energy spread and the quality of the optics, the current depends (very nearly) only on the voltage independent reduced brightness Br of the e-source.The current into a probe that contains the gaussian image dg of an electron source of reduced brightness Br is Ip = BrVp(π/2 dgα)2. Since a probe without current is not very useful, dg must be greater than zero for sources of less than infinite brightness. The diameter of the probe will not dramatically increase until dg≃δp≃δa. Thus the current into a diffraction limited probe can be as much as Ip = BrVp(π/2 λ/2α α)2 = Brπ2/32 h2/me = Br 10−18 A.

Proposal for a race-track microtron with high peak current

In order to obtain high gain in a free electron laser a high-quality electron beam with high peak current is required. It is well-known that a microtron is able to produce a high-quality beam having low emittance and small energy spread (1%). Because a circular microtron has a limited high-current capability a race-track design is adopted for providing flexibility, better beam quality and of course higher peak current in the microbunch. Space charge problems may be severe in a microtron. It can be shown that bunching on certain specific subharmonic frequencies will lead to a strong reduction of the space charge problems. The general layout of our microtron design will be presented. The characteristics are: energy 25 MeV, micropulse 10° of the rf frequency of 3 GHz. Our aim is to come beyond the present state of the art with the following characteristics: relative energy spread 0.001, emittance 3 mm mrad, current in the micropulse 100 A, macropulse length 50 μs and subharmonic bunching at 1:64.

Microtron accelerator for a free electron laser

The electron beam parameters of an energy tunable (11–22 MeV) circular microtron accelerator are measured to be close to that required for free electron laser (FEL) operation in the far infrared. Average currents of 30 mA are measured at an energy of 17 MeV during the macropulse (14 μs duration and 50 Hz repetition rate) in the 25 orbit microtron. The 3 GHz rƒ electron beam peak current is expected to be 20–40 times the macropulse average value. The relative energy spread of the electron beam is inversely proportional to the number of the microtron orbits and is expected to be about 0.2%. The radial emittance of the electron beam is measured after extraction from the microtron and has a value of about π cm mrad with the axial emittance one third this value. Electron beam extraction from the 25th orbit is nearly 100% efficient. The area of the electron beam is observed to be less than 1.0 cm 2 in a 270 G helical magnetic field of 20 cm periodicity and 10 m length. These preliminary results were recently obtained with the use of single crystal LaB 6 cathodes. These single crystal cathodes result in increases in the macropulse length by factors of ∼ 4 compared to results using sintered cathodes.

Issues in storage-ring design for operation of high-gain FEL

A high-gain free electron laser (FEL), placed in a special by-pass of a storage ring, can provide tens of megawatts of coherent power at wavelengths shorter than 1000 Å. The requirements on beam quality (few hundred amperes of peak current, emittance of the order of 10 −8 m·rad, and relative energy spread less than 0.002), are demanding but lie within the limits of modern storage ring technology. In this paper, we study various issues in storage ring design and FEL physics for operation of a high-gain FEL. Topics included are the requirements on beam parameters for FEL operation, coherent instabilities and intrabeam scattering effects in the storage ring, lattice design, and FEL performance computed by 2-D simulations.

Free-electron laser beam quality

Free-electron laser beam quality is defined as the ratio of the current density to the relative axial energy spread. The approach to obtaining high-quality electron beams in some representative accelerators is examined and compared to the beam brightness. Beam quality is found to be a decreasing function of beam current, unless axial current compression (i.e., subharmonic bunching) or high current density cathodes are used. Effects due to magnetic perturbations are enhanced at high beam density.

Energy spread measurements on the ACO storage ring

Absolute and relative energy spread measurements have been made on the ACO storage ring using two different methods. The first one consists of a spectral analysis of the electron bunch length based on the proportionality of the longitudinal bunch length versus energy spread. The second one constitutes a direct measurement using the on axis synchrotron radiation emitted by an optical klystron. The two measurement techniques present a very high signal/noise ratio allowing time resolved records. Examples are presented of the energy spread time evolution in the FEL oscillator and of harmonic generation experiments.

A double focusing magnet system for a medical linear electron accelerator

A double achromatic magnet system is described which bends electrons of moderate to high clinical energy (2 MeV to 30 MeV) through 112.5° onto a target. The X-ray or electron beam emerging from the target is used for medical therapy. By providing quadrupole and sextupole corrections to the bending magnets, a spot size of 2 × 2 mm 2 at the target is obtained as well as double achromatic behaviour, for electron beams with a large relative energy spread of 10%, an initial width and height of 5 mm, and with an initial divergence spread of ± 1 mrad. The corrections have been introduced on the basis of field measurements in the median plane of the bending system, and subsequent trajectory calculations. The effects agree with numerical results obtained by running a second-order particle transport code on the predicted magnet shape.

Electron Bunch Space Charge Influence upon Its Energy Spread and Sizes under the Motion in Free Space

The influence of Coulomb repulsion in electron bunch upon its energy spread and sizes under the motion in free space after leaving the linac is considered. Space charge field calculations based upon the model of charged ellipsoid, were carried out in the range of electron energies from 1.0 to 10.0 MeV and beam pulse currents from 0.2 to 10 mA. The bunch travelling path being 2.5 m longitudinal Coulomb repulsion results in relative energy spread of a bunch from 10-4 to 2.10-2. The effect examined should be taken into account when electron linac with high energy resolution is designed.

Effect of the energy spectrum of an electron source on the attraction of the electrons into the accelerating mode in a betatron

The energy spread of the electrons in a betatron has a stabilizing influence on the motion of the beam at all stages of the accelerating cycle. The possibility of electrons with a variety of energies being captured by an azimuthally-symmetrical magnetic field is considered in this paper. It is shown that, for both external and internal injection, electrons with specific values of the projection of the moment of momentum Mz of the system on the z axis may be captured by the magnetic field, the corresponding spread being finite ΔC≠ 0. Accepting this picture of electron capture, estimates are made for the greatest possible relative energy spread of the electrons which may be captured in case of external injection (17.4%), and for the relative energy spread of the “zero” electrons (those having C = 0), which amounts to 1.12%. The range of finite motion of the electrons (the boundaries of which should be included within the cross section of the vacuum chamber in order to obtain the maximum amount of accelerated charge) is also determined.

Unimolecular Decomposition of Chemically Activated Ethyl-d2 Radicals

Addition of H atoms to trans-ethylene-d2 produces chemically activated ethyl-d2 radicals. This choice of the reactant ethylene permits all subsequent secondary processes of the hot ethyl radicals to be counted. The system has been studied between —78° and 160°C, and in most detail at room temperature, as a function of ethylene pressure. Experimental details are similar to those given in previous work on butyl. The relative energy spread of the formed ethyl radicals is here effectively greater than that observed for butyl radical since the thermal spread is not superimposed upon a minimum excess above the critical energy, as was the case for butyl. Nevertheless, relative to high-temperature thermal decompositions, the product ethyl radicals are comparatively monoenergetic, especially at the lowest temperature. The observed rate constant for decomposition ka was evaluated from the amounts of stabilization vs decomposition by C–H rupture. The decomposition rate is compared with theoretical expressions. An isotope effect appears to exist for transfer of H and D in the disproportionation of ethyl-d2 radicals at 25°.