Beam Current Profile Research Articles

Rapid measurement of charged particle beam profiles using a current flux grating

The principle and physics issues of charged particle beam diagnostics using a current flux grating are presented. Unidirectional array of conducting channels with interstitial insulating layers of spacing d is placed in the beam path to capture flux of charge and electronically reproduce an exact beam current profile with density variation. The role of secondary electrons due to the impinging particle beam (both electron and ion) on the probe is addressed and a correction factor is introduced. A 2-dimensional profile of the electron beam is obtained by rotating the probe about the beam axis. Finally, a comparison of measured beam profile with a Gaussian is presented.

Design of an emittance exchanger for production of special shapes of the electron beam current

Recently, considerable attention has been focused on electron beam current profile shaping by means of a transverse beam mask followed by an emittance exchanger (EEX). This setup can transform a transverse particle distribution into a longitudinal particle distribution. We investigate the EEX technique with application to production of a double triangular drive and a trapezoidal main bunches for high transformer ratio, high brightness dielectric wakefield accelerators. We perform numerical optimization of two realistic configurations: a double dogleg and a chicane. In this paper we report sample designs and discuss the effects of the beam line nonlinearities, beam space charge, and coherent synchrotron radiation on a current profile of the output beam.

Thermal analysis of titanium drive-in target for D–D neutron generation

Thermal analysis was performed for a titanium drive-in target of a D-D neutron generator. Computational fluid dynamics code CFX-5 was used in this study. To define the heat flux term for the thermal analysis, beam current profile was measured. Temperature of the target was calculated at some of the operating conditions. The cooling performance of the target was evaluated by means of the comparison of the calculated maximum target temperature and the critical temperature of titanium.

Observation of Narrow-Band Terahertz Coherent Cherenkov Radiation from a Cylindrical Dielectric-Lined Waveguide

We report experimental observation of narrow-band coherent Cherenkov radiation driven by a subpicosecond electron bunch traveling along the axis of a hollow cylindrical dielectric-lined waveguide. For an appropriate choice of dielectric wall thickness, a short-pulse beam current profile excites only the fundamental mode of the structure, producing energetic pulses in the terahertz range. We present detailed measurements showing a narrow emission spectrum peaked at 367 + or - 3 GHz from a 1 cm long fused silica capillary tube with submillimeter transverse dimensions, closely matching predictions. We demonstrate a 100 GHz shift in the emitted central frequency when the tube wall thickness is changed by 50 microm. Calibrated measurements of the radiated energy indicate up to 10 microJ per 60 ps pulse for an incident beam charge of 200 pC, corresponding to a peak power of approximately 150 kW.

Generation and Measurement of Relativistic Electron Bunches Characterized by a Linearly Ramped Current Profile

We report the first successful attempt to generate ultrashort (1-10 ps) relativistic electron bunches characterized by a ramped longitudinal current profile that rises linearly from head to tail and then falls sharply to zero. Bunches with this type of longitudinal shape may be applied to plasma-based accelerator schemes as an optimized drive beam, and to free-electron lasers as a means of reducing asymmetry in microbunching due to slippage. The scheme used to generate the ramped bunches employs an anisochronous dogleg beam line with nonlinear correction elements to compress a beam having an initial positive time-energy chirp. The beam current profile is measured using a deflecting mode cavity, and a pseudoreconstruction of the beam's longitudinal phase space distribution is obtained by using this diagnostic with a residual horizontal dispersion after the dogleg.

SU-DD-A1-05: Turn-On Dose and Transit Time Adjustments in Treatment Planning for the Axxent® Electronic Brachytherapy System

Purpose: To analyze the dosimetric impact of x‐ray source turn‐on time and inter‐dwell position transit times for application of the Axxent® Electronic Brachytherapy System to APBI. Materials and Methods: At the first dwell position, the treatment timer starts after the source has ramped‐up to full operating voltage and beam current (50 kV, 300 μA), so planned dose‐delivery time does not account for a small “turn‐on” dose. Radial dose functions were calculated with MCNP5 for operating voltages from 20 to 50 kVp. Turn‐on dose was estimated by temporally averaging these distributions using the voltage and beam current ramp profiles. For subsequent dwell positions, the timer starts when the source begins moving to the next dwell position so elapsed time includes the transit time. (The source remains on during the time between dwell positions, typically 0.7 seconds for a 0.5 cm step). Dose contribution during transit was estimated using Varian BrachyVision™ by subtracting the transit time from the second and subsequent dwell positions, then adding extra dwell positions at midpoints between original positions with times equal to the transit time. Results: The composite turn‐on dose profile from Monte Carlo results was equivalent to 2 seconds of additional time at the first dwell position with source operation at 50 kVp. This corresponds to < 0.5% of a typical treatment time. Whether or not transit time is accounted for, the planned doses at prescription points 1 cm outside of a typical balloon agree to within an average of 0.1% with a standard deviation of 0.2%. Conclusions: Turn‐on dose may be approximated in treatment planning by adding 2 seconds to the first dwell time. Dose during source transit may be ignored when using a balloon applicator for APBI. Research sponsored by Xoft, Inc.

The new external microbeam facility at the 5 MV Tandetron accelerator laboratory in Madrid: beam characterisation and first results

This paper describes the new external microbeam on the 15° beamline of the 5 MV Tandetron accelerator recently installed at the CMAM in Madrid. The focusing and beam extraction system was supplied by Oxford Microbeams Ltd. and consists of a high precision quadrupole doublet with an interchangeable Kapton window exit nozzle and front-viewing video microscope. The sample is positioned in the beam using a stepper motor stage. The beam current and beam profile have been determined under different experimental conditions. A simple method based on the signal processing of ion-induced luminescence from quartz targets has been used to determine the beam profile in two dimensions simultaneously, without scanning. This is the first step in the development of a real time beam profile monitoring system, which could be used as part of an automated beam focusing procedure. The beam line will be primarily devoted to archaeometry and cultural heritage studies. As an example we report the characterisation of two Tang appearance antique porcelains.

Low convergent confined-flow Pierce gun for a space TWT

An approach for designing an electron gun for a high efficiency, high linearity space traveling wave tube (TWT), has been presented. A low convergent (<10 : 1) Pierce electron gun of beam perveance 0.43 μP has been designed for a high gain, high linearity and high efficiency C-band 60 W Space TWT using in-house developed two-dimensional FDM based gun and collector simulation code PIERCE. In this gun, the first anode (isolated from the ground anode) has been kept nearly 100 V above the ground anode to act as an ion barrier for increasing cathode life and to regulate beam current over the lifetime of the tube. An M-type dispenser cathode of diameter 3.20 mm has been used for cathode loading of less than 1.0 A/cm2 and heater wattage around 3.0 W. The magnetic focusing with integral-pole-piece barrel assembly and periodic-permanent magnets (PPM) have been designed using in house developed two-dimensional FDM based code SUNMAG. The practical problem of linking requisite cathode flux to the cathode for confined flow of the electron beam with low convergence factor has been sorted out by gradually increasing the PPM magnetic field. The magnetic field has been increased in steps from the gun and over the first five magnets varying from Brillouin field (B B) value to twice B B for achieving the electron beam with scalloping less than 10%. Agreements between the simulated results and the experimental results for the beam current and magnetic field profile have been achieved within 8%. The dynamic beam transmission (under rf operation of the tube) has been achieved better than 98% in the tube.

Cumulative beam breakup in linear accelerators with arbitrary beam current profile

An analytical formalism for the solution of cumulative beam breakup in linear accelerators with arbitrary time dependence of beam current is presented, and a closed-form expression for the time and position dependence of the transverse displacement is obtained. It is applied to the behavior of a single bunch and to the steady-state and transient behavior of dc beams and beams composed of pointlike and finite-length bunches. This formalism is also applied to the problem of cumulative beam breakup in the presence of random displacement of cavities and focusing elements, and a general solution is presented.

Phase space growth during RF capture in the GSI heavy ion synchrotron SIS-18

This study reports on the optimization of the radio frequency capture phase during the operational cycle of the SIS-18 synchrotron at Gesellschaft für Schwerionenforschung, Darmstadt, Germany. The ion species studied were 238U+28 and 238U73+ at an injection energy of 11.4 MeV/u. The longitudinal relative momentum spread derived from Schottky spectra of the coasting beam at injection provides a value of |Δp/p0|full-width ∼ 5 × 10−3. Simulation results from the synchrotron tracking code ESME (FermiLab) were compared with beam-current profile measurements obtained from a pickup. To gain further insight, the Tomography program (European Organization for Nuclear Research) has been used to derive the longitudinal phase space development from waterfall plots of the measured beam current profile, which may then be compared against simulation. Possible causes of this nonadiabaticity are discussed and solutions are proposed.

Emittance Measurement of Axisymmetric High-Current Electron Beam Confined by a Longitudinal Magnetic Field

We propose a method for emittance measurement of an axisymmetric high-current electron beam confined by a longitudinal magnetic field. In this method, the beam current is measured by a multi-layer Faraday cup to obtain the radial beam current profile using Abel inversion. Based on the theory of a beam in a uniform focusing channel, the transverse electron temperature is uniquely defined by the shape of the current density profile. Therefore, measuring the current density profile enables the determination of the beam emittance. Because the method does not require to use slits, the measurement has been markedly simplified. The method has been adopted to a test beam. The emittance is found to be 20.2π mm·mrad, which basically agrees with that obtained by a slit-slit method.

Simulation of topography evolution and damage formation during TEM sample preparation using focused ion beams

Our recently developed simulation code FIBSIM is applied to topics related to transmission electron microscopy (TEM) sample preparation using focused ion beams (FIB). FIBSIM combines dynamic Monte Carlo simulation of collision cascades with two-dimensional, cell-based topography simulation. The influence of the scanning mode and of the beam current profile on the evolution of the surface contour is investigated. Furthermore, amorphous zones in silicon samples and damaged regions are predicted for different beam energies of 10, 30 and 50 keV. The thickness of the predicted amorphous regions is in good agreement with experimental TEM data.

Electron emission from slow-wave structure walls in a long-pulse, high-power backward wave oscillator

Pulse shortening is a phenomenon common among all long-pulse, electron-beam-driven high-power microwave sources. Although the electron beam driving the source may continue to propagate through the interaction region of the device for several microseconds or more, the duration of the emitted microwave pulse is typically no more than /spl sim/100 ns. Most explanations of this phenomenon put forth involve the introduction of plasma into the interaction region and/or the degradation of beam quality. This paper describes experiments conducted on the University of New Mexico's Long-Pulse Backward Wave Oscillator (BWO) Experiment which investigate the behavior of beam electrons in the slow-wave structure (SWS) during microwave generation. A current probe having a small aperture at a variable radius is placed within the SWS to monitor the beam current profile at different radii as a function of time. Results from these experiments reveal the appearance of electrons between SWS ripples at times corresponding to the peaks of the microwave pulses in separate experiments. A drop in the main beam current is observed shortly thereafter. The source of the electrons within the ripples is thought to be field emission or secondary electron emission from the SWS walls or emission from plasmas generated there.

High-resolution ion beam profiler for ion implanters

A novel ion beam profiler has been developed for measuring a two-dimensional current distribution of an ion beam. The system consists of two parallel disks rotating at different speeds and mounted in front of a single Faraday cup. The cup intercepts the beam current transmitted through a small movable aperture, formed by the intersection of precise spiral slits positioned on the two disks. As the disks rotate, the intersection of the slits describes a sequence of radial lines across an ion beam. Consequently, a set of beam current profiles, each along a different radial line, is obtained at the output of the Faraday cup. Since only unidirectional rotary motion is employed for beam scanning, the data acquisition process is fast and no conductive disk cooling is needed for cyclical operation of the profiler. Preliminary tests have shown that the radial resolution of the system is better than 1 mm for beam images of 60 mm × 60 mm.

A 130–260 GHz, 1 MW free electron maser for fusion

We report on design work and testing of several main components of the FOM-Fusion-FEM project. Measurements on the electron gun are presented, showing a uniform beam current profile and a low halo current. A time-dependent simulation study of the interaction between the electron beam and the mm-waves shows that, for proper undulator tapering, longitudinal mode competition results in a quasi single-mode regime. Further, some high power and detailed low power measurements on mock-ups of the waveguide system are presented, showing that the reflection and outcoupling systems perform well.

A 1 MW, 130–250 GHz free-electron maser for fusion

We report on design work and testing of several main components of the FOM-Fusion-FEM project. Measurements on the electron gun are presented, showing a uniform beam current profile and a low halo current. A time-dependent simulation study of the interaction between the electron beam and the mm-waves shows that, for proper undulator tapering, longitudinal mode competition results in a quasi single-mode regime. Further, some high power and detailed low power measurements on mock-ups of the waveguide system are presented, showing that the reflection and outcoupling systems perform well.

A theory of relativistic electron beam bunching by the plasma wake-field effects

Modulated current profile of a relativistic electron beam is theoretically investigated in terms of the propagation distance in a chamber containing a diffused plasma, which exerts wake-field effects on the beam. Neglecting the beam-head erosion, it is found that the beam current profile at a specified propagation distance is expressed in terms of only the time t0, at which the beam segment enters the chamber. Energy modulation along the beam pulse is also calculated.

A method for calculating the current density of charged particle beams and the effect of finite source size and spherical and chromatic aberrations on the focusing characteristics

A method for calculating current density of a focusing system including the effects of finite source size, spherical, and chromatic aberrations is presented. Current density at the target plane can be determined by calculating the origin of trajectories at the object plane, for each target point. The relation between object and image points is calculated by numerical integration of the ray equation and using the analytical results from geometrical aberration theory. The focusing characteristics for current density distributions are also investigated. A focusing position which gives the maximum axial current density does not give a good beam current profile (measured by a knife edge) in the case of large aberrations, because the aberrations cause the current distribution to have a long tail.

Control of ion beam current density and profile for high current ion implantation systems

One solution for the problems of the local temperature increase and the charge-up of a wafer during high current ion implantation is to decrease the beam current density while maintaining the total beam current necessary on the wafer. To solve these problems, a method, that maintains the beam spatial profile on a wafer by controlling the normalized perveance and the pole face angle of the analyzing magnet for various beam currents, has been developed experimentally. As a result, under the experimental conditions of 1 to 8 mA of 35 keV As +, the beam uniformity, defined as [average current density]/[maximum current density], became more than 0.2 and the beam profile on the wafer was maintained by using a suitable value of the perveance.

Deceleration and ion beam optics in the regime of 10–200 eV

A deceleration lens system for low energy ion beam transport applications in the region of 10–200 eV has been designed, constructed and experimentally characterized. In this ion beam system, the target is required to be in an almost field-free region at the exit of the lens to allow for maximum target maneuverability. The lens design was optimized using the simulation data obtained with chden, a computer program for calculating ion trajectories with space-charge included. The design chosen in this work was a five-electrode lens which can be operated in either a simplified two-electrode deceleration mode or, when the best system performance is required, in a five-electrode decelerator-focusing mode. The performance of the lens was experimentally evaluated by using a 3 keV pure argon ion beam with a beam size of about 1 mm2 and beam current of about 10μA which was generated by a modified Colutron Ion Gun Model G-2 unit. The lens decelerated the incident beam to 10–200 eV and transported it to either a Faraday cage for beam current and beam profile measurements or to a quadrupole mass spectrometer equipped with a cylindrical ion energy analyzer for beam energy and mass characterization. The results indicated that the lens could deliver an ion current density of approximately 100 μA/cm2 at ion energies higher than 50 eV. The available current density dropped with decreasing energies but remained at more than 10 μA/cm2 at 10 eV.