DC Beam Research Articles

Effect due to Cs injection upon current oscillation of the beam extracted from the J-PARC negative hydrogen ion source

A high-speed emittance measurement system installed at the J-PARC RF negative hydrogen (H-) ion source test stand can construct emittance diagrams of the beam of a specific frequency component. The driving 2 MHz RF power produces small amplitude oscillating components at 2 MHz and at higher harmonic frequencies in the total H- ion beam detected by a Faraday installed downstream of the emittance measurement system. Magnitudes of oscillating component at 2 MHz and 4 MHz frequencies become appreciable as the slits of the emittance measurement system separate the H- ion beam into the position/angle resolved phase space elements. The reconstructed emittance diagrams of H- ion beams for the operation condition with continuous supply of Cs realizing large H- ion current to total extraction current ratios indicate that 2 MHz and 4 MHz frequency oscillating beam components propagate surrounding the center DC beam. Both 2 MHz and 4 MHz oscillating beam components comprise two to three beam particle groups in the phase space. The DC component of the H- ion beam exhibit a single peak in the angular distribution. Meanwhile, oscillating components often take the minima where the DC H- ion beam takes the maximum in the angular distribution diagram. Thus, the regions that produce large amplitude oscillations of the H- ion beam are considered located at the circumference of the beam extraction aperture. One possible explanation for observing larger amplitude beam oscillation can be the enhanced H- ion surface production at the plasma facing the wall of the beam extraction aperture. The operation without the Cs delivery to the source increased electron current coextracted with H- ions and changed the structure of the beam emittance diagram. In the direction perpendicular to the magnetic fields created by a magnetic filter and electron suppression magnets, the peak of the 4 MHz oscillating beam components appeared at the center of the DC beam. This observation suggests the extraction of H- ions formed in the central region by the volume production process near the internal RF antenna. Mechanisms that can excite and diminish the beam fluctuation amplitudes of the extracted H- ion beam are discussed.

Ultra-Sharp Angular Filtering of Far-Field Signals With Graded-Index Multilayers

Dielectric multilayers with graded index treat the propagating waves impinging at different angles very differently, due to the occurred total internal reflection. Such an ultra-sharp change in the response can be exploited for filtering, manipulation and tailoring of spatially distributed electromagnetic signals. Various utilities of the proposed filter such as power splitting, spatial DC isolation, beam shifting and low-profile energy interaction are demonstrated. The regarded setup can be vital as component in the operation of integrated photonic devices dealing with optical signal processing and analog information control.

Hybrid macroparticle algorithm for modeling space charge

A space charge algorithm is constructed that is a hybrid between envelope and multiparticle models. The transverse dynamics is simplified by tracking the transverse envelope of each macroparticle. The equations of motion are derived from the Hamiltonian-fluid formulation of the Vlasov Poisson system. A novel electrostatic self-field solver is derived that solves directly for the self-consistent on-axis potential and linear defocusing force, including longitudinal and beam pipe boundary conditions. The implementation of the algorithm, hyper3d, is presented. It is tested against the particle-in-cell code warp, using various configurations of a periodic focusing structure with rf cavities. The required number of macroparticles is reduced substantially compared to standard particle-tracking codes. This model is well adapted to cases where the transverse dynamics is linear and where the details of the longitudinal dynamics are the principal point of interest; an example is where a dc beam is converted to a bunched beam.

Improving a PGNAA Technique to Detect Heavy Metals in Solid Samples

Prompt gamma-ray neutron activation analysis (PGNAA) is a useful approach for determining the concentrations of a variety of elements in natural materials, either online or in situ, without affecting their chemical forms in matter. The current research aimed to improve the yield of a portable PGNAA setup using a dc beam of 2.5 MeV neutrons and a CeBr3 detector to record gamma rays from neutron inelastic scatterings. It is impossible to avoid the superimposition of heavy metal gamma rays and those from the detector’s element. However, tests were carried out to improve the signal-to-background ratio. By assessing the minimum detectable concentrations (MDC) of chrome, titanium, and zinc in soil samples, the effectiveness of the new optimization was confirmed. The study shows an improvement in the MDC values.

Design and Experiment of a Hollow Beam Electron Optics System for Ka-Band Extended Interaction Klystrons

In this article, the design and experiment of a hollow electron beam (HEB) electron optics system (EOS) for Ka-band extended interaction klystrons (EIKs) are presented. To realize the fast switch of the current emission at a low cutoff voltage, an electron gun with two focus electrodes (FEs) is chosen. The emitting surface of cathode is a ringed spherical surface. The first focus electrode (FE1) is around the cathode, and the second focus electrode (FE2) is set in the central hole of the cathode. The current emitted from the cathode surface is about 2A, when the operating voltage is 25 kV. Three simulation codes, Egun, Superfish, and CST, are used for designing the electron gun and the permanent magnet focusing system (PMFS). Some sensitivity analyses with respect to the PMFS’s critical parameters are presented. The simulation shows the dc beam transmission is 100%. The electron beam, confined by 6000 Gauss uniform field, propagates through the drift tube of 37 mm with good laminarity and small ripple. The emission current can be suppressed when the voltage of the FEs is −3.4 kV. According to the results of simulation, the Ka-band EIK had been constructed and the hot-test experiments had been fulfilled in succession. The experimental results exhibit that the dc beam transmission is about 99% and the beam can sustain 98% transmission at high frequency (HF) operation. In addition, the output power is higher than 8 kW in the bandwidth of over 100 MHz.

Pushing the capture limit of thermionic gun linacs

Although accelerator technology has matured sufficiently, state-of-the-art X-ray linacs for radiotherapy and cargo-scanning capture merely 30-50% of the electrons from a thermionic cathode, requiring a higher cathode current and leaving uncaptured electrons to cause problems due to back bombardment, shortening of cathode life, etc. Any solution to increase capture should be effective, simple, reliable, compact, and low cost in order to be adopted by industry. To address this, we present the design of a 6 MeV high capture efficiency S-band electron linac that captures 90% of the initial DC beam. This linac does not require any extra parts that would increase the cost as the high efficiency is achieved via a low-field-amplitude in the first bunching cell to decrease the number of backstreaming electrons, to velocity bunch the electron beam, and recapture backstreaming electrons. Under the low field amplitude, any electrons launched at decelerating phases travel backward with low speeds, thus most of them can catch the next RF cycle, and get re-accelerated/recaptured. As the electron speed is low, the cell length is also shorter than existing linacs. Such a short field is achieved by the use of asymmetric cells with differential coupling to the side-coupled cells. Our novel design has implications for all commercial high current thermionic gun linacs for increasing beam current and increasing cathode lifetime.

Design of 325 MHz RF grid-controlled high voltage thermionic cathode electron gun

The electron gun with high repetition rate and high average current has a very wide range of applications. This paper presents the design of a microwave grid-controlled high-voltage thermionic electron gun working in CW mode with a bunch repetition rate of 325 MHz and elaborates the experimental principles of this kind of electron guns. Firstly, simulation software EGUN, POISSON (Poisson Superfish) and GPT (General Particle Tracer) are used to accomplish the structure design of a300kV high-voltage DC electron gun and beam dynamics verification.Secondly, to feed the microwave into the gap between the cathode and the grid of the electron gunefficiently, design of a power supply with a scheme of impedance matching from the radio frequency power source to the cathode is completed.Accordingly, a 325 MHz dual-mode coaxial power supply device is designed, and its feasibility is verified and analyzed..

Demonstration of electron cooling using a pulsed beam from an electrostatic electron cooler

Cooling of hadron beams is critically important in the next generation of hadron storage rings for delivery of unprecedented performance. One such application is the electron-ion collider presently under development in the US. The desire to develop electron coolers for operation at much higher energies than previously achieved necessitates the use of radio-frequency (rf) fields for acceleration as opposed to the conventional, electrostatic approach. While electron cooling is a mature technology at low energy utilizing a dc beam, rf acceleration requires the cooling beam to be bunched, thus extending the parameter space to an unexplored territory. It is important to experimentally demonstrate the feasibility of cooling with electron bunches and further investigate how the relative time structure of the two beams affects the cooling properties; thus, a set of four pulsed-beam cooling experiments was carried out by a collaboration of Jefferson Lab and Institute of Modern Physics (IMP). The experiments have successfully demonstrated cooling with a beam of electron bunches in both the longitudinal and transverse directions for the first time. We have measured the effect of the electron bunch length and longitudinal ion focusing strength on the temporal evolution of the longitudinal and transverse ion beam profile and demonstrate that if the synchronization can be accurately maintained, the dynamics are not adversely affected by the change in time structure.13 MoreReceived 17 July 2020Accepted 18 November 2020DOI:https://doi.org/10.1103/PhysRevAccelBeams.24.012801Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam coolingAccelerators & Beams

Efficacy of radial matching section in enhancing RFQ cooler acceptance and optimization of its injection optics

Radio Frequency Quadrupole (RFQ) cooler is a device used for reducing transverse and longitudinal emittance of ion beams. The cooler employs a time varying field and as a result of which the acceptance ellipse, as seen by the beam while entering the cooler, is dependent on the RF phase. Acceptance ellipses having different orientations corresponding to the changing RF phase have a much smaller phase independent central overlapping area, so the real transmission for an injected DC beam into the RFQ cooler gets reduced. In RFQ linacs radial matching section has long been used for matching “time independent” DC input beam to the “time dependent” RF structure. Our simulation study shows that a similar radial matching section at the entrance of RFQ cooler can enhance the transmission for a DC beam into the RFQ cooler. Optimization of length of the radial matching section and the corresponding deceleration and injection optics shows an enhancement of more than two times in the area of time independent acceptance ellipse for ions over a mass range of 0A=4 to 240. Such innovative design could play a significant role in experiments with rare ion beams which are produced in miniscule quantities.

Development and testing of radio frequency modulated electron Gun at VECC, Kolkata

A superconducting electron linac (e-Linac) photo-fission driver is under construction at VECC for the upcoming rare isotope beam facility called ANURIB. The first stage consists of an injector linac based on 1.3 GHz superconducting radio frequency technology designed for accelerating 10 MeV, 2 mA CW electron beam. An rf modulated thermionic electron gun (E-Gun) has been developed and tested for this injector linac. The E-Gun will allow continuous beam operation up to 300 keV with around 3 pC charge per pulse at 650 MHz. Electron beam with RF modulation is extracted from the gun and intensity, energy, size of the DC beam as well as trans-conductance of the electron source are measured. The design of some major components for the electron gun and low energy beam transport line is discussed and results of beam tests are presented.

Development of a downsized proton accelerator system for compact neutron sources

In order to facilitate the widespread use of neutrons in industrial applications, it is important to provide a compact, semi-ready-made neutron source that a company and a factory can easily install. We are therefore developing a compact neutron source system utilizing a downsized proton accelerator as an entry level model for industrial use and materials development, which consists of a proton source, a four-vane radio frequency quadrupole linear accelerator (RFQ) and a lithium target for neutron production by the Li(p,n)37Be47 reaction. An RFQ is suitable to accelerate protons in the low energy region because it accepts DC beams extracted from the proton source and accelerates effectively to the design energy with bunching and focusing using the electric fields. Furthermore, it is possible to design the RFQ with a higher frequency than conventionally used owing to technological advances in high power solid-state RF amplifiers. We investigated the specifications for 500 MHz and 650 MHz through beam trajectory and electromagnetic simulations and fabricated a 2.5 MeV prototype of the 500 MHz type with a 2.45 GHz ECR proton source.

A new fast time of flight detector for single ion counting to high flux radioactive beams at ISOLDE

Beam diagnostics at the Isotope Separator On-Line DEvice (ISOLDE) consists mainly of wire SCanners and Faraday Cups (FC) coupled to charge sensitive picoammeters. Presented here is the design, simulations, and results of a new FC combined Secondary Electron Emission (SEE) multiplication detector for the beam energies found at ISOLDE (up to 60 keV). It expands the region of sensitivity from the current noise limit (≃0.25 pA) to individual ion counting. The detector is capable of counting rates of 105 ion ms−1 (≃10 pA) for DC or bunched beams with a high sensitivity timing resolution of σt = 0.10 ± 0.03 ns for individual ions. The device also operates as a conventional FC with integrated current measurements on the SEE plate for non-time sensitive operation with efficiency ε = 94% ± 1%. The detector was tested both at the ISOLDE Offline 2 facility and the high resolution separator beam line at the ISOLDE on-line facility where bunched and DC beams from ISCOOL were used.

Detection of sulfur in soil samples using 2.5 MeV neutron activation

Sulfur concentrations in soil samples containing 4.4–13.47 wt% sulphur were measured in neutron inelastic scattering using dc beams of 2.5 MeV neutrons from a DD neutron generator. The measurements were carried out using 2230 keV prompt gamma rays from sulfur using a CeBr3 detector. The minimum detectable concentration (MDC) of sulfur was 0.68 ± 0.21 wt%. The present MDC value is about one-fifth of an earlier reported value of 3.50 wt% for 3.2 MeV neutron beams. This study has shown an improvement in the sulfur MDC achieved using 2.5 MeV neutron beam.

Stability Study in Dielectric Ring Loaded Periodic Interaction Structure for a Megawatt Class Gyrotwystron

In this paper, the stability and design studies of an X-band gyrotwystron using a distributed loss technique at its output waveguide section are presented. The periodic dielectric loading (PDL) in the output section of the amplifier suppresses the self-generated backward-wave parasites including TE11, TE21, and second harmonic TE02 modes. These parasites are investigated using the small-signal linear theory. The PDL technique improves not only the stability of the amplifier but also its gain. The PDL waveguide provides a transmission loss of 0.63 dB/cm to the desired TE01 operating mode at 10 GHz that is good enough to stabilize the operation of gyrotwystron against parasites. The nonlinear simulation predicted an RF output power of ~ 25 MW in the TE01 mode for the dc beam of 220 A, 440 kV.

FRAC: Fringing-RF-field-activated dc-to-pulse converter for low-energy ion beams.

We developed a new type of dc-to-pulse converter, called FRAC (Fringing-RF-field-Activated dc-to-pulse Converter) for low-energy ion beams electrostatically accelerated from an ion source. FRAC is based on a radio-frequency quadrupole (RFQ) linear trap technique and works in principle under ultrahigh vacuum conditions. Ions continuously injected into FRAC are decelerated by an alternating longitudinal electric field produced in a distorted radio frequency field around the edge region of RFQ rods. These ions accumulate in FRAC for a significantly long time. This edge effect appears most notably when the energy of incoming ions exceeds the injection barrier potential by less than a few eV and the energy spread is quite small. The ions stacked during the FRAC operation period are ejected as a high intensity pulsed beam. We investigated the performance of FRAC and the capability of some FRAC operation methods developed to enhance the dc-to-pulse conversion efficiency. The maximum conversion efficiencies achieved were 22% and 5.6% at FRAC operation frequencies of 10 and 1 Hz, respectively. The number of ions contained in an output beam pulse with a duration of 500 μs was in practice 1.6 × 109 ions/pulse at an injected dc beam intensity of 4.6 nA and an operation frequency of 1 Hz.

Automatic detection of recoil-proton tracks and background rejection criteria in liquid scintillator-micro-capillary-array fast neutron spectrometer

We describe an analysis procedure for automatic unambiguous detection of fast-neutron-induced recoil proton tracks in a micro-capillary array filled with organic liquid scintillator. The detector is viewed by an intensified CCD camera. This imaging neutron detector possesses the capability to perform high position-resolution (few tens of μm), energy-dispersive transmission-imaging using ns-pulsed beams. However, when operated with CW or DC beams, it also features medium-quality spectroscopic capabilities for incident neutrons in the energy range 2–20 MeV.In addition to the recoil proton events which display a continuous extended track structure, the raw images exhibit complex ion-tracks from nuclear interactions of fast-neutrons in the scintillator, capillaries quartz-matrix and CCD. Moreover, as expected, one also observes a multitude of isolated scintillation spots of varying intensity (henceforth denoted ”blobs”) that originate from several different sources, such as: fragmented proton tracks, gamma-rays, heavy-ion reactions as well as events and noise that occur in the image-intensifier and CCD. In order to identify the continuous-track recoil proton events and distinguish them from all these background events, a rapid, computerized and automatic track-recognition-procedure was developed.Based on an appropriately weighted analysis of track parameters such as: length, width, area and overall light intensity, the method is capable of distinguishing a single continuous-track recoil proton from typically surrounding several thousands of background events that are found in each CCD frame.

Sawtooth-wave prebuncher with dual-gaps in Linac injector for HIRFL-SSC

An RFQ structure is normally composed of radial matcher, shaper, gentle buncher and accelerator section with changing cell geometry. Bunching is started in the shaper, and adiabatic bunching is done in gentle buncher section. The beam preforms from DC beam to bunch beam through the RFQ and the longitudinal emittance for the ions linacs is defined initially in the RFQ, in which the beam bunch has been shaped. In the present SSC-Linac injector, an RFQ has been designed to accelerate the continuous beam from 3.728 keV/u to 143 keV/u. The heavy ions beam is injected into the SSC (Separated Sector Cyclotron) with the kinetic energy of 1.025 MeV/u after four IH DTLs. The rf frequency of the SSC is 13.417 MHz, and the frequency of the heavy ions RFQ is set to four times of the rf frequency of the SSC. In order to increase the longitudinal capture efficiency of the SSC and suppress the longitudinal emittance at the exit of RFQ, an external MHB (Multi-Harmonics Buncher) is proposed in front of the RFQ. The fundamental frequency of the MHB is the same as the rf frequency of the cyclotron. The scheme of dual-gaps prebuncher with the sawtooth waveform is firstly carried out through multi-harmonics synthetic technology. The multi-particle beam dynamic simulations of the MHB have been done by the BEAMPATH code.

Fundamental cavity impedance and longitudinal coupled-bunch instabilities at the High Luminosity Large Hadron Collider

The interaction between beam dynamics and the radio frequency (rf) station in circular colliders is complex and can lead to longitudinal coupled-bunch instabilities at high beam currents. The excitation of the cavity higher order modes is traditionally damped using passive devices. But the wakefield developed at the cavity fundamental frequency falls in the frequency range of the rf power system and can, in theory, be compensated by modulating the generator drive. Such a regulation is the responsibility of the low-level rf (llrf) system that measures the cavity field (or beam current) and generates the rf power drive. The Large Hadron Collider (LHC) rf was designed for the nominal LHC parameter of 0.55 A DC beam current. At 7 TeV the synchrotron radiation damping time is 13 hours. Damping of the instability growth rates due to the cavity fundamental (400.789 MHz) can only come from the synchrotron tune spread (Landau damping) and will be very small (time constant in the order of 0.1 s). In this work, the ability of the present llrf compensation to prevent coupled-bunch instabilities with the planned high luminosity LHC (HiLumi LHC) doubling of the beam current to 1.1 A DC is investigated. The paper conclusions are based on the measured performances of the present llrf system. Models of the rf and llrf systems were developed at the LHC start-up. Following comparisons with measurements, the system was parametrized using these models. The parametric model then provides a more realistic estimation of the instability growth rates than an ideal model of the rf blocks. With this modeling approach, the key rf settings can be varied around their set value allowing for a sensitivity analysis (growth rate sensitivity to rf and llrf parameters). Finally, preliminary measurements from the LHC at 0.44 A DC are presented to support the conclusions of this work.

Harmonic Content in the Beam Current in a Traveling-Wave Tube

In a klystron, charge overtaking of electrons leads to an infinity of ac current on the electron beam. This paper extends the klystron theory of orbital bunching to a traveling-wave tube (TWT). We calculate the harmonic content of the beam current in a TWT that results from an input signal of a single frequency. We assume that the electron orbits are governed by Pierce’s classical three-wave, linear theory. The crowding of these linear orbits may lead to charge overtaking and, therefore, harmonic generation on the beam current, as in a klystron. We analytically calculate the buildup of harmonic content as a function of tube length from the input, and compare the results with the CHRISTINE code. Good agreement is found. Also found is the surprisingly high level of harmonic contents in the electron beam current, even when the TWT operates in the small signal regime. A dimensionless bunching parameter for a TWT, $X={(2P_{\rm{ in}}/(P_{b} C))}^{1/2}$ , is identified, which characterizes the harmonic content in the ac beam current, where $P_{{\textrm {in}}}$ is the input power of the signal, $P_{b}$ is the dc beam power, and $C$ is Pierce’s gain parameter.

GHz laser-free time-resolved transmission electron microscopy: A stroboscopic high-duty-cycle method

A device and a method for producing ultrashort electron pulses with GHz repetition rates via pulsing an input direct current (dc) electron beam are provided. The device and the method are based on an electromagnetic-mechanical pulser (EMMP) that consists of a series of transverse deflecting cavities and magnetic quadrupoles. The EMMP modulates and chops the incoming dc electron beam and converts it into pico- and sub-pico-second electron pulse sequences (pulse trains) at >1GHz repetition rates, as well as controllably manipulates the resulting pulses. Ultimately, it leads to negligible electron pulse phase-space degradation compared to the incoming dc beam parameters. The temporal pulse length and repetition rate for the EMMP can be continuously tunable over wide ranges.Applying the EMMP to a transmission electron microscope (TEM) with any dc electron source (e.g. thermionic, Schottky, or field-emission source), a GHz stroboscopic high-duty-cycle TEM can be realized. Unlike in many recent developments in time-resolved TEM that rely on a sample pumping laser paired with a laser launching electrons from a photocathode to probe the sample, there is no laser in the presented experimental set-up. This is expected to be a significant relief for electron microscopists who are not familiar with laser systems. The EMMP and the sample are externally driven by a radiofrequency (RF) source synchronized through a delay line. With no laser pumping the sample, the problem of the pump laser induced residual heating/damaging the sample is eliminated. As many RF-driven processes can be cycled indefinitely, sampling rates of 1–50GHz become accessible. Such a GHz stroboscopic TEM would open up a new paradigm for in situ and in operando experiments to study samples externally driven electromagnetically. Complementary to the lower (MHz) repetition rates experiments enabled by laser photocathode TEM, new experiments in the multi-GHz regime will be enabled by the proposed RF design. Because TEM is also a platform for various analytical methods, there are infinite application opportunities in energy and electronics to resolve charge (electronic and ionic) transport, and magnetic, plasmonic and excitonic dynamics in advanced functional materials. In addition, because the beam duty-cycle can be as high as ~10−1 (or 10%), detection can be accomplished by commercially available detectors.In this article, we report an optimal design of the EMMP. The optimal design was found using an analytical generalized matrix approach in the thin lens approximation along with detailed beam dynamics taking actual realistic dc beam parameters in a TEM operating at 200keV.