Beam Recirculation Research Articles

The Continuous Electron Beam Accelerator Facility at 12 GeV

This review paper describes the energy-upgraded Continuous Electron Beam Accelerator Facility (CEBAF) accelerator. This superconducting linac has achieved 12 GeV beam energy by adding 11 new high-performance cryomodules containing 88 superconducting cavities that have operated cw at an average accelerating gradient of 20 MV/m. After reviewing the attributes and performance of the previous 6 GeV CEBAF accelerator, we discuss the upgraded CEBAF accelerator system in detail with particular attention paid to the new beam acceleration systems. In addition to doubling the acceleration in each linac, the upgrade included improving the beam recirculation magnets, adding more helium cooling capacity to allow the newly installed modules to run cold, adding a new experimental hall, and improving numerous other accelerator components. We review several of the techniques deployed to operate and analyze the accelerator performance and document system operating experience and performance. In the final portion of the document, we present much of the current planning regarding projects to improve accelerator performance and enhance operating margins, and our plans for ensuring CEBAF operates reliably into the future. For the benefit of potential users of CEBAF, the performance and quality measures for the beam delivered to each of the experimental halls are summarized in the Appendix. Published by the American Physical Society 2024

Beam recirculation and mode-order conversion via compact Mach-Zehnder-Fano interferometers.

In this Letter, photonic crystal (PC) waveguide-based interferometer design is studied; spectral as well as temporal analyses have been conducted. Intentional structural modifications inside the interferometer trigger Fano resonances, allowing for extraordinary optical effects, such as enhanced beam recirculation and mode-order conversion. The proposed Mach-Zehnder-Fano interferometer is compatible with conventional silicon-on-insulator (SOI) technology and consists of two arms: the lower arm, with no point defects, creates continuum states, whereas the upper arm, including a Fano defect, creates discrete states. The PC waveguide channel with intentional point defects, which possess effective discreteness, creates the required phase retardation of the propagating beam, enabling Fano resonance excitations. Such a type of PC waveguide-based interferometer allows for the realization of compact mode-order converters with a broad 3 dB-bandwidth of 65 nm (at the wavelength range of λ = 1517 - 1582 nm) and efficient optical switching as well as sensing capabilities, operating at optical telecommunication bands, favoring inherent beam recirculation characteristics.

Beam-Breakup Studies for the 4-Pass Cornell-Brookhaven Energy-Recovery Linac Test Accelerator

The Cornell Brookhaven Energy Recovery Linac (ERL) Test Accelerator (CBETA) [1] is currently under construction at Cornell University’s Wilson Laboratory. The primary structures in CBETA for beam recirculation include the Main Linac Cryomodule (MLC) and the Fixed Field Alternating Gradient (FFAG) beamline. As the electron bunches pass through the MLC cavities, higher order modes (HOMs) are excited. The recirculating bunches in turn excite HOMs further, and this feedback loop can give rise to beam breakup instability (BBU). We will first explain how BBU occurs and how we simulate the effect. Then we present the simulation results on how BBU limits the maximum achievable current of CBETA, and the potential ways to improve the threshold current.

Harmonic Frequency Locking in the Multifrequency Recirculating Planar Magnetron

This paper presents experimental results that demonstrate the first known instance of harmonic frequency locking in a magnetron. The prototype crossed-field high-power microwave source, termed the multifrequency recirculating planar magnetron, consisted of two planar cavity arrays having design frequencies near 1 and 2 GHz, respectively, coupled by smooth-bore electron beam recirculation bends. The magnetron was driven by the MELBA-C modified Marx generator using −300-kV pulses supplying up to 3 kA for approximately 300 ns. Using a 0.17-T axial magnetic field, the harmonic frequency-locked state simultaneously produced microwave pulses of 32± 3 MW at 0.984± 0.001 GHz and 13±2 MW at 1.970±0.002 GHz. In addition, changes in the relative phase difference between the magnetron slow wave structures (SWSs) were correlated with changes in the axial magnetic field magnitude. This correlation, along with the results from experiments that tested each individual planar cavity array in isolation, suggested the locking was facilitated through the second harmonic content of the 1-GHz modulated electron beam recirculating from the 1-to 2-GHz SWS. Analysis of shots conducted near 0.17 T showed a ±17° shot-to-shot variation in the average relative phase difference between the SWSs, and the average variation in this phase difference during a locked shot was ±8°.

Thomson scattering upgrade in RFX-mod: alternative designs for injection and collection optics

RFX-mod RFP is now under shutdown and it is undergoing a deep modification in the plasma magnetic boundary, with changes in the vacuum vessel, first wall and stabilizing shell. A diagnostics upgrade is also foreseen; in particular, an improvement in the signal level of the main Thomson Scattering Diagnostics becomes crucial for the diagnostic operation in low density RFP and tokamak discharges. Launching and reflecting optics for a double passage of the laser beam in the equatorial plane is needed, as well as a manipulator for retroreflector substitution and precise positioning. Analysis of the expected energy density on the optics is presented; stray light generation, positioning issues and optical damage risk are examined. Moreover, beam retro-reflection allows in principle re-injection in the edge Thomson Scattering system, lying at another toroidal position; this possibility is analyzed focusing on the reflected beam profile and pointing stability. Alone, laser beam recirculation can provide a doubling in the signal level, while a further 30% gain can be obtained maximizing the collection optics transmission: an alternative, simplified design featuring a 3 elements objective is presented and discussed, the major drawback being a non-planar image surface.

Diagnostics of Local Electron Beam Losses in Microtron-recuperator on Novosibirsk Free Electron Laser Beamline via Detection of Induced X-rays

The Novosibirsk Free Electron Laser (NovoFEL) is based on a four-turn microtron-recuperator. To ensure its stable operation and radiation generation, it is necessary to provide a stable mode of electron beam recirculation and minimize beam losses on the vacuum chamber wall on all the beam way in the accelerator beamline. To this end it is necessary to know the longitudinal distribution of these losses along the beamline.A system for registration of beam losses was created. The system applies optical fibers placed along the vacuum chamber and nearby it. If local electron beam losses occur somewhere in the vacuum chamber, electrons falling to the vacuum chamber wall cause generation of X- and gamma rays. This radiation in turn originates optical emission in an optical fiber nearby the region of electron “precipitation” on the chamber wall. Then, executing some transformation and processing of the time dependencies of signals from these optical fibers, one can obtain the longitudinal distribution of electron beam losses along the accelerator line in all its four turns.

X-ray sources by energy recovered linacs and their needed R&D

In this paper we review the current state of research on energy recovered linacs as drivers for future X-ray sources. For many types of user experiments, such sources may have substantial advantages compared to the workhorse sources of the present: high energy storage rings. Energy recovered linacs need to be improved beyond present experience in both energy and average current to support this application. To build an energy recovered linac based X-ray user facility presents many interesting challenges. We present summaries on the Research and Development (R&D) topics needed for full development of such a source, including the discussion at the Future Light Sources Workshop held in Gaithersburg, Maryland on September 15–17, 2009. A first iteration of an R&D plan is presented that is founded on the notion of building a set of succeedingly larger test accelerators exploring cathode physics, high average current injector physics, and beam recirculation and beam energy recovery at high average current. Our basic conclusion is that a reviewable design of such a source can be developed after an R&D period of five to ten years.

Recent developments in electrostatic accelerator technology at NEC

The 5 MV S-series Pelletron, the terminal ripple reduction system, one knob terminal voltage controller and ion source development will be discussed. The current experiment in MeV dc ampere intensity electron beam recirculation will also be reviewed.

Further studies on beam breakup growth reduction by cavity cross-couplings in recirculating accelerators: effects of long pulse length and multiturn recirculation

Cavity cross-coupling was recently found to reduce beam breakup (BBU) growth in a recirculating accelerator known as the Spiral Line Induction Accelerator (SLIA). Here, we extend the analyses in two respects: long beam pulse lengths and a SLIA upgrade geometry which accelerates a 10 kA, 35 ns beam to 25 MeV via a 70 cavity, 7 turn recirculation. We found that when the beam pulse length τ exceeds the beam's transit time τ′ between cross-coupled cavities. BBU growth may be worsened as a result of the cross-couplings among cavities. This situation is not unlike other long pulse recirculating accelerators where beam recirculation leads to beam breakup of a regenative type. Thus, the advantage of BBU reduction by cavity cross-coupling is restricted primarily to beams with τ<τ′, a condition envisioned for all SLIA geometries. For the 70 gap, 7 turn SLIA upgrade, we found that cavity cross-coupling may reduce BBU growth up to factors of a thousand when the quality factor Q of the deflecting modes are relatively high (like 100). In these high Q cases, the amount of growth reduction depends on the arrangement and sequence of beam recirculation. For Q⪅20, BBU growth reduction by factors of hundreds is observed, but this reduction is insensitive to the sequence of beam recirculation. The above conclusions were based on simple models of cavity coupling that have been used in conventional microwave literature. Not addressed is the detail design consideration that leads to the desired degree of cavity coupling.

Operation of an isochronous beam recirculation system

Abstract The design and performance of a highly achromatic and isochronous system of magnets which transport a pulsed electron beam for the purpose of beam recirculation is described. The beam energy capability of the Bates electron linear accelerator has been increased from 400 to 750 MeV by the addition of this system. The isochronicity is achieved by the use of reverse bends and dipole pole edge rotations. A high degree of symmetry in the optical design and the use of sextuple corrections allow an energy band pass of 6%. The system has been used to supply beam for nuclear physics experiments with average currents of over 30 μA.

Progress Report on Construction and Testing of a 3 MeV, DC, Ampere Intensity Electron Beam Recirculation System

Construction is underway on a 3 MV electrostatic accelerator designed to recirculate D.C. electron beams with intensities up to 4 amperes. The test facility includes a 3 MV, SF6 insulated vertical accelerator with parallel accelerator and decelerator tubes, diagnostic equipment, and vacuum and beam line components to bend the beam 180° and return it to the terminal. Special consideration has been given to cathode, electron gun, electron collector, and optical design to allow beam recovery efficiency of better than 99.99% at currents up to 4 amperes.

Accelerator Development at Bates

The past year has seen the completion of a major expansion of the Bates Accelerator Laboratory. A second experimental hall, South Hall, and several magnetic spectrometers have been constructed. The accelerator's maximum energy has been raised from 400 to 750 MeV by means of beam recirculation. A current two-year project for the fabrication of an additional RF transmitter plus a 30% increase in RF peak power capability will increase energy further to ca. 1 GeV. During the same period pulse-to-pulse beam sharing between the high-resolution spectrometer area and South Hall will become available. In January 1981 the Laboratory submitted their ''Proposal for a Long-Range Expansion Program'' to DOE-NSF. The proposed development consists of three stages. Stage I calls for the addition of a pulse stretcher ring to furnish a CW beam to the South Hall beam lines. Additional experimental space for internal target experiments and photon tagging research are also included. Stage II increases the accelerator energy to 2.1 GeV (at 140 microamps) by means of a five-pass head-to-tail recirculator. Stage III is, at this time, a plan rather than a proposal. It increases accelerator energy to 4 GeV by extending the accelerator length and power and adds another pulsemore » stretcher ring and three new experimental areas for the higher energy work. This paper discusses each of these stages in detail and recommends their funding and scheduling.« less

Progress on the NBS-LANL CW Microtron

The NBS-LANL racetrack microtron (RTM) currently under construction at the National Bureau of Standards is a demonstration accelerator to determine the feasibility of, and to develop the technology necessary for building high-energy, high-current, continuous beam (CW) electron accelerators using beam recirculation through room temperature rf accelerating structures. Parameters of the RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15 or 16; final beam energy - 185-197 MeV; maximum current - 550 #x003BC;A; rf frequency - 2380 MHz. At present, the electron gun and 100 keV beam transport line are operational, and most other major subsystems are in the construction or installation phase. Exceptions are the rf structure (under development), the 5 MeV beam transport line (in engineering design), and the extraction beam line (in conceptual design). Our studies of the original candidate accelerating structure, the disk-and-washer, have led to the discovery of beam steering modes which render this structure unsuitable for the RTM without at least substantial further development beyond the scope of the project. The most promising alternate for meeting the design goal of CW operation at 1.5 MeV/m is the side-coupled structure. A shunt impedance of 80 MΩ/m has been measured in a test section of side-coupled structure at 2380 MHz, adequate cooling has been designed, and a 2.7 m long section of this design is under construction. The electron optics of the RTM have been studied in detail.

The NBS-LASL CW Microtron

The NBS-LASL racetrack microtron (RTM) is a joint research project of the National Bureau of Standards and the Los Alamos Scientific Laboratory. The project goals are to determine the feasibility of, and develop the necessary technology for building high-energy, high-current, continuous-beam (cw) electron accelerators using beam recirculation and room-temperature rf accelerating structures. To achieve these goals, a demonstration accelerator will be designed, constructed, and tested. Parameters of the demonstration RTM are: injection energy - 5 MeV; energy gain per pass - 12 MeV; number of passes - 15; final beam energy - 185 MeV; maximum current 550 ?A. One 450 kW cw klystron operating at 2380 MHz will supply rf power to both the injector linac and the main accelerating section of the RTM. The disk and washer standing wave rf structure being developed at LASL will be used. SUPERFISH calculations indicate that an effective shunt impedance (ZT2) of about 100 M?/m can be obtained. Thus, rf power dissipation of 25 kW/m results in an energy gain of more than 1.5 MeV/m. Accelerators of this type should be attractive for many applications. At beam energies above about 50 MeV, an RTM should be considerably cheaper to build and operate than a conventional pulsed rf linac of the same maximum energy and time-average beam power. In addition, the RTM provides superior beam quality and a continuous beam which is essential for nuclear physics experiments requiring time-coincidence measurements between emitted particles.

Initial Operation of the MIT Electron Linear Accelerator

Initial operating experience with the various high power sub-systems is discussed, and the results of electron beam tests at progressive stages of completion of the linac are presented. A highly stable analyzed beam containing greater than 85% of the accelerated current has been demonstrated routinely using ±0.11% analyzing slits at both the 20 MeV and 120 MeV stations. Of particular importance, especially for beam transportation and future considerations of beam recirculation, has been the achievement of an extremely low emittance. Beam measurements at the 20 MeV station have confirmed that essentially all of the accelerated current is contained within an emittance of 7±2×10-4 πmoc-cm; i.e., ~0.2 mm mrad at 20 MeV. System checkout is currently in progress to bring the linac up to its maximum operating condition of 430 MeV at a duty factor of 1.8 per cent.