RF buncher cavity for polarized He-3 beam at BNL
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43 PARTICLE ACCELERATORS↗
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A low-frequency buncher field-variation study on a 750 keV H – beam to increase Drift Tube Linac capture
We report low-frequency (such as 16.77 MHz) RF bunchers are widely used in RF accelerator systems for longitudinal compression of pulses into a single RF bucket, which increases instantaneous beam intensity for time-dependent studies. In this study, the dependency of capture into a 201.25 MHz Drift Tube Linac (DTL) was measured as a function of gap voltage for a 16.77 MHz buncher on chopped H - beam (approximately 25 ns at 750 keV, 10 mA peak current). The multiparticle code PARMILA was used to simulate the phase-space distribution of the 10 mA, 750 keV, H - beam at the entrance to DTL with a wide range of the Low-Frequency Buncher (LFB) field (10 kV to 35 kV). The measurement and simulation indicated that the DTL capture could be dilute (reduced) for a non-optimized buncher field to a pre-configured beamline geometry. The data shows that changing the bunch field while keeping the incoming beam current and energy constant does not significantly alter the beam’s emittance. However, downstream beam capture into the DTL is changed for a non-optimized phase-space bunching distance with the buncher field.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Study of the effect of loop inductance on the RF transmission line to cavity coupling coefficient
Coupling of RF power is an important aspect in the design and development of RF accelerating structures. RF power coupling employing coupler loops has the advantage of tunability of β, the transmission line to cavity coupling coefficient. Analytical expressions available in literature for determination of size of the coupler loop using Faraday’s law of induction show reasonably good agreement with experimentally measured values of β below critical coupling (β ≤ 1) but show large deviation with experimentally measured values and predictions by simulations for higher values of β. In actual accelerator application, many RF cavities need to be over-coupled with β > 1 for reasons of beam loading compensation, reduction of cavity filling time, etc. This paper discusses a modified analytical formulation by including the effect of loop inductance in the determination of loop size for any desired coupling coefficient. The analytical formulation shows good agreement with 3D simulations and with experimentally measured values. It has been successfully qualified by the design and development of power coupler loops for two 476 MHz pre-buncher RF cavities, which have successfully been conditioned at rated power levels using these coupler loops.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Optimization of a buncher for High Energy H- Beam Transport [Slides]
Accelerators and beam transport systems typically contain numerous beamline devices, diagnostics, system study, and upgrade. Routinely TRACE, MADX and TRANSPORT codes are used for the beam envelope prediction and optimization. Unknown problems of accelerators and beam-transport are addressed based on R&D, such as LFB voltage variation effect study. A nominal diagnostic have used in non-traditional beam transport path to address scientific question (such as use of a BPM in the air). High energy beam operation is a methodical process and routine R&D is required to maintain high standards to deliver beam.
43 PARTICLE ACCELERATORS↗
Multi-beam linear accelerator EVT
A novel electron multi-beam accelerator is presented. The accelerator, short-named EVT (Electron Voltage Transformer) belongs to the class of two-beam accelerators. It combines an RF generator and essentially an accelerator within the same vacuum envelope. Drive beam-lets and an accelerated beam are modulated in RF modulators and then bunches pass into an accelerating structure, comprising uncoupled with each other and inductive tuned cavities, where the energy transfer from the drive beams to the accelerated beam occurs. A phasing of bunches is solved by choice correspond distances between gaps of the adjacent cavities. Preliminary results of numerical simulations and the initial specification of EVT operating in S-band, with a 60 kV gun and generating a 2.7 A, 1.1 MV beam at its output is presented. Furthermore, a relatively high efficiency of 67% and high design average power suggest that EVT can find its use in industrial applications.
43 PARTICLE ACCELERATORS↗
MEBT Bunchers: System Design Document (SDD)
The LAMP Medium Energy Beam Transport (MEBT) transfers bunched beam at the energy 3 MeV from RFQ to the Drift-Tube Linac (DTL) entrance. The beam particles (protons or H- ) in the LAMP MEBT have velocity β = v/c = 0.08, where v is the beam velocity, c is the speed of light. The MEBT bunchers keep beam bunches from spreading longitudinally as they propagate through the MEBT, where some unwanted bunches are removed by a chopper to create a required beam pattern. The MEBT bunchers are RF cavities operating at the frequency 201.25 MHz; possible design options were considered in.
43 PARTICLE ACCELERATORS↗
Charge per Micro-Pulse Calculation for Ideal Lujan and WNR beams
A complete redesign of the LANSCE front-end is currently under development for the Los Alamos Modernization Project (LAMP). This includes the replacement of the traditional Cockroft-Walton injection system to the newer radiofrequency quadrupole (RFQ) standard. LANSCE accelerates both H- and H+ beams and therefore requires an unconventional application for RFQ systems. A new facility could consider using an independent RFQ for each beam species. However, with the tight space of the LANSCE injector facility, it would prove difficult to instrument. Our initial studies have shown that a single RFQ could be used. The negatively- and positively-charged direct current (DC) beams are shaped, bunched, and accelerated out of phase to each other, with very little interaction. This would work for all possible H+ beam gates and most H- Beam gates. However, a challenge arises with the Weapons Neutron Research (WNR) beam structure that requires the acceleration of bunched beams at higher charge density per pulse. The Long Bunch Enable Gate (LBEG) used for the Lujan center is the best example of a standard injector beam gate. The beam gate, or Macro-Pulse (MP), is 625-us long, separated by at least 8.3 ms. A chopper is used to dice the MP into 1750 mini-Pulses (mP), which are 290-ns wide. After chopping, the MP contains 100,000 micro-Pulses (uP), which are generated by the linac acceleration structure modulated at 201.25 MHz; that is, the micro-Pulses are separated 5 ns apart from each other. The H+ beam structure will be very similar once a chopper is added to its transport. However, the Multi-Pulse Enable Gate (MPEG) used for WNR is significantly different in structure. In this case, the mP is chopped to be only 35-ns wide. This width is then rotated into a single 201.25 MHz RF bucket using the Low Frequency Buncher (LFB). This is proving difficult for the RFQ design in the LAMP upgrade. We began investigating the amount of current sent to WNR for two reasons. Firstly, we wanted to find a solution to avoid using the Low Frequency Buncher to simplify the LAMP RFQ design. It is simple to inject and accelerate a 35 mA DC beam in the LAMP RFQ (this is done in numerous accelerator facilities). However, injecting a short bunch with a larger current (once compressed to 5 ns to meet the RFQ frequency) may be difficult. Second, we have a technique called PSR2WNR in which we do not employ the Low Frequency Buncher, but rather accumulate 5 LBEG uP (5 ns) in the PSR and deliver the accumulated beam to WNR every 1.8 us. As a result, we could provide to WNR with five accumulated LBEG uPs every shot. The goal here is to create a rapid kicker with a frequency response of 555 kHz. DARHT-II and other accelerator facilities have constructed kickers of this type for other uses, and our engineers consider this technology as feasible.
43 PARTICLE ACCELERATORS↗
Four-harmonic buncher for radioactive and stable beams switching at the ATLAS facility [Four-harmonic buncher for radioactive and stable beams combination at the ATLAS facility]
Here, the Argonne Tandem Linac Accelerator System (ATLAS) is planning an upgrade to a multi-user facility to simultaneously accelerate both stable beams from an ECR ion source and radioactive beams from an Electron Beam Ion Source charge breeder. The ATLAS Multi-User Upgrade will need to provide high transmission of ion beams with a mass-to-charge ratio up to 7 as it enables more options to alternate radioactive and stable beams. Currently, the bunching is provided with a lumped circuit multi-harmonic buncher, which cannot effectively operate at the required amplitude of the saw-tooth voltage for ions with mass-to-charge ratio of 7 due to thermal issues. RadiaBeam Systems in collaboration with Argonne National Laboratory has designed a four-harmonic coaxial resonator buncher with the ATLAS fundamental frequency of 12.125 MHz, and a compact, less than 2-m-long, footprint, capable of providing the required voltage of 6.2 kV. This device allows fast and reliable pre-bunching of DC ion beams with the capability of fast switching of heavy-ion radioactive and stable beams for delivery to multiple users. In this paper, we will discuss the RF and engineering design considerations of this buncher.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
First on-line commissioning experiments at the St. Benedict facility
Nuclear beta decays provide an excellent probe of fundamental symmetries due to their mediation by the weak interaction. In particular, precise measurements of these decays provide constraints on the unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) quark-mixing matrix. While superallowed pure Fermi decays currently set the most precise limits, the alternative suite of superallowed mixed mirror decays has been ill-studied. These nuclei can provide an important consistency check of calculation and measurement methods employed for the pure Fermi decays, more critically needed now in the wake of a 2.4σ deviation from unitarity of the CKM matrix. In order to remedy the gap in data for mirror decays, the Superallowed Transition Beta-Neutrino Decay Ion Coincidence Trap (St. Benedict) facility is being commissioned at the University of Notre Dame’s Nuclear Science Laboratory (NSL). In this paper, we present first results of the commissioning of the St. Benedict facility on-line at the TwinSol radioactive beam facility. The results of initial commissioning experiments involving the St. Benedict gas catcher, RF carpet, RFQ ion guide and RFQ cooler-buncher will be presented.
beta decay↗
LLRF System Analysis for the Fermilab PIP-II Superconducting LINAC
PIP-II is a superconducting linac that is in the initial acceleration chain for the Fermilab accelerator complex. The RF system consists of a warm front-end with an RFQ and buncher cavities along with 25 superconducting cryo-modules comprised of cavities with five different acceleration \(β\). The LLRF system for the linac has to provide field and resonance control for a total of 125 RF cavities. Various components of the LLRF system have been tested with and without beam at the PIP-II test stands. The LLRF system design is derived from the LCLS-II project with its self-excited loop architecture used in the majority of the cryo-modules. The PIP-II beam loading at 2 mA is much higher than the LCLS-II linac. The control system architecture is analyzed and evaluated for the operational limits of feedback gains and their ability to meet the project regulation requirements for cavity field amplitude and phase regulation.
Varghese, P. [Fermilab] (ORCID:0009000925131983)↗
LLRF System for the Fermilab PIP-II Superconducting LINAC
PIP-II is an 800 MEV superconducting linac that is in the initial acceleration chain for the Fermilab accelerator complex. The RF system consists of a warm front-end with an ion source, RFQ and buncher cavities along with 25 superconducting cryo-modules comprised of five different acceleration \(\beta\). The LLRF system for the LINAC has to provide field and resonance control for a total of 125 RF cavities.The LLRF system design is in the final design review phase and will enter the production phase next year. The PIP-II project is an international collaboration with various partner labs contributing subsystems. The LLRF system design for the PIP-II Linac is presented and the specification requirements and system performance in various stages of testing are described in this paper.
43 PARTICLE ACCELERATORS↗
RF pulse shape control in the compact linear collider test facility
The Compact Linear Collider (CLIC) is a study for an electron–positron machine aiming at accelerating and colliding particles at the next energy frontier. The CLIC concept is based on the novel two-beam acceleration scheme, where a high-current low-energy drive beam generates RF in series of power extraction and transfer structures accelerating the low-current main beam. To compensate for the transient beam-loading and meet the energy spread specification requirements for the main linac, the RF pulse shape must be carefully optimized. This was recently modelled by varying the drive beam phase switch times in the sub-harmonic buncher so that, when combined, the drive beam modulation translates into the required voltage modulation of the accelerating pulse. In this paper, the control over the RF pulse shape with the phase switches, that is crucial for the success of the developed compensation model, is studied. The results on the experimental verification of this control method are presented and a good agreement with the numerical predictions is demonstrated. Implications for the CLIC beam-loading compensation model are also discussed.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
LCLS-II Gun/Buncher LLRF System Design
For a free electron laser, the stability of injector is critical to the final electron beam parameters, e.g., beam energy, beam arrival time, and eventually it determines the photon quality. The LCLS-II project's injector contains a VHF copper cavity as the gun and a two-cell L-band copper cavity as its buncher. The cavity designs are inherited from the APEX design, but requires more field stability than demonstrated in APEX operation. The gun LLRF system design uses a connectorized RF front end and low noise digitizer, together with the same general purpose FPGA carrier board used in the LCLS-II SRF LLRF system. The buncher LLRF system directly adopts the SRF LLRF chassis design, but programs the controller to run the normal conducting cavities. In this paper, we describe the gun/buncher LLRF system design, including the hardware design, the firmware design and bench test.
43 PARTICLE ACCELERATORS↗
The N = 126 factory: A new facility to produce very heavy neutron-rich isotopes
A new facility, the N = 126 factory, is currently under construction at Argonne National Laboratory. It will use multi-nucleon transfer reactions to create neutron-rich isotopes of very heavy elements for studies of interest to the formation of the A similar to 195 abundance peak in the r-process. This region of the nuclear chart is difficult to access by standard fragmentation or spallation reactions and as a result has remained mostly unexplored. The nuclei of interest, very neutron-rich isotopes around Z = 70-95, will be produced by multi-nucleon exchange of a high intensity 10 MeV/u heavy-ion beam on the most neutron-rich stable isotopes of heavy elements such as 198 Pt and 238 U. This reaction mechanism can transfer a large number of neutrons and create very neutron-rich isotopes with larger than mb cross-section. The reaction mechanism is a nuclear surface process and the reaction products come out close to the grazing angle, which makes them very difficult to collect. The N = 126 factory circumvents this difficulty by using a unique large high-intensity RF gas catcher, similar to the one currently in operation at CARIBU, to collect the target-like reaction products and transform them into a low-energy beam that will then be mass separated with a medium resolution electromagnetic separator (Delta M/M similar or equal to 1/1500), followed by an RFQ buncher and an MR-TOF (Delta M/M similar or equal to 1/100,000) system. The extracted radioactive beams will be essentially pure and available at low-energy for mass measurements with the CPT mass spectrometer or decay study with the X-array.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
Modeling a Nb3Sn Cryounit in GPT at UITF
Nb₃Sn is a prospective material for future superconducting RF (SRF) accelerator cavities. The material can achieve higher quality factors, higher temperature operation and potentially higher accelerating gradients (E_{acc} 96 MV/m) compared to conventional niobium. In this work, we performed modeling of the Upgraded Injector Test Facility (UITF) at Jefferson Lab utilizing newly constructed Nb₃Sn cavities. We studied the effects of the buncher cavity and varied the gun voltages from 200-500 keV. We have calibrated and optimized the SRF cavity gradients and phases for the Nb₃Sn five-cell cavities energy gains with the framework of General Particle Tracer (GPT). Our calculations show the beam goes cleanly through the unit. There is full energy gain out of the second SRF cavity but not from the first SRF cavity due to non-relativistic phase shifts.
Pokharel, S.↗
Comparison of Feschenko BSM and Fast Faraday Cup with Low Energy Ion Beams
A comparison between Fast Faraday Cup and Feschenko longitudinal bunch shape detectors was recently performed at HELIAC Advanced Demonstrator beamline at GSI. Feschenko bunch shape monitor (BSM) uses the time to space conversion by means of secondary electrons emitted from a wire correlated to a rf deflector, while the fast Faraday cup (FFC) measures the deposited charge in a cup geometry matched to 50 Ohm. The FFC design aims to minimize the bunch shape dilution due to field polarization and secondary electrons produced on irradiation. An He¹⁺ with 100 µA average current and 1.4 MeV/u kinetic energy is utilized for this comparison. A buncher upstream of the detectors was operated to focus the beam longitudinally. The results are discussed in this contribution.
43 PARTICLE ACCELERATORS↗
Modeling for the phased injector upgrade for 12 GeV CEBAF
As a follow on to the 12 GeV upgrade to the Continuous Electron Beam Accelerator Facility, the front end of the DC photo-gun-based injector has gone through a phased upgrade. The first phase focused on the beamline between the gun and the RF chopper system, and the second phase addresses the beamline after the RF chopper system including replacing the capture section and quarter cryomodule with a new booster module containing a 2-cell and 7-cell cavity string. Throughout the design process, we maintained and developed three models, one for the existing injector and one for each of the upgrade phases. With these models, we evaluated proposed hardware upgrades, evaluated and determined optimized beamline element positions, developed buncher voltage requirements, and settings for optimal injector running. In this paper, we will describe the models and results from these various studies and provide a brief summary of Phase 1 commissioning.
Kazimi, R.↗
Phase space manipulations for optimization of an inverse free electron laser
The Inverse Free Electron Laser (IFEL) has developed into a mature advanced accelerator with the ability to produce mono energetic high brightness beams accelerated at gradients beyond those of conventional RF technology. By copropagating a relativistic electron beam and a high power laser in a strongly tapered undulator, resonant energy exchange can be maintained over large distances. To optimize the number of electrons accelerated in an IFEL, one must prepare the initial electron beam phase space such that a majority of particles are injected at the accelerating phase of the ponderomotive potential. We present a scheme for optimization of the Rubicon IFEL accelerator trapping by preparing the initial electron beam phase space. Utilizing two cascaded modulator-chicane pre-bunchers simulations predict 95% capture of the injected electron beam.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗