Sergey S. Kurennoy

Meander-line current structure for SNS fast beam chopper

A new current structure for the fast traveling-wave 2.5 MeV beam chopper for the Spallation Neutron Source (SNS) project has been proposed. It is based on the meander-folded straight or notched stripline with separators. Detailed electromagnetic modeling with MAFIA has been used to optimize the structure design and parameters. The time-domain 3D MAFIA simulations predict the structure rise and fall times around 1 ns. A notched meander line with a dielectric substrate has been developed to accommodate both electromagnetic and mechanical requirements.

Beam coupling impedances of obstacles protruding into beam pipe

The beam coupling impedances of small obstacles protruding inside the vacuum chamber of an accelerator are calculated at frequencies for which the wavelength is large compared to a typical size of the obstacle. Formulas for a few important particular cases including both essentially three-dimensional objects like a post or a mask and axisymmetric irises, are presented. These results allow simple practical estimates of the broadband impedance contributions from such discontinuities. {copyright} {ital 1997} {ital The American Physical Society}

Efficient accelerating structures for low-energy light IONS

The radio-frequency quadrupole (RFQ) accelerator is the best structure immediately after an ion source for accelerating light-ion beams with considerable currents. On the other hand, the higher-energy part of the RFQ is known to be not a very efficient accelerator. We consider alternative room-temperature RF accelerating structures for the beam velocities in the range of a few percent of the speed of light - including H-mode cavities and drift-tube linacs - and compare them with respect to their efficiency, compactness, ease of fabrication, and overall cost. Options for the beam transverse focusing in such structures are discussed. Possible applications include a compact deuteron-beam accelerator up to the energy of a few MeV for homeland defense.

Progress with SNS fast beam chopper

The fast traveling-wave 2.5 MeV beam chopper for the Spallation Neutron Source (SNS) project has a new current structure based on the meander-folded notched stripline with separators. Measurements of the full-length structure prototype showed a good agreement with predictions of MAFIA electromagnetic modeling. The design of the final chopper units is completed, and they are being manufactured. Here we report on the final design of the chopper and on results of recent thermal and vacuum tests of the prototype chopper structure.

Normal-Conducting High Current RF Photoinjector for High Power CW FEL

An RF photoinjector capable of producing high average current with low emittance and energy spread is a key enabling technology for high power CW FEL. The design of a 2.5-cell, -mode, 700-MHz normal-conducting RF photoinjector cavity with magnetic emittance compensation is completed. With average gradients of 7, 7, and 5 MV/m in its three accelerating cells, the photoinjector will produce a 2.5-MeV electron beam with 3-nC charge per bunch and transverse rms emittance below 7 mm-mrad. Electromagnetic modeling has been used extensively to optimize ridge-loaded tapered waveguides and RF couplers, and led to a new, improved coupler iris design. The results, combined with a thermal and stress analysis, show that the challenging problem of cavity cooling can be successfully solved. Fabrication of a demo 100-mA (at 35 MHz bunch repetition rate) photoinjector is underway. The design is scalable to higher average currents by increasing the electron bunch repetition rate, and provides a path to a MW-class FEL. This paper presents the cavity design and details of RF coupler modeling.

Beam diagnostic suite for the SNS linac

The Spallation Neutron Source (SNS) is the next-generation pulsed neutron source to be built in the United States. The accelerator chosen to produce the 2 MW beam power on the neutron-producing target is an H− linear accelerator (linac) to 1 GeV, followed by a proton accumulator ring. The ring compresses the 1 ms long beam bunches from the linac to less than 1 μs. The linac is pulsed at 60 Hz with a 6% duty factor. Stringent control of the pulse structure and stability of the high-intensity H− beam is needed to minimize beam loss in the linac and to optimize injection into the accumulator ring. This requires a set of beam diagnostics that can operate at high peak currents (∼52 mA) with high sensitivity and minimum beam interception.

Electromagnetic modeling of beam position and phase monitors for SNS linac

Electromagnetic modeling of the beam position monitors (BPMs) for the Spallation Neutron Source (SNS) linac has been performed with MAFIA. The signal amplitudes and phases on the BPM electrodes are computed as functions of the beam transverse position using time-domain 3-D simulations with an ultra-relativistic beam. An analytical model is then applied to extrapolate the results to lower beam velocities. It is shown that while the signal phases on the individual electrodes for an off-axis beam can differ from those for a centered beam by a few degrees, the phase of the summed signal from all electrodes is insensitive to the beam transverse position inside the device. Based on the analysis results, an optimal BPM design with 4 one-end-shorted 60-degree electrodes has been chosen. It provides a very good linearity and sufficient signal power for both position and phase measurements, while satisfying the linac geometrical constrains and mechanical requirements.

LANSCE prototype beam position and phase monitor (BPPM) mechanical design

A prototype Beam Position and Phase Monitor (BPPM) beam line device is being designed to go in the LANSCE 805-MHz linear accelerator. The concept is to install two beam line devices in locations where their measurements can be compared with older existing Delta-T loop and wire scanner measurements. The purpose for the new devices is to measure the transverse position, angular trajectory, and central beam phase and energy of the LANSCE H+ and H- beams. The mechanical design of the new devices will combine features from previous LANL designs that were built for the LANSCE Isotope Production Facility, LANSCE Switchyard project, and those done for the SNS linear accelerator. This paper will discuss the mechanical design and fabrication issues encountered during the course of developing the BPPM.

CW RF Cavity Design for High-Average-Current Photoinjector for High Power FEL

This project is a coordinated effort among NAVSEA, LANL and AES, to develop a key enabling technology for high-power FEL called for by the High Energy Laser Joint Technology Office: a high current RF photoinjector capable of producing continuous average current greater than 100 mA. The specific aim is a π-mode, normal-conducting RF photoinjector, 5 nC of bunch charge, 100 mA of current (at 21.88 MHz bunch repetition rate) and emittance less than 10 mm-mrad. This level of performance will enable robust 100-kW-class FEL operation with electron beam energy <100 MeV, thereby reducing the size and cost of the FEL. This design is scalable to the MW power level by increasing the electron bunch repetition rate from 21.88 MHz (the 32nd sub-harmonic of 700 MHz) to a higher value. The major challenges are emittance control and high heat flux within the CW 700 MHz RF cavities. Preliminary results of RF cavity designs and cooling schemes are presented, including both high-velocity water and liquid-nitrogen cooling options. PACS codes: 41.60. Cr, 29.17+w, 29.27. Bd, 41.75. Fr

High-power electron beam injectors for 100 kW free-electron lasers

A key technology issue on the path to high-power FEL operation is the demonstration of reliable, high-brightness, high-power injector operation. We describe two ongoing programs to produce 100 mA injectors as drivers for 100 kW free-electron lasers. In one approach, in collaboration with the Thomas Jefferson National Accelerator Facility, we are fabricating a 750 MHz superconducting RF cryomodule that will be integrated with a room-temperature DC photocathode gun and tested at the Laboratory. In the other approach, in collaboration with Los Alamos National Laboratory, a high-current 700 MHz, normal-conducting, RF photo-injector is being designed and will undergo thermal management testing at the Laboratory. We describe the design, the projected performance and the status of both injectors.