Lawrence J. Rybarcyk

High power RF conditioning of the LEDA RFQ

We are preparing the radio frequency quadrupole (RFQ) for the Low Energy Demonstration Accelerator (LEDA) to accelerate beam. The LEDA RFQ accelerates a 100-mA CW proton beam from 75 keV to 6.7 MeV. We report our experience with high-power RF conditioning the RFQ, first with one klystron and then with two klystrons. The RFQ will dissipate 1.2 megawatts of RF power at design fields. This 350-MHz CW RFQ has peak fields on the vane tips of 33 MV/m. The average power dissipation is 13 watts/cm/sup 2/ on the outer walls of the RFQ near the high energy end. The power from each klystron is split 4 ways to lower the stress on the RF windows. Each klystron can produce 1.3 megawatts of RF power.

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.

Commissioning of the Low Energy Demonstration Accelerator (LEDA) radiofrequency quadrupole (RFQ)

Initial commissioning of a 6.7-MeV 100-mA RFQ is underway. The RFQ is part of LEDA, the He injector for the Accelerator Production of Tritium (APT) project. To benchmark the RFQ performance, beam physics experiments will be done with low and high current beams for both pulsed and cw beam operation. Commissioning efforts thus far have been limited to low-current pulsed beam LEDA operation. Measurements to fully characterize the RFQ will ultimately include the dependence of RFQ beam transmission on RFQ vane voltage, input beam energy, input match, and input transverse centroids. Other commissioning measurements for the RFQ will include output beam energy, phase, noise, transverse profiles, and transverse RMS emittances. This paper contains initial LEDA RFQ commissioning results, including RFQ pulsed output beam currents up to 40 mA.

Commissioning plan for a high-current proton linac

High-power proton linacs (E>500 MeV) are potentially useful for transmutation applications, such as the production of tritium, In production applications, high availability is essential. Achieving high availability requires an accelerator design that simplifies maintenance and accommodates commissioning procedures designed to minimize tune-up time. These are worthwhile goals for any accelerator, but the high beam powers (170 MW) and heavy beam loading of the Accelerator Production of Tritium (APT) linac introduce significant new challenges. This paper describes the commissioning plan, as developed to date.

Simulation studies of the LAMPF proton linac

The work presented here is an extension of our previous work. We have attempted to do a more complete simulation by including modeling of the low-energy beam transport (LEBT). No measurements of the longitudinal structure of the beam, except phase-scans, are performed at LAMPF. Transverse measurements include slit and collector emittance measurements, and wire scans to determine beam size and centroids. Comparison of simulations to beam loss data suggest that the primary causes of beam spill are incomplete longitudinal capture and the lack of longitudinal matching. Measurements to support these claims are not presently made at LAMPF. However, agreement between measurement and simulation for the transverse beam properties and transmissions serve to benchmark the simulations.

Development of a commissioning plan for the APT linac

The Accelerator Production of Tritium (APT) facility utilizes a high-intensity CW linear accelerator consisting of both normal-conducting and superconducting (SC) RF structures to accelerate a 100-mA CW proton beam to an energy of between 1030 and 1700 MeV depending upon tritium production needs. The accelerator will be commissioned in stages defined by these different normal and superconducting modules. Different commissioning modes developed to set the transverse and longitudinal beam parameters, require pulsed operation of the accelerator over a wide range of beam currents. These stages and modes and the different techniques utilized to tune the phase and amplitude of the modules are described. Beam dynamics simulations of the tuning process for the phase and amplitudes of the RF structures in the low energy (LE) linac will be presented.

Space charge in proton linacs

Space charge effects on beam dynamics in linear accelerators are discussed. Practical linac beam dynamics calculation methods which include space charge effects are discussed. Also, the status of beam performance experiments including space charge studies are summarized.

A new bunching scheme for increasing the LANSCE WNR peak beam current

The LANSCE linac simultaneously provides both H- and H+ beams to several user facilities. The Weapons Neutron Research (WNR) user facility is configured to accept the H- beam with a typical pulse pattern of one linac micro-pulse every 1.8 microseconds. This pattern is produced through a combination of chopping and bunching in the 750 keV beam transport. One downside of the chopping process is that the majority of the beam produced by the ion source during each WNR macro- pulse is discarded. By applying a longitudinal bunching action immediately following the ion source, simulations have shown that some of this discarded beam can be used to increase the charge in these micro-pulses. Recently, we began an effort to develop this buncher by superimposing 16.77 MHz RF voltage on one of the HVDC electrodes in the 80 kV column adjacent to the source. This paper describes the beam dynamics simulations, design and implementation of the RF hardware and the results of tests performed with the system.

Proposed beam diagnostics instrumentation for the LANSCE refurbishment project

Presently, the Los Alamos National Laboratory is in the process of planning a refurbishment of various subsystems within its Los Alamos Neutron Science Center accelerator facility. A part of this LANSCE facility refurbishment will include some replacement of and improvement to existing older beam-diagnostics instrumentation. While plans are still being discussed, some instrumentation that is under improvement or replacement consideration are beam phase and position measurements within the 805-MHz side-coupled cavity linac, slow wire profile measurements, typically known as wire scanners, and possibly additional installation of fast ionization-chamber loss monitors. This paper will briefly describe the requirements for these beam measurements, what we have done thus far to answer these requirements, and some of the technical issues related to the implementation of the instrumentation.

Electron cloud generation and trapping in a quadrupole magnet at the Los Alamos PSR

A diagnostic to measure electron cloud formation and trapping in a quadrupole magnet has been developed, installed, and successfully tested at PSR. Beam studies with this diagnostic show that the electron flux striking the wall in the quadrupole is comparable to or larger than in an adjacent drift. In addition, the trapped electron signal, obtained using the sweeping feature of diagnostic, was larger than expected and decayed very slowly with an exponential time constant of 50 to 100 mus. Experimental results were also obtained which suggest that a significant fraction of the electrons observed in the adjacent drift space were seeded by electrons ejected from the quadrupole.