Rodney C. McCrady

Status of the experimental studies of the electron cloud at the Los Alamos proton storage ring

The electron cloud (EC) at the Los Alamos Proton Storage Ring (PSR) has been studied extensively for the past several years with an overall aim to identify and measure its important characteristics, the factors that influence these characteristics, and to relate these to the two-stream (e-p) transverse instability long observed at PSR. Some new results since PAC2001 are presented

Experimental tests of a prototype system for active damping of the e-p instability at the lanl PSR

A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos national laboratory proton storage ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). Evidence obtained from these tests suggests that further improvement in performance is limited by beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizontal plane, which had no feedback. Here we describe the present system configuration, system optimization, results of several recent experimental tests, and results from studies of factors limiting its performance.

Stripper foil temperatures and electron emission at the Los Alamos proton storage ring

We have modeled the heating process of the PSR stripper foil and compared our results to observations that depend on the foil temperature. The foil is heated by the energy deposited by injected H - ions and stored protons passing through the foil. Secondary emission of electrons due to these foil hits results in a measurable current that we can use to benchmark our model. At higher beam intensities, thermionic emission of electrons dominates the foil current. Due to the extreme temperature dependence of the thermionic current this is a very sensitive indicator of the foil temperature and will be used to safeguard against overheating the foil in extreme beam conditions. We will present our best estimates of the foil temperature for different beam intensities

Halo measurements of the extracted beam at the Los Alamos Proton Storage Ring

The spatial beam density distribution beyond 2.5 to 3 standard deviations of the beam center is an important property for understanding the relatively small fractional losses of high intensity beams at the Los Alamos Proton Storage Ring (PSR) and transport lines to the neutron production target. This part of the distribution (sometimes referred to as beam halo) is not well determined by the LANSCE-standard wire scanner system nor is it yet reliably predicted by the simulation codes. To significantly improve the experimental determination of the beam halo, an improved wire scanner has been developed, tested and installed in the extraction line. To enhance the signal-to-noise ratio, an amplifier consisting of a wide dynamic-range, integrating amplifier, sample-and-hold circuit, log amplifier and line driver is located near the beam line. Offset errors at the input of the amplifiers are actively cancelled and timing gates are derived from a single input pulse. We will describe the prototype instrument, discuss our encouraging test results and report our experience with the instrument in the PSR extraction line.

Active Damping of the E-P Instability at the Los Alamos Proton Storage Ring

A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). The feedback system configuration, setup procedures, and optimization of performance are described. Results of several experimental tests of system performance are presented including observations of instability threshold improvement and grow-damp experiments, which yield estimates of instability growth and damping rates. A major effort was undertaken to identify and study several factors limiting system performance. Evidence obtained from these tests suggests that performance of the prototype was limited by higher instability growth rates arising from beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizonta...

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.

A digital ring transverse feedback low-level RF control system

A digital wide-band system for damping ring instabilities in an accelerator is presented. With increased beam intensity, the losses of an accumulator ring tend to increase due to the onset of various instabilities in the beam. An analog feedback damper system has been implemented at Los Alamos National Laboratory. This analog system, while functional, has certain limitations and a lack of programmability, which can be overcome by a digital solution. A digital feedback damper system is being designed through a collaborative effort by researchers at Oakridge National Laboratory, Los Alamos National Laboratory, and the University of Wisconsin. This system, which includes analog-to-digital converters, field programmable gate arrays and digital-to-analog converters can equalize errors inherent to analog systems, such as dispersion due to amplifiers/cables, gain mismatches, and timing adjustments. The digital system features programmable gains and delays, and programmable equalizers that are implemented using digital FIR and comb filters. The flexibility of the digital system allows it to be customized to implement different configurations and extended to address other diagnostic problems.

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.

The Effect of DTL Cavity Field Errors on Beam Spill at LANSCE

The Los Alamos Neutron Science Center (LANSCE) accelerator comprises two (H+ and H-) 750-keV Cockcroft-Walton style injectors, a 201.25-MHz, 100-MeV drift-tube linac (DTL) and an 805-MHz, 800-MeV coupled-cavity linac (CCL). As part of the LANSCE Risk Mitigation project a new digital low-level radio frequency (LLRF) control system is being deployed across the linac, starting with the DTL. Related to this upgrade, a study was performed where specific cavity field errors were simultaneously introduced in all DTL tanks about the nominal stable, low-spill, production set points to mimic LLRF control errors. The impact of these errors on the resultant beam spill was quantified for the nominal 100 μA, 800-MeV Lujan beam. We present the details of the measurement approach and results that show a rapid increase in total linac beam spill as DTL cavity field phase and amplitude errors are increased.

Operation of LANSCE Linear Accelerator with Double Pulse Rate and Low Beam Losses

In 2014 the LANSCE accelerator facility returned to 120 Hz pulse rate operation after a long period of operation at a 60 Hz pulse rate. Increased capabilities required careful tuning of all components of the linear accelerator. Transformation to the double pulse rate resulted in re-evaluation of tuning procedures in order to meet new challenges in beam operation. This paper summarizes experimental activity on sustaining high productivity of the accelerator facility while keeping beam losses along accelerator low. LANSCE ACCELERATOR FACILITY The LANSCE Accelerator facility has been in operation for more than 40 years. Currently it operates with four 800 MeV H beams and one 100 MeV proton beam (see Table 1). The accelerator facility is equipped with two independent injectors for H and H beams, merging at the entrance of a 201.25 MHz Drift Tube Linac (DTL). The DTL performs acceleration up to the energy of 100 MeV. After the DTL, the Transition Region (TR) beamline directs 100 MeV proton beam to the Isotope Production Facility (IPF), while H beam is accelerated up to the final energy of 800 MeV in an 805 MHz Coupled Cavity Linac (CCL). The H beams, created by different time structure of a low-energy chopper, are distributed in the Switch Yard to four experimental areas. The most powerful H beam, average current 100 μA, is accumulated in the Proton Storage Ring (PSR) and is extracted to the Lujan Neutron Scattering Center facility for production of moderated neutrons with meV-keV energy. Another H beam, as a sequence of short pulses, is delivered to the Weapon Neutron Research (WNR) facility to create un-moderated neutrons in the keVMeV energy range. The third H beam is shared between the Proton Radiography Facility (pRad) and the Ultra-Cold Neutron (UCN) facility. Between 2006 – 2014, the LANSCE accelerator was in operation at 60 Hz pulse rate to prevent excessive cathode emission and ceramic cracking in 201.25 MHz amplifiers feeding the DTL. The LANSCE Risk Mitigation Project [1] was initiated to replace three out of four 201.25 MHz amplifiers with newly developed RF power systems based on TH628L Diacrodes [2]. The first RF power system was replaced in 2014 enabling restoration of 120 Hz operation. The replacement of 201.25 MHz RF system will be completed in 2016. In addition, a new low-level RF control system was installed, and end-of-life CCL klystrons were replaced to insure further stable beam operation. ________________________ *Work supported by US DOE under contract DE-AC52-06NA25396 #batygin@lanl.gov Table 1: Beam Parameters at 120 Hz LANSCE Accelerator (number in brackets are given for previous 60 Hz operation) Area Rep. Rate (Hz) Pulse Length (μs) Current / bunch (mA) Average current