A very low output-impedance (~35 ohms) second-harmonic cavity system is being developed for high intensity proton accelerators [1]. The final amplifier is comprised of a grounded cathode scheme with a feedback loop from anode to grid. Due to the Miller effect, the grid voltage waveform is seriously distorted even if only a few percent of sub-harmonic or higher harmonic are mixed in the driver current. Such distortion is much enhanced by the beam loading. In order to eliminate the effect of this distortion upon the phase detector used to achieve precise cavity tuning, a swept bandpass filter was applied to the grid voltage at the phase detector input. Filter design details and the result of high power tests are reported.

High-power waveguide dampers have been designed and prototyped for the Short-Pulse X-ray (SPX) cavities at the Advanced Photon Source. The cavities will operate at 2.815 GHz and utilize the TM110 dipole mode. As a result, higher-order (HOM) and lower-order mode (LOM) in-vacuum dampers have been designed to satisfy the demanding broadband damping requirements in the APS storage ring. The SPX single-cell cavity consists of two WR284 waveguides for damping the HOMs and one WR284 waveguide for primarily damping the LOM where up to 2kW will be dissipated in the damping material. The damper designs and high-power experimental results will be discussed in this paper.

First beam test with the low output-impedance second harmonic cavity (LOI), was performed in April, 2011 using the ISIS synchrotron at 2.1×10 protons per pulse, which corresponds to the circulating beam current of 2.3~5.2A. The LOI was proven to be stable under such a high intensity beam. The output impedance of LOI was derived from the beam induced voltage across the cavity gap, resulting in 35ohm at 3.4MHz, which agrees well with the measurement by the network analyzer. Issues on the present LOI system and a plan for further beam experiments are also described.

The intense pulsed neutron source (IPNS) rapid cycling synchrotron (RCS) is used to accelerate protons from 50 MeV to 450 MeV, at a repetition rate of 30 Hz. The original ring design included two identical rf systems, each consisting of an accelerating cavity, cavity bias supply, power amplifiers and low-level analog electronics. The original cavities are located 180 degrees apart in the ring and provide a total peak accelerating voltage of ~21 kV over the 2.21-MHz to 5.14-MHz revolution frequency sweep. A third rf system has been constructed and installed in the RCS. The third rf system is capable of operating at the fundamental revolution frequency for the entire acceleration cycle, providing an additional peak accelerating voltage of up to ~11 kV, or at the second harmonic of the revolution frequency for the first ~4 ms of the acceleration cycle, providing an additional peak voltage of up to ~11 kV for bunch shape control. We describe here the hardware implementation and operation to date of the third rf cavity in the second harmonic mode.

The rapid cycling synchrotron (RCS) of the intense pulsed neutron source (IPNS) at ANL accelerates > 3.0 times 10 12 protons from 50 MeV to 450 MeV with 30-Hz repetition frequency. During the acceleration cycle, the rf frequency varies from 2.21 MHz to 5.14 MHz. Presently, the beam current is limited by a vertical instability. By analyzing turn-by-turn beam position monitor (BPM) data, large- amplitude mode 0 and mode 1 vertical beam centroid oscillations were observed in the later part of the acceleration cycle. The oscillations start in the tail of the bunch, build up, and remain localized in the tail half of the bunch. This vertical instability was compared with a head-tail instability that was intentionally induced in the RCS by adjusting the trim sextupoles. It appears that our vertical instability is not a classical head-tail instability [1]. More data analysis and experiments were performed to characterize the instability.

In the rapid cycling synchrotron (RCS), a single proton bunch (h=1) is accelerated from 50 MeV to 450 MeV in approximately 14.2 ms. The bunch experiences an instability shortly after injection (<1 ms). During the first millisecond, the beam is bunched but little acceleration takes place; therefore this period is similar to a storage ring mode of operation. Natural vertical oscillations (assumed to be tune lines) show the vertical tune to be rising toward the bare tune value, suggesting neutralization of space charge and a reduction of its detuning effects. Neutralization time near injection ranges from 250 mus - 500 mus, depending on the background gas pressure. Oscillations move from the LSB to the USB before disappearing. Tune measurements made with a recently installed ferrite- magnet pinger system show the horizontal chromaticity to be positive early but approaching zero later in the cycle; on the other hand, the vertical chromaticity is negative throughout the cycle. During pinger studies near injection, two vertical lines are observed. Neutralization of the beam space charge implies the generation of plasma in the beam volume early in the cycle which may then dissipate as the time-varying electric fields of the beam become stronger.

The Rapid Cycling Synchrotron (RCS) of the Intense Pulsed Neutron Source (IPNS) at ANL accelerates > 3.0 times 10 12 protons from 50 MeV to 450 MeV with 30-Hz repetition frequency. During each acceleration cycle, the RF varies from 2.21 MHz to 5.14 MHz. In order to improve capture efficiency, we varied the injection timing and the early RF voltage profiles. The experimental results are compared with simulation results obtained with the 1-D tracking code, CAPTURE. This allowed us to optimize injection time and the RF voltage profile for better capture efficiency. An optimized injection time and RF voltage profile was found that resulted in raising the capture efficiency from 85.1% to 88.6%. These studies have now also been expanded to include second harmonic RF during the capture and initial acceleration cycle in the RCS.

A wideband low-output-impedance RF system for the second harmonic cavity in the ISIS synchrotron has been developed by the collaboration between Argonne National Laboratory (US), KEK (Japan) and Rutherford Appleton Laboratory (UK). The system has less than 30 Ω over the frequency range of 2.7 - 6.2 MHz. High power test was performed with a ferrite-loaded second harmonic cavity at 50 Hz repetition rate. The maximum voltage of 12.6 kV peak per accelerating gap was obtained stably at earlier period of an operation cycle. A beam test with this system is planned at the ISIS synchrotron.

The Rapid Cycling Synchrotron (RCS) at IPNS is a sixperiod combined function synchrotron with a magnet structure of D00FDF0 which was commissioned and demonstrated operation up to 1x10 protons/pulse in 1979[1]. Operation was intermittent over the next couple years during which the extraction was changed to provide beam to the present IPNS target. Initial attempts to increase the charge/pulse in 1981 were limited by a headtail instability that had a threshold just under 1x10 protons/pulse. Tune measurements revealed that the horizontal chromaticity became slightly positive near the end of the acceleration cycle. The trim-sextupole power supplies were changed from dc to programmable to adjust the chromaticity, allowing an intensity increase to 2x10 protons/pulse[2]. Limited experiments of adding 2harmonic rf to spread the bunch longitudinally were performed in early 1983 and it was proposed to add a 3 rf cavity to improve beam handling as a route to higher charge per pulse[3]. Between 1983 and 1985, improvements to control loops[4] and the introduction of a cavity-to-cavity phase modulation (PM) or “scrambler” to broaden the bunch longitudinally[5] provided another intensity increase to slightly greater than 3x10 protons per pulse. Funding problems prevented development of a third rf cavity and the “scrambler” proved a low-cost cure to the instability threshold at 2x10 protons/pulse. Thus by 1985, the RCS had reached its original design goal of operating at 3x10 protons pulse. Funding constraints indefinitely postponed further attempts to get closer to the RCS’s space charge limit for a uniformly distributed beam of 5.6 x10 protons/pulse. Funding for U.S. DOE user facilities improved in the late 1990's and operational imperatives at IPNS changed from year-to-year survival to ensuring operation for the indefinite future with ever-increasing scientific capabilities. Neutron-scattering instrument improvements offered the most cost-effective route to significant gains, several of which have achieved data-rate increases of more than a factor of ten through adding detectors, neutron guides and other enhancements. For the accelerator system the goals were more modest, increase beam-on-target by increasing operating time (to ~ 30 weeks/year) and increase protons/pulse while maintaining or improving reliability. Initially, the emphasis had to be on updating or replacing those subsystems that were becoming obsolete (spares no longer available or nearing end-of-life) and controlling losses so that operating hours could increase without increasing the activation of components. Calculations[6] showed that improving synchrotron rf system capabilities could reduce beam losses, and had the further advantage of providing redundancy to improve reliability. Thus, fifteen years after the original proposal, work started on a third rf system[7] for the RCS. More recently, improvements in diagnostics and better computer modeling have let us take a more detailed look at what limits the charge-per-pulse in the RCS and begin studies to increase the threshold current of the instability.

In the ISIS facility at Rutherford Appleton Laboratory (RAL) in the UK, a second target station project is under construction. Four 2nd harmonic RF cavities will be installed in the ISIS synchrotron to increase the trapping efficiency, and to mitigate the space charge detuning, allowing a 50 % increase in accelerated beam intensity. A very low output-impedance RF system for the 2nd harmonic cavity has been developed by a collaboration of RAL, Argonne National Laboratory (US) and KEK (Japan). The system consists of a 240 kW triode as a final amplifier with plate-to-grid feedback. The measured output-impedance is less than 30 ohms over the frequency range of 2.7 - 6.2 MHz, which agrees well with calculations. High power tests were performed using swept-frequency at 50 Hz repetition rate. The operation is almost stable, and more than 12 kV peak-to-peak was obtained. The voltage gain of the final amplifier is 25 – 30. This decreases gradually with frequency due to the decreasing input-impedance of the triode. A test with beam is planned at ISIS in the near future.

The IPNS RCS is a rapid cycling synchrotron used to accelerate protons from 50 MeV to 450 MeV, 30 times per second. Currently, two single-gap, ferrite-loaded coaxial cavities, located 180/spl deg/ apart, provide a total peak accelerating voltage of approximately 21 kV over the 2.2 MHz to 5.1 MHz revolution frequency band. An amplifier chain, which includes a 2 kW predriver, a 20 kW driver and a 100 kW final, drives each cavity. A third RF system, consisting of a cavity, cavity bias supply, and amplifier chain, is currently under construction. When complete, this upgrade will provide flexibility in operation that is expected to enhance reliability (i.e., three cavity operation at higher total accelerating voltage, three cavity operation at lower voltage per cavity, or two cavity operation with an on-line spare). In addition, the third cavity will provide an experimental station for second harmonic RF cavity studies. We report progress to date.

The Intense Pulsed Neutron Source (IPNS) Rapid Cycling Synchrotron (RCS) delivers 450-MeV protons in 70 ns pulses at 30 Hz to a heavy-metal target producing spallation neutrons for material science research. The average current extracted from the RCS is 15 /spl mu/A with a peak intensity of 10 amps. The large circulating currents in the RCS generate oscillations in the bunch which are presently controlled by modulating the phase of the two RF cavities. By adding second harmonic (SH) rf, the bunch length can be increased reducing the peak current. Simulations suggest that a 20-40 percent increase in extracted current should be achievable. The simulation program allows for phasing between fundamental and SH RF voltages. Initial studies to optimize phase indicate the need to maximize bucket area early in the acceleration cycle, whereas the bunching factor is more significant later in the cycle.