Thomas W Hardek

Producing persistent, high-current, high-duty-factor H− beams for routine 1 MW operation of Spallation Neutron Source (invited)a)

Since 2009, the Spallation Neutron Source (SNS) has been producing neutrons with ion beam powers near 1 MW, which requires the extraction of ∼50 mA H(-) ions from the ion source with a ∼5% duty factor. The 50 mA are achieved after an initial dose of ∼3 mg of Cs and heating the Cs collar to ∼170 °C. The 50 mA normally persist for the entire 4-week source service cycles. Fundamental processes are reviewed to elucidate the persistence of the SNS H(-) beams without a steady feed of Cs and why the Cs collar temperature may have to be kept near 170 °C.

Performance of the H−Ion Source Supporting 1‐MW Beam Operations at SNS

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory reached 1‐MW of beam power in September 2009, and now routinely operates near 1‐MW for the production of neutrons. This paper reviews the performance, operational issues, implemented and planned mitigations of the SNS H− ion source to support such high power‐level beams with high availability. Some results from R&D activities are also briefly described.

rf improvements for Spallation Neutron Source H- ion source.

The Spallation Neutron Source at Oak Ridge National Laboratory is ramping up the accelerated proton beam power to 1.4 MW and just reached 1 MW. The rf-driven multicusp ion source that originates from the Lawrence Berkeley National Laboratory has been delivering approximately 38 mA H(-) beam in the linac at 60 Hz, 0.9 ms. To improve availability, a rf-driven external antenna multicusp ion source with a water-cooled ceramic aluminum nitride (AlN) plasma chamber is developed. Computer modeling and simulations have been made to analyze and optimize the rf performance of the new ion source. Operational statistics and test runs with up to 56 mA medium energy beam transport beam current identify the 2 MHz rf system as a limiting factor in the system availability and beam production. Plasma ignition system is under development by using a separate 13 MHz system. To improve the availability of the rf power system with easier maintenance, we tested a 70 kV isolation transformer for the 80 kW, 6% duty cycle 2 MHz amplifier to power the ion source from a grounded solid-state amplifier.

Intense Pulsed Neutron Source (IPNS-I) Accelerator RF System

RF equipment constructed for the IPNS-I accelerator system, formerly called the Zero Gradient Synchrotron (ZGS) Booster II, is described in this report. The accelerator is a first harmonic rapid cycling machine intended to accelerate 3 × 1012 protons from 50 MeV to 500 MeV, 30 times per second. The RF system produces a peak accelerating voltage of 22 kV over a 2.2 MHz to 5.3 MHz frequency band. Two single gap ferrite loaded cavities are located 180° around the accelerator and operated 180° out of phase to provide the desired voltage. High level equipment, low level subsystems and control equipment are covered as well as modifications incorporated after start-up.

Development of Fast High-Power Rf Vector Modulator Employing Tem Ferrite Phase Shifters

Recent charged-particle accelerator projects employ high power RF distribution systems to power superconducting radio frequency (SRF) accelerating cavities. Each cavity usually requires several hundred killowatts of pulsed power at relatively high duty cycle (roughly up to 10% in high-power proton linear accelerators) in low UHF bands for production of the particle beam. So far, one amplifier to one cavity configuration is common, since precise vector control of RF is done at the low power inputs of the amplifiers. If a fan-out configuration that feeds many cavities with a single very high-power amplifier klystron is realized, significant cost savings can be achieved in construction and installation. The fan-out configuration, however, requires independent control of RF amplitudes and phases to the cavities at high power level. A prototype high-power RF vector modulator for such applications has been built and tested. The vector modulator employs a quadrature hybrid and two fast ferrite phase shifters in square coaxial transverse electromagnetic (TEM) transmission lines. The square coaxial format can provide the power-handling capability and thermal stability. RF properties of the design and results of high power system testing of the design are presented.

Status of the spallation neutron source superconducting RF facilities

The spallation neutron source (SNS) project was completed with only limited superconducting RF (SRF) facilities installed as part of the project. A concerted effort has been initiated to install the infrastructure and equipment necessary to maintain and repair the superconducting Linac, and to support power upgrade research and development (R&D). Installation of a Class 10/100/10,000 cleanroom and outfitting of the test cave with RF, vacuum, controls, personnel protection and cryogenics systems is underway. A horizontal cryostat, which can house a helium vessel/cavity and fundamental power coupler for full power, pulsed testing, is being procured. Equipment for cryomodule assembly and disassembly is being procured. This effort, while derived from the experience of the SRF community, will provide a unique high power test capability as well as long term maintenance capabilities. This paper presents the current status and the future plans for the SNS SRF facilities.

Design and high power processing of RFQ input power couplers

A RF power coupling system has been developed for future upgrade of input coupling of the RFQ in the SNS linac. The design employs two coaxial loop couplers for 402.5 MHz operation. Each loop is fed through a coaxial ceramic window that is connected to an output of a magic-T waveguide hybrid through a coaxial to waveguide transition. The coaxial loop couplers are designed, manufactured, and high power processed. Two couplers will be used in parallel to power the accelerating structure with up to total 800 kW peak power at 6% duty cycle. RF and mechanical properties of the couplers are discussed. Result of high power RF conditioning that is performed in the RF test facility of the SNS is presented.

The spallation neutron source accumulator ring RF system

The spallation neutron source (SNS) accumulator ring is a fixed-frequency proton storage ring located at the output of the SNS linear accelerator (Linac). Its purpose is to redistribute the 1 millisecond long H-beam pulses from the SNS Linac into high-intensity 695 nanosecond long pulses of protons for delivery to the neutron target. The RF bunching system controls longitudinal beam distribution during the accumulation process and maintains a 250+ nanosecond gap required for beam extraction. The RF system consists of three stations which operate at the beam revolution frequency of 1.05 MHz and a fourth station providing a second harmonic component at 2.1 MHz. The beam pulse at extraction consists of 1.6e14 protons representing a peak beam current of 52 amperes. The system utilizes four 600 kW tetrodes to provide the RF current necessary to produce the 40 kV peak fundamental frequency bunching voltage and to control phase and amplitude at high beam current. A 20 kV peak second harmonic voltage is intended to control longitudinal beam distribution to control the peak circulating current. In this paper we review the design concepts incorporated into this heavily beam-loaded RF system and discuss its commissioning status.

Simulation study and initial test of the SNS ring RF system

The rfsimulator code was developed for the study of the Spallation Neutron Source (SNS) dual-harmonic ring RF control. It uses time-domain solvers to compute beam- cavity interactions and FFT methods to simulate the time responses of the linear RF system. The important elements of the system considered in the model include beam loading, dynamic cavity detuning, circuit bandwidth, loop delay, proportional-integral controller for feedback and adaptive feed forward, stochastic noise, width-in-turn loop parameter change, beam current fluctuation, and bunch leakage. As the beam power increases, beam loss in the ring goes up and thus precise control of the bunching RF phase and amplitude is required to limit beam loss. The code will help in the development of a functional RF control and in achieving the goal of minimizing beam loss in the accumulator ring.

Development and testing of high power RF vector modulators

A fan-out RF power distribution system can allow many accelerating cavities to be powered by a single high-power klystron amplifier. High-power vector modulators can perform independent control of amplitudes and phases of RF voltages at the cavities without changing the klystron signal. A prototype high-power RF vector modulator employing a quadrature hybrid and two ferrite phase shifters in coaxial TEM transmission lines has been built and tested for 402.5 MHz. RF properties of the design and results of high power testing are presented.