Nowadays the designer of a new rf control system has access to a wealth of powerful digital, analog, and rf circuitry. The requirements for the rf control system have changed from only controlling the amplitude and phase of the accelerating field to the required degree to stability. Additional tasks include exception handling and extensive build-in diagnostics while pursuing issues related to reliability, operability, and maintainability. Also operation close to the performance limit must be supported while maximizing the availability of the accelerator. With many accelerator projects in planning or under construction several state-of-the art rf control designs have evolved. This paper will present an overview of this new technology and discuss its performance.

/pdf/state-of-the-art-in-rf-control-25ipiu1gv0.pdf

State of the art in rf control

Superconducting cavities exhibit a high susceptibility to mechanical perturbations due to the narrow bandwidth of the cavities. Significant phase and amplitude errors can be induced by the frequency variations driven by microphonics and Lorentz force detuning especially in the case of pulsed operation where mechanical resonances of the cavity can be excited. While the relativistic electron and positron beams in the TESLA linacs will permit the control of the vector-sum of many cavities driven by one klystron, the non-relativistic proton beam in the linacs for SNS and the JAERI/KEK Joint Project limits the number of cavities that can be controlled by one klystron. Considerable experience of RF control at high gradients (>15 MV/m) with pulsed RF and pulsed beam has been gained at the TESLA Test Facility in which presently 16 cavities are driven by one klystron. The RF control system employs a completely digital feedback system to provide flexibility in the control algorithms, precise calibration of the vector-sum, and extensive diagnostics and exception handling capabilities. Presently under study is a piezotranslator based active compensation scheme for the time varying Lorentz force detuning which if successful will reduce RF power requirements at gradients >25 MV/m considerably and provide improved field stability.

/pdf/achieving-phase-and-amplitude-stability-in-pulsed-1mb9z9hrk9.pdf

Achieving phase and amplitude stability in pulsed superconducting cavities

The requirements for RF Control Systems of Superconducting Linear Colliders are not only defined in terms of the quality of field control but also with respect to operability, availability, and maintainability of the RF System, and the interfaces to other subsystems. The field control of the vector-sum of many cavities driven by one klystron in pulsed mode at high gradients is a challenging task since severe Lorentz force detuning, microphonics and beam induced field errors must be suppressed by several orders of magnitude. This is accomplished by a combination of local and global feedback and feedforward control. Sensors monitor individual cavity probe signals, and forward and reflected wave as well as the beam properties including beam energy and phase while actuators control the incident wave of the klystron and individual cavity resonance frequencies. The operability of a large llrf system requires a high degree of automation while the high availability requires robust algorithms, redundancy, and extremely reliable hardware. The maintenance of the llrf demands sophisticated on-line diagnostics for the llrf sub-systems to minimize downtime.

Digital Low-Level RF Conctrols for Future Superconducting Linear Colliders

Spallation Neutron Source (SNS) accelerator includes a nominally 1000 MeV, 2 mA average current linac consisting of a radio frequency quadrupole (RFQ), drift tube linac (DTL), coupled cavity linac (CCL), a medium and high beta superconducting (SC) linac, and two buncher cavities for beam transport to the ring. Los Alamos is responsible for the RF systems for all sections of the linac. The SNS linac is a pulsed proton linac and the RF system must support a 1 msec beam pulse at up to a 60 Hz repetition rate. The RFQ and DTL utilize seven, 2.5 MW klystrons and operate at 402.5 MHz. The CCL, SC, and buncher cavities operate at 805 MHz. Six, 5 MW klystrons are utilized for the CCL and buncher cavities while eighty-one 550 kW klystrons are used for the SC cavities. All of the RF hardware for the SNS linac is currently in production. This paper will present details of the RF system-level design as well as specific details of the SNS RF equipment. The design parameters will be discussed. One of the design challenges has been achieving a reasonable cost with the very large number of high-power klystrons. The approaches we used to reduce cost and the resulting design compromises will be discussed.

/pdf/the-rf-system-design-for-the-spallation-neutron-source-1wsvubk948.pdf

The RF system design for the Spallation Neutron Source

When initial beam studies at the Spallation Neutron Source (SNS) indicated a need for better compensation of the effects of beam-loading, a succession of rapid-prototyping and experimentation lead to the development of a simple yet successful adaptive feed forward (AFF) technique within a few weeks. We describe the process and first results.

/pdf/adaptive-feed-forward-beam-loading-compensation-experience-3qgqplv6aq.pdf

Adaptive Feed Forward Beam-Loading Compensation Experience at the Spallation Neutron Source Linac

The SNS linac RF control system (RFCS) is currently in development. A system is being installed in a superconducting test stand at Jefferson Laboratory presently. Two systems will soon be installed at Oak Ridge National Laboratory (ORNL) and more are due to be installed early next year. The RF control system provides field control for the entire SNS linac, including an RFQ and 6 DTL cavities at 402.5 MHz as well as three different types of cavities at of 805 MHz: 4 CCL cavities, 36 medium beta superconducting (SRF) cavities, and 45 high beta superconducting cavities. In addition to field control, it provides cavity resonance control, and incorporates high power protect functions. This paper will discuss the RFCS design to date, with emphasis on the challenges of providing a universal digital system for use on each of the individual cavity types. The RF control system hardware has been designed to minimize the amount of changes for all of the applications. Through software/firmware modification and changing a couple of frequency-dependent filters, the same control system design can be used for all five cavity types. The SNS is the first to utilize SRF cavities for a pulsed high-current proton accelerator, thereby makingmore » RF control especially challenging.« less

SNS LINAC RF control system.

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SNS LINAC RF control system.