K. Fong

RF control system for ISAC II superconducting cavities

The RF control system for the superconducting cavities of the ISAC II project is a hybrid analogue/digital system. Each system consists of a self-excited feedback loop with phase-locked loops for phase and frequency stabilization. Amplitude and phase regulation, as well as tuning control, are performed using digital signal processors. Special pulsing circuitry is incorporated into the system for fast punching through multipactoring. This paper describes the RF control system, the characteristics of the feedback loops, and the experience gained in operating this system.

Commissioning of the TRIUMF ISAC RF system

The ISAC RF system at present consists of a Radio Frequency Quadrupole accelerator, five Drift Tube Linear Accelerators, six bunchers, two choppers and a bunch rotator. The RFQ operates at the fundamental frequency of 35.36 MHz, while the DTLs operate at the third harmonic frequency of 106.08 MHz. The operating power ranges from 45 W to 120 W for the choppers, 1 kW to 20 kW for the DTLs and bunchers, and 80 kW for the RFQ. These cavities have been commissioned to operate synchronously with both closed-loop amplitude and phase regulation, as well as automatic tuning of the cavities. This paper gives a brief summary of the commissioning experience.

RF control systems for the TRIUMF ISAC RF structures

The ISAC RF system consists of a radio frequency quadrupole accelerator, five drift tube LINACs, six bunchers, two choppers, and a bunch rotator. An additional buncher will be added in the near future. They run at frequencies ranging from 5.89 to 106.08 MHz, and have cavity Q values ranging from 400 to in excess of 5000. All have to run synchronously and be individually amplitude and phase regulated. A novel system that provides closed-loop amplitude, phase, and tuning control for each cavity using a single feedback circuit is employed. This system, as well as the frequency synthesis and distribution system, is described in this paper.

The ISAC-II upgrade at TRIUMF - progress and developments

TRIUMF is proceeding with a major upgrade to the ISAC project, ISAC-II, that includes the addition of 43 MV of heavy ion superconducting linear accelerator and an ECR charge state booster. An initial installation of 18 MV of mid beta cavities (/spl beta/ = 5.8%, 7.1%) is due for commissioning in 2005. The paper will describe the superconducting linac program at TRIUMF including the status of the production cavities, the design of the medium beta cryomodule and a summary of the activities of the SCRF laboratory.

Sawtooth wave generation for pre-buncher cavity in ISAC

Two phase-locked loops operating with a reference frequency of 11.667 MHz are used to generate the Fourier components for a sawtooth waveform with a repetition frequency of 11.667 MHz. These frequencies are combined at signal level, amplified and used to power a set of parallel plates inside the pre-buncher cavity up to 600 V/sub p-p/. The resultant sawtooth waveform is feedback regulated by I/Q regulation of its individual Fourier components.

RF amplifier choice for the ISAC superconducting linac

A superconducting linac is being commissioned at TRIUMF as an extension to the existing room temperature accelerator of exotic ions at ISAC. It will increase the isotope final energy from 1.5 to 6.5 MeV/u. Acceleration is accomplished in 40 bulk niobium quarter wave superconducting cavities operating at 106 and 141 MHz. Each cavity is energized from an independent RF amplifier with power rating up to 1 kW cw. Both vacuum tube and solid-state amplifiers were considered as a viable option for the drivers. The paper compares many important parameters of these 2 amplifiers such as reliability, serviceability, capital and maintenance costs, as well as operating characteristics: gain linearity, phase noise, phase drift and others. Test results of prototypes of both types of amplifiers and 1-year operational experience of 20 tube amplifiers are discussed. Based on that the amplifier design recommendations are formulated.

Impact of the cyclotron RF booster on the 500 MeV proton beam production

The TRIUMF cyclotron routinely accelerates ∼220 μA of H− ions, extracting protons simultaneously to four external beam lines. The radioactive beam facility ISAC, now operating at 10–20 μA ∼500 MeV protons, will soon require up to 100 μA at 500 MeV. The CHAOS experiment on the π+, π− secondary beam line also requires a high intensity beam (∼140 μA, 500 MeV) but with a short (2 ns) bunch length. High current operation with 2 ns beam has been facilitated by the 4th harmonic auxiliary acceleration cavity [4]. The 2 ns beam structure is now achieved by phase compression as the energy gain per turn increases near extraction. The paper focuses on improvements in the reliability of this cavity and its rf coupler. The higher energy gain per turn also reduces H− stripping losses (by about 33%) in the high energy region, hence increases the allowed beam intensity for a given beam activation. The total current will soon be increased to about 300 μA to allow for ISAC requirements.

Operating experience with the new TRIUMF RF control system

The 23 MHz RF control of the TRIUMF cyclotron has been replaced by a new VXI control system based on digital signal processing. It provides amplitude and phase regulation of the cyclotron dee voltage, as well as other functions such as power-up sequencing, spark and high VSWR protection. Modularity of the hardware is achieved by the VXI architecture, and in the software by object oriented programming. It is expected that this will result in a considerably longer MTBF, and shorter fault diagnosis and repair times, than the equipment it replaces. The new system has now been in operation for over two months. The results of commissioning, testing, and early operating experience are presented.

Initial operating experience with the auxiliary accelerating cavity for the TRIUMF cyclotron

A 92-MHz auxiliary accelerating cavity has been installed in the TRIUMF cyclotron. The cavity operates at the fourth harmonic of the main RF frequency with a planned peak voltage of 150 kV. At full power it will almost double the present energy gain per turn in the 400-500 MeV range, reducing by 25% the stripping loss of the H/sup -/ beam. Low-current beam tests have been conducted at voltages of up to 90 kV and a maximum voltage of 145 kV has been attained. The cavity has also been used to flattop the integrated energy gain per turn. A description of the cavity design and a summary of the operating experience are given.<<ETX>>

Tuner control in TRIUMF ISAC 2 superconducting RF system

The TRIUMF ISAC 2 superconducting RF system operates in self-excited, phase locking mode. A mechanical tuner is used to minimize the required RF power. The tuner derives the tuning information from the phase shift around the self-excited loop. Its accuracy is however reduced by phase drift in amplifiers and cables due to thermal effects. The cross product between the amplitude and the phase errors is used to detect this drift. The signal derived from the cross product does not depend on any initial value adjustment. Furthermore, no special adjustment is required and the measurement can be performed online during beam production.