N. F. Ziegler

Design Considerations for the ORNL 25 MV Tandem Accelerator

Construction of a 25 MV tandem electrostatic accelerator is now planned as part of a new heavy-ion facility at the Oak Ridge National Laboratory. The design of this accelerator incorporates several unusual features. The most important of these are a folded design, in which the low-energy and high-energy acceleration tubes are contained within a single column structure, and a digital control system. Motivations for these design features are discussed in conjunction with a brief description of the accelerator.

Calibration of the HHIRF tandem accelerator energy-analyzing magnet☆

Abstract A description is given of the calibration of the Holifield Heavy Ion Research Facility tandem accelerator energy-analyzing magnet. Two methods were used to measure beam energies: a novel time-of-flight technique using a variable frequency buncher; and an alpha-particle reaction-energy technique. Calibrations, accurate to better than 0.04%, were obtained for 11 different magnet excitations corresponding to mass-energy products between 28 and 298.

Improved voltage performance of the Oak Ridge 25URC tandem accelerator

Abstract Installation of compressed geometry acceleration tubes and associated changes in the corona voltage grading system have resulted in significant improvement in voltage performance of the Oak Ridge 25URC tandem accelerator. Details of the final phase of this work and initial tests on the modified accelerator are provided.

Control System for the Holifield Heavy Ion Research Facility Beam Buncher

A beam buncher has been developed to produce very short pulses of beam from the 25 MV accelerator for injection into ORIC. The buncher is a two-harmonic double-drift klystron-type operating over a frequency range of 4.5 to 14.5 MHz and has produced 1.1-ns pulse widths of 16O in tests on the ORNL EN tandem accelerator. Both rf voltage on, and relative phase between, the two buncher drift tubes must be accurately controlled to maintain the required narrow pulse width. A control system has been developed which accomplishes these tasks as well as automatic tuning of the buncher resonators, monitoring system parameters, and remote adjustment of various mechanical devices. Input commands and monitoring signals are CAMAC-compatible so that the system can eventually be included in the computer-based control system for the accelerator. In addition to the requirement for a very narrow beam pulse, the pulses must arrive at the ORIC dee gap with precise phasing. A closed-loop control for this task is being constructed using the beam pulse phase detector circuit developed at Argonne National Laboratory.

Tests of compressed geometry acceleration tubes in the Oak Ridge 25URC tandem accelerator

Abstract In an effort to improve further the voltage performance of the Oak Ridge 25URC accelerator, the original acceleration tubes will be replaced with NEC compressed geometry acceleration tubes. In this paper, we report on tests in the 25URC accelerator of two prototype compressed geometry acceleration tube designs. One of the designs utilizes a novel aperture which provides enhanced electron and ion trapping.

The Holifield Heavy Ion Research Facility

The Holifield Heavy Ion Research Facility has been in routine operation since July 1982. Beams have been provided using both the tandem accelerator alone and a coupled mode in which the Oak Ridge Isochronous Cyclotron is used as an energy booster for tandem beams. The coupled mode has proved to be especially effective and has allowed us to provide a wide range of energetic beams for scheduled experiments. In this report we discuss our operational experience and recent development activities.

Tests of compressed geometry NEC acceleration tubes

Abstract Tests have been performed in the 3 MV Pelletron test machine at NEC on a compressed geometry tube which increases the insulating length of the tube by eliminating the heated electrode assemblies (∼ 2.5 cm thick) at the end of each tube section. Some insert electrodes are changed to provide some trapping of secondary ions. The geometry tested provided an 18% increase in live ceramic in the tube. The compressed geometry tube allowed a terminal voltage of 3.55 MV on the 3 MV column at normal gradients of 30.3 kV/tube gap. The tube was also conditioned to more than 4 MV and remained stable in voltage with few sparks and with low X-ray levels for days at about 4 MV. This same performance could be achieved with or without arc discharge cleaning.

Arc discharge conditioning test on the oak ridge 25URC accelerator

Abstract A hydrogen arc discharge has been run in the top five units (fifteen 20 cm tube sections) of the ORNL tandem accelerator to test the effectiveness of this method of cleaning the high voltage electrodes. The discharge was maintained in both the high- and low-energy tubes for a period of six hours. The arc current in each tube was 4.0 A and the hydrogen pressure was nominally 100 mTorr. The arc discharge does not appear to have significantly affected the operating voltage of the top five units of the accelerator. However, the voltage conditioning behavior of the tested units is markedly different.

The 4-MeV Separated-Orbit Cyclotron

The Separated-Orbit Cyclotron Experiment (SOCE) will extend and complement earlier theoretical and experimental studies and will provide a unique facility for the evaluation of an operating SOC system. The six-sector, fourturn accelerator will provide maximum energies of 4 MeV for protons and deuterons and 8 MeV for 3He++ and 4He++ ions. The output energy is variable over a 2:1 range by adjustment of the magnetic field and acceleration of the ions at the appropriate harmonic of the ion frequency. Ions are injected into the SOC at one quarter the final energy. The injection system consists of a duoplasmatron ion source, a 500-kV dc accelerator, and a three-cavity linear accelerator. Proton currents in the 10-to 20-mA range are predicted. The principal characteristics of the accelerator are given in Table I. All of the major components have been fabricated and delivered on site except the injectors linac cavities, which are expected shortly. The photograph in Fig. 1 shows the accelerator as seen from the control area on a mezzanine about 40 feet away. The SOC sector magnets and rf cavities are in the approximate location, with the dc injector components in the background. The tops of the magnet yokes have been temporarily removed to permit completion of pole tip alignment. An rf power amplifier will be mounted on the outer wall of each cavity. One of the PA units can be seen in the background (Fig. 1) on a test stand with a water-cooled dummy load.

Coaxial Cavities for Separated Orbit Cyclotrons

Coaxial cavities are recommended over TM cavities for use in separated-orbit cyclotrons (SOCs) under 100 MeV primarily because they are smaller and construction tolerances are less stringent. At some energy above 50 MeV, however, the TM cavity1 becomes more efficient. Particles can gain energy in a coaxial cavity with arbitrary spacing between gap centers; however, maximum energy gain occurs for a spacing of 0.5s?. In a multi-particle SOC a spacing can be selected to provide acceleration of several different particles. It becomes impractical to define modes in a coaxial cavity relative to the beam motion since the primary TEM mode is defined relative to the direction of wave propagation in the transmission line. Thus TE modes in this paper are likewise defined. The TE modes in a coaxial cavity produce an accelerating voltage which varies with machine radius. Three coaxial cavities have been constructed at ORNL. RF measurements on a 1/4-scale model of a 50-MeV cavity and on the SOCE model have been completed. The 50-MeV prototype cavity is now ready for testing.