Dee Gap Research Articles

Determination of the shape of the dee electrodes in the SINR minicyclotron used as AMS for 14C dating

The exact shapes of the “sandwich” dee electrodes in the Shanghai Institute of Nuclear Research (SINR) minicyclotron used as accelerator mass spectrometer (AMS) are determined based upon their effects on both the energy gain and the phase shift of the particles in acceleration mode, in order to meet the demands of both injecting, accelerating and extracting radioactive particles and discriminating background particles. Besides, two additional effects induced by the asymmetry of the electric force at two sides of a dee gap under the high-harmonic operation will be described.

Model Study on the Reduction of RF Leakage in the TRIUMF Cyclotron

Since the 1 MW RF cavity of the TRIUMF cyclotron is contained completely inside the vacuum chamber, RF leakage from the resonator cavity into the tank vacuum chamber can cause undesirable heating and damage to uncooled structures and diagnostic equipment. The cavity traversed by the beam outside the resonators has been successfully simulated with the two dimensional code SUPERFISH and the quasi-three-dimensional code RFQ3D. Numerical calculations using these codes and precise 1:10 model measurements show that the power coupling between the operating mode of the resonant cavity and the parasitic modes of the beam cavity depends on the voltage distribution along the dee gap, as well as on the frequencies and the field patterns of the parasitic modes. These parameters are quite sensitive to the geometries and imperfections of the centre post, the flux guides an the cantilevered hot arms. Adjustments of these geometries have achieved a significant reduction in the RF leakage in the model.

Modelling the TRIUMF RF Cavity Using the Code RFQ3D

Development work on the TRIUMF cyclotron RF cavity requires an efficient code to study the effects on voltage distributions of resoaator geometry nonuniformity, including misalignment of the cantilevered cavity walls, the end flux guides and the centre post. A quasi-three-dimensional code, RFQ3D, although originally written for radio frequency quadrupole cavity studies, has been successfully adapted to simulate the TRIUMF cavity. The code uses four one-dimensional coupled transmission line equations to calculate the voltage distributions along the dee gap and allows for local variations in the transmission line inductance and capacitance. The effects of individual resonator components on the accelerating voltage profile and the RF leakage for both the fundamental and third harmonic frequencies are calculated and compared with precise 1:10 model measurements.

Determination of the RF Leakage Field in the Vacuum Tank of the TRIUMF Cyclotron

In the TRIUMF cyclotron the dees are partially defined by the two halves of the quasi-circular vacuum chamber, which completely contains the 1 MW resonating cavity. A stray electric field or RF exists inside the dees and has occasionally caused problems to probes or other tank equipment. In order to understand the nature of this field a precise 1:10 scale metal model of the entire tank and resonator system has been built and is being investigated. Various resonances have been identified: the push-pull mode at 226 MHz and the push-push mode at 233 MHz; cross modes along the dee gap resonate at 243 MHz and 253 MHz. In the quasicircular tank region outside of the main resonating cavity the Tm310 and Tm410 modes are mainly responsible for the configuration of the leakage field since they are closer to the operating frequency. The measurements are in reasonable agreement with the results of numerical relaxation calculation and with measurements in the cyclotron tank.

Improvements of the central region of the Eindhoven AVF cyclotron

Electric field components in the acceleration gap of the central region of the Eindhoven AVF cyclotron have been determined, for several different geometries of the cyclotron centre, employing the magnetic analogue technique. Effects of changes of the puller shape have been investigated. Measurements and numerical calculations show that a substantial improvement of the axial focusing properties of particles at the first few turns is obtained, when at the position of the orbit the electric field component along the dee gap is constant or has a positive derivative with respect to radius. Tests with a new puller in the cyclotron have shown an increase of the beam current of at least a factor three with respect to the old situation.

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.

COMA - A Linear Motion Code for Cyclotrons

COMA, a computer program written in FORTRAN IV, accurately calculates the relative motion of up to 5000 particles over the complete energy range of the TRIUMF cyclotron at a cost about 1% of our numerical integration codes. In addition it can simulate the action of diagnostic probes and slits. COMA uses linear transformation matrices, obtained from an equilibrium orbit code, that include coupling between radial and longitudinal motion. The energy gain and vertical focusing forces at the dee gap are given by an improved analytical model.

Central Region Orbit Dynamics in the TRIUMF Cyclotron

Orbit dynamics in the central region have been studied with the aim of optimizing the design for beam quality and RF phase acceptance. Axial ion motion near the centre is dominated by the strong phase dependent RF electric forces at the dee gaps; the consequent restriction of the phase acceptance to ions lagging the RF voltage peak can be lifted both by using an asymmetric dee gap geometry and by the addition of third harmonic to the RF fundamental. A transition from six- to three-sector magnet geometry at the centre to increase the axial focusing was also tried, but was found to give rise to unacceptable ±9° phase oscillations if the dee gap were oriented to avoid the usual gap crossing resonance. Other effects producing coherent radial oscillations, such as first harmonic magnetic field components and dee-voltage asymmetries, have been assigned tight tolerances. The proposed design also makes possible near perfect centring for ions of all phases. Numerical orbit tracking has shown that a beam with the emittance of the external ion source can be accelerated to 20 MeV within 0.25 ? in. mrad for an RF phase band at least 25° wide: in the absence of inflationary effects at higher energies this would yield a 500 MeV beam with ±600 keV energy resolution.

Second-harmonic acceleration in the cyclotron

It is shown that steady acceleration is possible in the cyclotron, for ions which take an even number of radio-frequency cycles for one revolution, if the centre of revolution is to one side of the dee gap. The acceleration of HD+ ions, taking two radio-frequency cycles to complete one revolution, has been observed and confirmed by several methods. The contribution of this process to the production and acceleration of multiply charged heavy ions is discussed.

Electric Fields within Cyclotron Dees

Formulas for the electric field strength parallel to and perpendicular to the magnetic field for an idealized cyclotron dee system are derived by use of the Schwarz-Christoffel transformation. A convenient method for computing numerical values for any geometry characterized by the ratio of dee gap to dee height is presented and applied.