Isochronous Field Research Articles

Operation of the Orsay Cevil and Its Extraction Systems

It is possible to obtain an isochronous magnetic field over a wide range of average induction for a given ion by changing the structure of the electro-magnet. This new structure allows great reduction of correcting adjustements. We obtained, at Orsay, encouraging results by the use of shims with variable permeability and damping gaps. Only two correction coils were necessary to make the average field isochronous to within 30 gauss, for all the ions. The internal beam tests confirmed the good quality of the magnetic field and verified the theoretical calculations ; the intensity of the multicharged ion beam was generally greater than the initial evaluation. Two extractors are at present set up : an electrostatic deflector and a stripping extractor. This latter device perserves the optical qualities of the beam. Furthermore, one or more simultaneous beams can be obtained with an excellent efficiency, and with various energies, depending on the radius.

Computer programs for an AVF cyclotron

A coherent group of magnetic field measurements and computer programs has been developed. With the measurements we determine the field shape and the influence of the trimming coils. The results are delivered as punched tape. With the computer programs the field shape measurements are reduced to tables of Fourier coefficients and the orbits studied in these fields. One of the results is the isochronous field. From the measurements of the trimming coil influence the optimum currents to approximate the isochronous field are calculated. About 100000 measured points have been evaluated; in the computer program the detection of erroneous points is emphasized. If possible these points are corrected by interpolation.

Beam-phase measurement studies

A non-intercepting system was developed for measuring the phaseof the internal beam of a cyclotron with respect to the r.f. voltage on the dee. The system has been used during initial tests of the Oak Ridge Isochronous Cyclotron to improve the isochronous field shape. It will also be used in beam studies and during operation to provide data to improve the computed setting of the trimming coils. Beam pulses are detected on an electrostatic pickup probe. The differentiated waveform is presented on a sampling oscilloscope from which the width and shape of the beam pulse can be deduced. The system is sensitive to average beam currents to below 10 μA; average beam currents up to 1 mA have been observed. The precision of phase measurement can extend to 2 deg at 22.5 Mc/sec. The probe design criteria, circuitry, and some results are discussed.

Fixed-point orbits in the vicinity of the vr = N/3, N/4, and N/2 resonances

The fixed-point (closed) orbits associated with sector resonances are significant because they substantially define the main features of the radial phase space diagram such as, for example, the stability limits. A fairly detailed and rather general calculation of these orbits is carried out for the v r = N /3, N /4, and N /2 resonances. In each case formulae are given for the fixed-point orbits which are suitable for numerical calculations. Among the general results obtained are the following. (1) The net fourth-order “frequency-shifting” term in the Lagrangian is essentially zero for an isochronous field at all energies. (Thus, the N /4 resonance, and to a lesser extent the N /5 resonance, are unstable.) (2) The cubic terms in the Lagrangian, all by themselves, can produce not only the N /3 resonance, but also the N /4 and higher-order resonances as well. (3) The behavior of the N /2 resonance resembles more closely the N /4 resonance than the N /3 case.

On the duty cycle of cyclotrons

The poor (1–6%) duty cycle of present day cyclotrons is mainly due to: 1. r.f. extraction from the ion source; 2. r.f. defocusing caused by part of the r.f. phase; 3. loss of phase due to relativistic effects; 4. phase bunching at small radii; and 5. inefficient extractors. Possible methods of improvement are: 1. d.c. extraction from the ion source; 2. introduction of focusing grids; 3. introduction of isochronous fields; 4. squaring of the r.f. wave form; and 5. Mobley magnets and other debunchers.

Variable Energy Spiral Ridge Cyclotron

A spiral ridge cyclotron which can accelerate protons from 3.5 to 15 Mev, deuterons from 7 to 14 Mev, He3++ from 11 to 37 Mev, and alpha particles from 15 to 27 Mev is described. The required magnetic field is obtained from four spiral shims and five correction coils placed on each of the pole faces of the magnet. The spiral shims produce a maximum rms flutter of 0.07 which provides sufficient vertical focusing beyond a radial distance 6 in. from the center of the cyclotron. The two innermost correction coils provide magnetic focusing up to 6 in. and the three outer coils build up the isochronous field. Focusing effects of slits, placed on the dee and the dummy dee in the path of the particles for the first four revolutions, are demonstrated. The machine can deliver 1 ma of proton beam at the extraction radius, 18.5 in.