J. Rainwater

Nevis Synchrocyclotron Conversion Program - RF System

The for the revised Nevis Synchrocylotron magnet will increase from 18 kG at the center to 20 kG at 80 in. radius. This gives 27.48 MHz at injection, and 19.23 MHz for extraction (550 MeV, 76 in. radius). The resonator will be tuned so that fmin = fextraction including a slow RF turnoff where df/dt ? 0 for adiabatic energy damping. The peak dee voltage ? 30 kV with the maximum near injection. The repetition rate will be 300 Hz with < 50% RF duty factor. The single dee and dee stem structure will be ~ 150 in. wide and 163 in. long. One of the three pairs of sector-iron shims is to be incorporated into the dee and the assembly supported on cooled alumina structural insulators. The resonator frequency is varied by two rotating capacitors located at the end of the dee stem, which is split outside the magnet circle. Excitation of the resonator will be provided by a grounded-grid, triode oscillator having the anode and cathode coupled to the resonator via coupling loops and transmission lines of proper length and impedance. A series tube modulator will permit the slow linear RF turnoff needed for adiabatic energy damping of the phase oscillations before extraction.

Increasing Synchrocyclotron Currents and the Space Charge Limit

Until MacKenzie recently emphasized the dominating role of space charge beam blowup at small radii in synchrocyclotrons, the importance of this factor was not appreciated. Much largely futile effort had been expended on attempts to increase average beam currents by improving other factors. Lawsons extension of MacKenzies treatment for ordinary cyclotrons is considered in more detail, with numerical examples and a detailed discussion of the extension of the theory to synchrocyclotrons. In addition, a simpler semi‐empirical formula is obtained which gives a good match to existing currents in all present synchrocyclotrons. Current gains up to a factor ∼100 should be feasible, and a discussion is given as to how such an improvement might be gained by modifications of existing synchrocyclotrons.

Nevis Synchrocyclotron Slow Neutron Velocity Spectrometer

A high-intensity, high-resolution neutron velocity selector system, employing the Nevis synchrocyclotron as a source of pulsed neutrons, is described. Detector counts are accumulated in a 2000-channel analyzer with 0.1-μsec channel width. Punch card data reduction techniques are discussed. Resonance spectra of PbI2 obtained using a 35-m flight path provide an example of the resolution and illustrate the use of the self-indication capture γ-ray detector scheme extensively employed. A planned flight path of 200 m is expected to provide a resolution of <1 nsec/m for neutron energies above 1000 ev.

Status of the Nevis Synchrocyclotron Modification

After 20 years of operation as a conventional synchrocyclotron producing 0.4 ?A time average internal beam current of protons (increased to 1.6 ?A in 1967) of ~ 385 MeV, we stopped operation in September 1970 for the major modification for which we have been planning since 1965. The major study program and the conversion project are supported by a

SYNCHROCYCLOTRON 200-METER FLIGHT PATH NEUTRON VELOCITY SPECTROMETER

3.9 M grant from the NSF. The conversion retains the basic cyclotron magnet (2000 tons of Fe and ~ 300 tons of Cu coils), but adds a 10-in. thick Fe band around the outside magnet perimeter to lower the return path reluctance. A new larger vacuum chamber is used, with auxilliary excitation coils. Pole iron within 30 in. of the median plane is replaced by a new configuration which includes N=3 symmetry sector iron with a small median plane gap. = 18 kG near the center and 20 kG near 80 in. radius. Strong azimuthal magnetic field flutter begins at < 2 in. radius to give strong focusing ?z, and ?r?1. We expect to obtain ~ 550 MeV proton energy, with high extraction efficiency for a 10 to 40 ?A time average external beam (~ 50% duty factor), when operation starts later this year. Additional details are given in companion papers in these Proceedings. The system will still be a synchrocyclotron and will have a 300 Hz FM repetition rate, with a new RF system, etc.

The Nevis Synchrocyclotron Conversion Project

Our synchrocyclotron neutron time‐of‐flight spectrometer now uses a 200‐m flight path and two 2000‐channel analyzers. The many important modifications and improvements described in this paper have been made as a result of operating experience. The present operation uses ∼20 nsec bursts at an instantaneous intensity ∼5×1018 evaporation neutrons per second and a 60‐cps repetition rate. The resolution for energies above 1 keV is now limited by the time analyzers which have 100‐nsec detection channel widths, giving a resolution of 0.5 nsec/m in the keV region.

A Stabilized Ionization Gauge Circuit with Vacuum Tube Voltmeter

The status of the Columbia University Nevis Synchrocyclotron modification program is presented. The machine will be converted to a three-fold symmetry spiral sector focussing AVF synchrocyclotron, having a long duty factor 550 MeV external proton beam. The time average external beam intensity is expected to be between 5 ?A and 40 ?A. The reasons leading to the particular approach of this conversion program are given.

Operation Status of the Nevis Synchrocyclotron

A complete control circuit for use with the usual triode ionization gauge tube is described. A modified Ridenour Lampson type circuit is employed to stabilize grid and plate voltage, as well as emission current. A simple vacuum tube voltmeter circuit permits the use of a relatively rugged 0–1 milliameter to read plate current. Special provision is made for outgassing, and for leak hunting.

Nevis Synchrocyclotron Beam Status Report

The Nevis Synchrocyclotron (S.C.) has been running on a fairly regular basis since October, with peak beam intensity of 2.2 ..mu..A. It had also run from late January through mid-April 1976. The current operating status of the accelerator is reported, and new developments of the past year are outlined.

Recent Experimental Neutron Resonance Spectroscopy Results as a Test of Statistical Theories of Short and Long Range Order for Level Spacings

The full energy beam with vertical width 0.25 in. and projected intensity 12 W.A is a major step toward fulfilling the design goals (20 W.A) of the accelerator. We anticipate that an increase in intensity by a factor of 2 will be obtained by extending our injection time from 10 W±s (at present) to 25 ¿s. By varying the timing of the source trigger, we have verified that this time window is indeed available for injection. This injection time extension will require modifying the ion source pulser. We also expect to be able to extract higher ion currents by modifying the source tip to bring the plasma closer to the extraction hole. Calculations on the operation of the extraction system indicate that we should be able to exceed 75% extraction efficiency with the vertical and radial widths measured. The adiabatic turn-off of the RF voltage, with concurrent compression of energy spread, has not yet been tested, but we expect that it will improve the extraction efficiency. We are presently installing the extraction system, and anticipate extracting beam in April 1975.