R. C. Cohen

The Nevis Synchrocyclotron Conversion Project

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.

Extraction Studies for the Modified Nevis Synchrocyclotron

Computer studies of a regenerative beam extraction system using magnetic peeler-regenerator have been performed. We have also investigated the use of a time varying first harmonic magnetic bump as a beam stretcher. The results indicate, that we can expect a turn separation of the order of 0.8 in. at the mouth of the magnetic channel. The time varying bump makes it possible to stretch the external beam over most of the RF-off time of the acceleration cycle. For the, magnetic channel, a current configuration has been found which does not perturb the main cyclotron field outside itself and reduces the field by 4 kG inside the channel. The step from no field to full field is only 0.2 in. This 4 kG field extending over a distance of 20-in. is sufficient to lead the beam out of the machine.

Operation Status of the Nevis Synchrocyclotron

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.

Nevis Synchrocyclotron Beam Status Report

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.

The Current Extractions Channel of the Nevis Synchrocyclotron Conversion Project

A current channel has been designed (computer) with a total septum thickness including supports of 0.150 in. It provides a field jump of over 6 kG and a gradient of over 1.5 kG/in. The direction of the gradient is such that the channel field superimposed on the fringing field of the cyclotron magnet produces a radially focusing field. The heat load and the power dissipation in the septum will be greatly reduced by having the septum flare out from 0.125 in. at the upstream end of the channel to about 0.600 in. at the downstream end. This reduces the power consumption in the septum to 40 kW.

A Pulsed Synchrocyclotron Ion Source Using a Single Cold Cathode

During a development program to improve the beam current accepted by the Nevis Synchrocyclotron, a new source using a single cold cathode was developed. The change to a single cold cathode has resulted in a marked improvement in the performance of the source compared to operation as a PIG source with the same chimney and extraction geometry. The extracted H+ current was greater than 90% compared to about 70% for the PIG source, and the vertical emittance was improved by over a factor of 2. Pulsed beam currents of up to 80 mA were extracted from a slit of 1.6 mm by 6.25 mm high. Although the extracted current was less than the current from the PIG source (¿ 150mA), the charge per pulse accepted by the accelerator (as measured by an internal probe at a radius corresponding to 30 MeV) increased from 20 to as high as 100 nanocoulombs/pulse.

Status of the Nevis Synchrocyclotron

A general description of the various systems, i. e. the central region, the RF system, the extraction system, etc., of the Nevis Synchrocyclotron Conversion Project is presented. Recent progress is described and future plans are outlined.

Beam Monitoring in the Extraction Region of the Nevis Synchrocyclotron

A secondary emission monitor has been designed and tested for use in beam profile measurements of the extracted proton beam of the Nevis Synchrocyclotron. The monitor will provide a non-destructive simultaneous measurement of the vertical and horizontal position distributions, with a resolution of ~ .1 in. A residual gas ionization counter has been also designed. This counter will measure the radial profile of the accelerated beam at the parking radius.

Design of Secondary Beam Channels for the Nevis Synchrocyclotron Conversion Program

The modification program includes the design of two pion channels characterized by 0?Po?250 MeV/c with ±1% ??P/Po?±10% and 200? Po?400 MeV/c with ±0.5%??P/Po?±3% and a stopped muon channel with low contamination and average polarization ?70%. Two computer programs instrumental in their design are described. BARTELBE utilizes a function minimizing code in conjunction with first order beam optics to simultaneously optimize any number of desired conditions relating to e.g., a desired optical system and momentum resolution, and a beam acceptance volume. MAGSYS is a Monte Carlo code which generates each pion according to given input parameters and follows its history through a system. In the output are e.g. detailed pion and muon beam profiles and muon polarization.