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The past year has seen the completion of a major expansion of the Bates Accelerator Laboratory. A second experimental hall, South Hall, and several magnetic spectrometers have been constructed. The accelerator's maximum energy has been raised from 400 to 750 MeV by means of beam recirculation. A current two-year project for the fabrication of an additional RF transmitter plus a 30% increase in RF peak power capability will increase energy further to ca. 1 GeV. During the same period pulse-to-pulse beam sharing between the high-resolution spectrometer area and South Hall will become available. In January 1981 the Laboratory submitted their ''Proposal for a Long-Range Expansion Program'' to DOE-NSF. The proposed development consists of three stages. Stage I calls for the addition of a pulse stretcher ring to furnish a CW beam to the South Hall beam lines. Additional experimental space for internal target experiments and photon tagging research are also included. Stage II increases the accelerator energy to 2.1 GeV (at 140 microamps) by means of a five-pass head-to-tail recirculator. Stage III is, at this time, a plan rather than a proposal. It increases accelerator energy to 4 GeV by extending the accelerator length and power and adds another pulsemore » stretcher ring and three new experimental areas for the higher energy work. This paper discusses each of these stages in detail and recommends their funding and scheduling.« less

The beam energy capability of an electron linear accelerator can be extended by using a magnetic transport system to return the beam after passing once through the accelerator, to the injector end and then "recirculating" the beam through the accelerating structure several times. The most important property of the transport system is that it be a good achromat, since operability requirements and beam loading transients usually require at least a few percent energy bandwidth over which the beam will successfully traverse the accelerator during recirculation. The system must also be nearly isochronous, with linear phase-energy correlation corresponding to not more than a fraction of an RF wavelength over a few percent bandwidth and with a small second-order phase-energy correlation coefficient. Such a system to be described in this paper has been designed and is currently under construction for the purpose of recirculating the beam of the 400 MeV Bates Linear Electron Accelerator.

An isochronous recirculation system is in operation at the MIT-Bates Accelerator Center. It is used to transport the electron beam from the output of the LINAC back to the input, to be reinjected so that the electrons pass through the accelerator a second time. The design and the general operation of the recirculation system and the Linac has been discussed elsewhere. The Linac accelerates electrons to 400 MeV in one pass with a duty factor near 1%, within an energy spread of always less than 0.3% and an emittance of 10 /el mm-mr. With the recirculation system the accelerator energy capability has been extended to 750 MeV. The accelerator is designed with a progressive series of stop bands to prevent the propagation of transverse modes, thus raising the calculated threshold of beam break-up (BBU) to more than an order of magnitude above operation needs for both single pass and recirculated operation. The recirculation transport system (RTS) is isochronous and highly achromatic allowing for an energy band pass of 6% in which the beam is fully transmitted through the accelerator twice. The admittance of the accelerator is about 0.3 mm-mrad for the recirculated beam. In this paper we describe, qualitatively, observationsmore » of the operation of the recirculation system under a variety of conditions. In particular, both simultaneous and head-to-tail recirculation have been accomplished. The former was done under a variety of phasing conditions of the second pass beam relative to the first. The instrumentation used in the evaluation of the tests include a beam current monitor (toroid) which measures the peak current, and the Rf loads of the output of the traveling wave waveguides which measure effectively the product of the current in the waveguide and the energy gain in that section of waveguide.« less

A design study has been completed for the RF system of a pulse stretcher ring to be operated in conjunction with the Bates Linear Accelerator (BLA). The 390m circumference ring is to operate from 300 MeV to 1 GeV, with 80 mA injected current and 1 msec extraction time. The stretcher ring is for nuclear physics research and has radiation damping time and quantum lifetime long compared to the extraction time. The special requirements of the RF system have been considered. Dynamic behavior at the fundamental frequency has been investigated by computer simulation. Higher-order mode impedance limitation has been specified by an investigation of transverse multi-bunch multi-turn instabilities.

In the past few years, the pulse stretcher ring (PSR) concept has emerged as a viable method for extending the duty factor of pulsed linacs. The viability is a result of considerations such as cost and expected extracted beam quality. A few of the projects emphasize high average extracted beam intensity. This is accomplished through a high injection frequency (1 kHz) and relatively high injected peak currents (greater than or equal to80mA). To enhance the injection efficiency, the number of turns of injection are minimized so the ring is long and there are fewer turns for extraction. There is considerable experience with beam extraction over large numbers of turns from synchrotrons, but time uniform extraction over fewer turns has not been treated as thoroughly. In this paper, some of the extraction calculations made for the proposed MIT-Bates One-GeV Pulse Stretcher Ring are discussed. The theory used for the simulation procedure will be reviewed, the results summarized, and some points of operational interest mentioned.

The RF design objective for the MIT medium energy, high-duty-cycle linear accelerator has been to optimize and extend existing technology to obtain a degree of phase stability not previously achieved with high power RF systems. The RF system, which operates with a maximum average RF power of 80 kW per high power klystron (4 MW peak at 2% duty cycle to 1 MW peak at 6.5% duty cycle), is described. Accurate phase bridge measurements on the system indicate a pulse-top phase stability of better than 10 at 2856 MHz.

Accelerator operation during the first year of scheduled beam to physics experiments is described. Some details of the gun, RF, water, vacuum, control and monitoring systems are reviewed with reference to performance and reliability.