B.E. Clifft

An improved phase-control system for superconducting low-velocity accelerating structures

It is noted that microphonic fluctuations in the RF eigenfrequency of superconducting (SC) slow-wave structures must be compensated by a fast-tuning system in order to control the RF phase. The tuning system must handle a reactive power proportional to the product of the tuning range and the RF energy content of the resonant cavity. The accelerating field level of many of the SC cavities forming the ATLAS linac has been limited by the RF power capacity of the presently used p-i-n diode based fast-tuner. A new system has been developed, utilizing p-i-n diodes operating immersed in liquid nitrogen, with the diodes controlled by a high-voltage V-groove metal-oxide semiconductor (VMOS) FET driver. The system has operated at reactive power levels above 20 kVA, a factor of four increase over an earlier design. The increased capacity will permit phase stabilization of superconducting resonators at higher field levels. In particular, it has enabled the operation of very-low-velocity superconducting structures at gradients of more than 4 MV/m in the ATLAS positive-ion injector linac.<<ETX>>

The positive-ion injector of ATLAS: Design and operating experience

Abstract The recently completed positive-ion injector for the heavy-ion accelerator ATLAS is a replacement for the tandem injector of the present tandem-linac system. Unlike the tandem, the new injector provides ions from the full range of the periodic table. The concept for the new injector, which consists of an ECR ion source on a voltage platform coupled to a very-low-velocity superconducting linac, introduces technical problems and uncertainties that are well beyond those encountered previously for superconducting linacs. The solution to these problems and their relationship to performance are outlined, and experience in the operation of ATLAS with its new injector is discussed.

STATUS OF THE ATLAS ACCELERATOR

The construction of the ATLAS superconducting heavy‐ion linear accelerator is complete. The first beam acceleration occurred on March 22, 1985. The first experiment with the ATLAS linac took place during the week of October 7, 1985. The project was completed on time and within budget. Initial system performance has met our expectations.

Current status of ATLAS and proposed expansion to an exotic beam facility

The Argonne Tandem Linear Accelerator System (ATLAS) has been operating on a twenty-four hour, seven days a week schedule since the beginning of Fiscal Year 1994. Twenty-six different ion species ran during this period in 71 separate experiments. During the past year, there have been many projects undertaken to improve operation efficiency and upgrade various accelerator systems. There is also a new ECR ion source construction project underway. This paper covers, linac operation and new tuning techniques, the second generation ECR source construction project, the refrigerator system upgrade, an upgrade to the control system. Also described is a future expansion of ATLAS as an Exotic Beam Facility. ATLAS is a world class heavy ion accelerator with an estimated value of approximately

A low-cost non-intercepting beam current and phase monitor for heavy ions

80 million. A concept that would utilize ATLAS as the foundation for a facility to generate and accelerate radioactive beams is briefly discussed.

Status of the positive ion injector for atlas

A low cost ion beam measurement system has been developed for use at ATLAS. The system provides nondestructive phase and intensity measurement of passing ion beam bunches by sensing their electric fields. Bunches traverse a short tubular electrode thereby inducing displacement currents. These currents are brought outside the vacuum jacket where a lumped inductance resonates electrode capacitance at one of the bunching harmonic frequencies. This configuration yields a basic sensitivity of a few hundred millivolts signal per microampere of beam current. Beam induced radiofrequency signals are summed against an offset frequency generated by the master oscillator. The resulting difference frequency conveys beam intensity and bunch phase information which is sent to separate processing channels. One channel utilizes a phase locked loop to stabilize phase readings during microsecond beam dropouts. The other channel uses a linear full-wave active rectifier circuit which converts sine wave signal amplitude to a DC voltage representing beam current. Plans are in progress to install this new diagnostic at several locations in ATLAS which should help shorten the tuning cycle of new ion species.

A Study of Beam Chopping Options for the ATLAS Positive Ion Linac

The positive ion injector project will replace a High Voltage Engineering Corp. model FN 9 MV tandem electrostatic accelerator as the injector into the ATLAS superconducting heavy ion linear accelerator. It consists of an electron cyclotron resonance (ECR) ion source on a 350-kV platform injecting into a linac of individually phased superconducting resonators which have been optimized for ions with velocities as low as ..beta.. = 0.009. The resulting combination will extend the useful mass range of ATLAS to projectiles as heavy as uranium, while increasing the beam currents available by a factor of 100. (2 refs., 2 figs., 1 tab.)

First operational experience with the positive-ion injector of ATLAS

Unbunched beam components from the injection beam bunching system must be removed prior to acceleration in the ATLAS Positive Ion Injector Linac (PII). A sine wave chopper has been used for this purpose up to now. Such a device can have a significant detrimental effect on the longitudinal and transverse beam emittance of heavy-ion beams which can be sufficiently severe to limit the overall beam quality from the ATLAS accelerator. A study of the optimum chopper configuration and chopper type was undertaken as part of a new ion source project for ATLAS. A transmission line chopper and a two harmonic chopper were investigated as alternatives to the conventional sine wave chopper. This paper reports the results of that investigation and discusses the design of the selected transmission line chopper.

Operational status of the uranium beam upgrade of the ATLAS accelerator

Abstract A Positive-Ion Injector (PII) designed to enable ATLAS to accelerate all stable nuclei has been completed and successfully tested. This new injector system consists of an ECR source on a 350-kV platform coupled to a 12-MV superconducting injector linac formed with four different types of independently-phased 4-gap accelerating structures. The injector linac is configured to be optimum for the acceleration of uranium ions from 0.029 to ≈ 1.1 MeV/u. When ions with q/A > 0.1 are accelerated by PII and injected into the main ATLAS linac, CW beams with energies over 6 MeV/u can be delivered to the experimental areas. Since its completion in March 1992, PII has been tested by accelerating 30 Si 7+ , 40 Ar 11+ , 132 Xe 13+ , and 208 Pb 24+ . For all of these, transmission through the injector linac was ∼100% of the pre-bunched beam, which corresponds to ∼60% of the DC beam from the source. The accelerating fields of the superconducting resonators were somewhat greater than the design goals, and the whole system ran stably for long periods of time.

Operational experience of the atlas accelerator

The Positive-Ion Injector (PII) for ATLAS is complete. First beams from the new injector have been accelerated and used for experiments at ATLAS. The POI consists of an ECR ion source on a 350-kV platform and a low-velocity superconducting linac. The first acceleration of uranium for the experimental program has demonstrated that the design goals of the project have been met, Since the summer of 1992, the new injector has been used for the research program approximately 50% of the time. Longitudinal beam quality from the new injector has been measured to be significantly better than comparable beams from the tandem injector. Changes to the mix of resonators in the main ATLAS accelerator to match better the velocity profile for heavy beams such as uranium are nearly complete and uranium energies up to 6.45 MeV per nucleon have been achieved. The operating experience of the new ATLAS facility will be discussed with emphasis on the measured beam quality as well as achieved beam energies and currents.<<ETX>>