Michigan State University Cyclotron Research Articles

Slow Current Discharges and Quenching of the MSU Superconducting Cyclotron Gas-Stopper Magnet

The Michigan State University cyclotron gas-stopper magnet generates a strong focusing cyclotron magnetic field that permits an ion beam of up to 100 MeV/u to enter the magnet and have most of its energy reduced by low-pressure helium gas. The magnet has two superconducting coil cryostats mounted within warm iron poles and a warm split iron return yoke. The coils are connected in series using room temperature cables. The peak magnet stored energy is 3.5 MJ. The magnet has been tested to its full operating current of 180 A. During tests, the magnet discharged inadvertently three times. Two low-current discharges caused the magnet current to decay through the power supply system. A discharge at 180 A quenched the magnet, because the quench protection system caused the magnet to discharge across a 1.25-Ω resistor. This fired the cold quench protection diodes within the cryostat, quenching both coils. The magnetic field in the gap was measured as the magnet discharged. For the two low-current discharges, the magnetic field followed the current in the external magnet circuit. During a quench, the magnetic field decay rate was much slower than the external circuit current decay rate.

Cooling and Operation of the MSU Cyclotron Gas-Stopper Magnet

The two coils of the cyclotron gas-stopper magnet at Michigan State University (MSU) were each cooled down and filled with liquid helium made from 300-K gas separately using three PT415 coolers in the fall of 2014. The magnet was first powered in late 2014. During the process of powering the magnet, discharges occurred because the interlock system caused the magnet to discharge. As the magnet was powered to higher currents, the coil positions were changed using the cold mass support adjustments to ensure that forces on the cold mass supports were within the design limits. The force pattern on the cold mass supports was expected to change as the iron pole pieces saturated. Changes in the coil inductance were expected and observed as the iron pole pieces saturated. This paper describes the process that brings the magnet current up to its full current.

A Cyclotron Magnet Case Study: Would Replacing the LTS Coils With HTS Coils Make Sense?

The cyclotron gas-stopper magnet (CGSM) at Michigan State University (MSU) was used as model for a study for whether HTS conductor is feasible for use in cyclotrons. The outside diameter of the CGSM split warm iron magnet yoke is 4 m, with a pole radius of 1.1 m. The desired field shape is obtained by shaping the iron pole profile. Each superconducting coil is in a separate cryostat. The two coils are connected in series through the warm electrical connection. Each coil is wound and potted within an 80 mm × 80 mm cross section and mounted within a 304 stainless steel helium vessel that is designed to carry magnetic forces to the magnet cold mass supports without excessive deflection. The iron is split, so that the cyclotron chamber and RF extraction system can be mounted between the two poles. This magnet has been successfully cooled down and tested at MSU to its 180-A operating current. One can ask the following question, “If one built a cyclotron magnet the size of the MSU cyclotron gas-stopper magnet, would replacing the existing LTS coils with HTS coils using a standard second generation HTS tape make technical sense or economic sense?” The report will explore the technical and economic issues of replacing an existing LTS coil with an HTS coil of the same size and shape, using 2G YBCO tape conductor commercially available in 2015 in place of the Nb-Ti used in the magnet.

The Michigan State University Cyclotron Laboratory: Its Early Years

The Michigan State University Cyclotron Laboratory was founded in 1958 and over the years grew in stature, becoming the highest-ranked university-based program in nuclear science. Its K50 cyclotron had unmatched capability as a light-ion accelerator and helped to define what a modern cyclotron could do to advance our understanding of nuclei. This paper describes the first twenty years of the Cyclotron Laboratory’s evolution and gives some insight into the cultural characteristics of the laboratory, and of its early members, that led it to thrive.

Henry Gabriel Blosser

Henry Gabriel Blosser

Lifetime Improvements of Heavy Ion Source Cathodes

The cathode lifetime for a cold cathode Penning heavy ion source has been increased to = 24 hrs. (a factor of 7 increase) by the use of hafnium cathodes when accelerating a carbon beam on the Michigan State University cyclotron. Compared to tantalum, the hafnium cathodes are operated at reduced power (≃ 2 kW) to avoid melting them. Thus, while the carbon beam intensity achieved with Ta cathodes is 3 to 10 times higher than that from the Hf cathodes, this is no advantage for experiments which need low beam currents (e. g. due to count rate limitations). For injection into a K=800 cyclotron the higher intensity for lower charge states from a lower power source is expected to favor Hf cathodes. Lowering the arc power for Ta cathodes, thus reducing beam intensity, does not substantially lengthen cathode lifetime. Source feed gas composition strongly influences Hf cathode performance. Extended lifetimes were achieved with CO while Ne gas resulted in very short lifetimes, suggesting the difference may be due to a chemical reaction between the Hf cathodes and an element in the feed gas.

A Quick Change Axial Cold Cathode Ion Source

An axial cold cathode heavy ion source has been built and successfully operated on the Michigan State University Cyclotron. This ion source was designed to minimize the time to change cathodes and thereby increase the cyclotron efficiency. The ion source change time (i.e. the time to change the cathode and restrike the arc) can be less than 15 minutes. In particular, all source parts that need to be cleaned at the end of a source lifetime are easily removed and clean pieces substituted. No internal ion source vacuum and water seals are broken. The ion source operates at gas flows less than 1 cc/min. and typically at 1/2 cc/min. This reduced gas usage results in a good accelerating chamber pressure and small attenuation of the accelerated beam to the external beam stop. The external ion beam intensity for a given particle and charge state are comparable to the intensity obtained from Oak Ridge and Berkeley.

The neutron time-of-flight facility at Michigan State University

A new neutron time-of-flight facility at the Michigan State University Cyclotron Laboratory incorporates a magnetic beam swinger permitting measurement of angular distributions with a stationary detector. A large volume (2.7 l) liquid scintillation detector with a time resolution of less than 200 ps was developed to exploit the long flight paths (4–32 m) made possible by the swinger. For 33 MeV neutrons an overall time (energy) resolution of 550 ps (≅150 keV) has been achieved.

Response functions of organic scintillators to high energy neutrons

Two cylindrical liquid scintillators of dimensions 5 cm × 5 cm and 12.5 cm × 12.5 cm, filled with NE213, were calibrated with high energy neutrons from E n = 3 MeV to E n = 75 MeV at the time-of-flight facility associated with the Michigan State University Cyclotron. Pulse shape discrimination was used on each detector to separate the protons and alphas produced by neutron interactions from the electrons produced by gamma rays. Response functions for monoenergetic neutrons from about 2 MeV to 75 MeV have been determined. These response functions are very different from the calculated response using the Monte Carlo method. The implications of these calibrations for measurements on high energy neutrons using liquid scintillators are discussed.

Improvements in the Michigan State University Cyclotron RF System

A new all solid state dee voltage amplitude regulator has been built for the MSU Cyclotron. It should lead to a factor of two improvement in the energy resolution of the external beam with a consequent increase in the ratio of beam-on-target to internal beam for most experiments. The RF drive to the final amplifier is regulated at a 10 mW level via an FET tetrode amplifier (Fig. 1). The gain of the FET is controlled by a signal proportional to the difference between a reference voltage and the sum of demodulated signals from each dee. Protective circuits are included which remove RF drive to the final amnlifier in the event of severe dee sparking, and limit final grid and anode voltages as guards against clumsy cyclotron operators. Peak-to-peak noise levels of the dee voltage of less than 0.07% are achieved and with careful adjustments noise levels of less than 0.05% have been obtained. Since the cyclotron is usually operated with internal beam pulse-widths of less than 0.5 nsec. corresponding to an energy spread due to phase width alone of 0.04%, noise on the dee voltage makes a significant contribution to the over-all energy resolution of the external beam.

Use of a Sigma-7 in a Nuclear Physics Laboratory

Advanced features of the Scientific Data Systems Sigma-7 computer are being used at the Michigan State University Cyclotron Laboratory to produce a system which meets all of the needs of the laboratory. The computer is capable of large off-line tasks such as distorted wave Born approximation calculations and large on-line jobs such as 16K channel pulse height analysis and particle identification. Several off-line jobs can be time-shared with safe real-time operation using a supervisor program called JANUS.

Lithium-Drifted Germanium for Charged Particle Spectroscopy

The Michigan State University Cyclotron Laboratory has undertaken a program of develoning lithium-drifted germanium for hiphresolution charged particle soectroscopy. The charged particle spnectroscopy has been concerned with reactions leading to protons, deuterons, and tritons in the energy range of 25 to 90 MeV. The emphasis of this program has been two-fold in that both the response of the detector and the packaging of the detector are considered. The side-entrv geometry Ge(Li) detector was shown to be capable of 60 keV (FWHM) with 40-MeV protons. The major limitation in the resolution for this geometry is attributed to a non-uniform window (3-12?) on the surface of the detector. The electrodes of a side entry detector were segmented and a single crystal ?E - E counter telescope was achieved. The best resolution was achieved with a thin-window surface barrier Ge(Li) detector and measured to be 34 keV (FWHM) for 40 MeV protons. A slit was used and shown to give cleaner spectra over a limited range in excitation energy. A peak to valley ratio of 5000:1 was obtained using this thin slit. The proton radiation damage of Ge(Li) was studied at 40 MeV and shown to be an appreciable problem bv 1010 protons/cm2.

Theoretical and experimental beam studies for the Michigan state university cyclotron

Numerical orbit studies using Michigan State University cyclotron fields have been performed to determine the validity of use of the separated longitudinal equations of motion under conditions obtained in actual cyclotron operation. These results have been applied in a systematic study of phase measurement procedures using frequency and trim coil detuning. Direct experimental measurements were performed to determine the phase and phase width of the beam, as well as the betatron oscillation frequencies v r and v z , as a function of radius. From the phase history data the radial dependence of the rf dee voltage has been obtained, and the nominal initial phase ϕ 0 has been determined to be +20° ± 1° for the MSU central region geometry. Use of necessary computer calculations with this result gives an rf starting time τ 0 of −28° ± 1° for maximum beam intensity. Results of experimental measurements are compared with pre-computed orbit behavior where possible.

The Beam Transport System of the Michigan State University Cyclotron

The Beam Transport System of the Michigan State University Cyclotron

Control of the Michigan State University Cyclotron

To operate the MSU cyclotron, controls are set to pre-computed values prepared by a computer program known as SETOP; to obtain a complete set of instructions the experimenter need only specify the particle of interest and a field identification number roughly proportional to energy. The program uses this information along with measured field data and a pre-computed set of ideal fields to determine the desired operating point in a straightforward series of calculations: (1) the fields at the desired excitation are synthesized by interpolation in the measured field data, (2) an average field is obtained using a modified least squares fit of the desired average field by the trim coils, and (3) the rf frequency is determined so as to minimize the energy spread of the beam in a single turn at the extraction energy. This paper describes the procedure used in SETOP to determine such points, and discusses the principle features of these computations. A typical SETOP sheet is presented along with instructions for its use. Results of beam measurements are in excellent agreement with computed beam behavior.