9 INCH CYCLOTRON MAGNET

The key most element of any cyclotron is the uniform magnetic field which is used to bend the accelerated ions on their outward spiraling course to the target or collector. The most accessible magnet was a Varian V-3400 NMR magnet. It is of the typical H-frame design. Slight modifications were made to utilize the V-3400. Mounting the magnet sideways on a table created a horizontal gap at a reasonable work height. New pole tips were machined from 1020 rolled steel into cylinders, maximizing the diameter. The poles are nine inches in diameter and create a gap of 2.1875 inches. The maximum field obtainable from this geometry is 1.2 Tesla. The operating field value was determined by the oscillator. For reasons that will be discussed later the operating frequency is 13.56+0.03 MHz. Using the cyclotron frequency relationship:

f = qB/2p m

a magnetic field of 0.889 Tesla was determined to be the operating field value. This was a welcome operating value, as the magnet need only be run at 70 percent of its maximum values, reducing the chance of coil failure by pressing the tolerances.

The magnet weighs 1780 pounds, it requires 40 volts at 168 amps, 7Kilowatts, to produce the maximum field of 1.2 Tesla. Only 28 volts at 114 amps, 3.2 kW, is required at the operating value of 0.889 Tesla. Water cooling is used to remove the heat generated by the coils, the inlet pressure is approximately 38 PSI and flow rate is no less than 4 GPM. The pressure is controlled with an inline pressure regulator and the flow

Fig.1 Magnetic Field Calibration of V-3400

rate is monitored with an impeller driven magnetic pick-up digital flow meter.

Power is obtained from a Sorenson DCR-40-250A DC 10kW power supply. It has input requirements of 208Vrms 3-Phase at 60 Amps and is only forced air cooled. The DCR-40-250A has the ability to be "programmed" remotely, either by an external variable voltage or current source or a variable resistance. This option ultimately allowed the magnetic field to be computer controlled by interfacing a Fluke 4210 BCD programmable DC voltage source. The method of controlling the 4210 is via the IEEE-488 standard, also know as HPIB. The output range of the 4210 is 0 to 10.000 volts in 1 millivolt steps, while the input programming voltage required by the DCR-40-250A is 0 to approximately 8.00 Volts at maximum output. Since the maximum magnetic field of 1.2 Telsa requires a maximum programming voltage of 6.00 volts into the DCR-40-250A, theoretically the magnetic field could be adjusted to one part in six thousand or 2 gauss. Practically though, the field control was less than the theoretical as variations in cooling water temperature would change the resistance of the coils. The DC power system being voltage regulated then caused changes in the magnet current and of course the magnetic field.

A magnetic field calibration to determine the field as a function of applied current was done in order simplify the data collection. A precision shunt of 1.00 mW was inserted into the magnet DC power lead, a 5 digit Keithly DVM measured the voltage drop across the shunt. A Bell 620 Hall Effect Gaussmeter measured the field, while another Keithly DVM measured the 620s recorder output. The Hall Effect probe was located centered, flat against the surface of the bottom pole piece. An HP85 HPIB based computer was employed to slowly ramp the magnetic field while, while reading the values of the two meters. The data was then recorded to an IBM PC disk via an RS232 link. Fig.1 shows the calibration plot.

Fig2.Radial profile of magnetic field.

A radial measurement of the magnetic field was carefully made to determine the field fringing. Fig.2 shows the radial profile of the magnetic field while the magnet is energized with 100.00 amperes.

Uniformity of the magnetic field is extremely precise. The surfaces of the two poles are parallel with 0.001 inches. As will be seen later, the poles and yoke are slightly deflected due to the extreme pull of the electromagnetic force at high fields. No shimming of the magnetic field to increase the beam current has been attempted yet, however there are future plans to do so.


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