A 200 kev Electrostatic Accelerator

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1 A 200 kev Electrostatic Accelerator

2 P.Brady, B. Winey, and M.Yuly Department of Physics Houghton College Houghton, NY 14744

3 I. Abstract A small 200 kev electrostatic electron accelerator is being constructed. The preliminary design used a glass acceleration tube with external copper equipotential rings to provide a uniform electric field, but charge that accumulated on the glass eventually deflected the beam. In order to solve this problem, a new design is being tested which is made up of 51 pairs of alternating aluminum and plastic rings, with inside diameters of 3.8 cm and 5.1 cm respectively. The differing inner radii of the rings ensure that the electrons will only strike the aluminum rings, and can then be removed.

4 II. History and Motivation A small electrostatic accelerator is being constructed for use as an X-ray source, and possibly, with the later addition of an ion source, to generate neutrons. In the initial design, a Van de Graaff generator produced a high voltage, which was used to accelerate electrons leaving a tungsten cathode down a cm glass tube of diameter 3.8 cm as shown in figure 1. Copper equipotential rings spaced evenly along the length of the glass tube created a uniform electric field. However, it was discovered that charge accumulated on the glass and eventually deflected the beam. To remedy this, the new design is made up of 51 pairs of aluminum and plastic rings. Due to the difference between the inner radii of the aluminum and plastic rings, electrons in the beam will only strike the aluminum rings and be removed from the wall of the tube.

5 III.Computer Simulation Simulations of the acceleration were run using Simion 3D v7.0 to study focusing and determine the location of the cathode which produced the smallest beam spot. A computer model was created using the size and potential of the aluminum rings, flanges and high voltage terminal as shown.

6 IV. Operation A vacuum system consisting of a fore-pump, liquid nitrogen (LN 2 ) cold-trap, and diffusion-pump is used to evacuate the accelerating tube to approximately 10-7 torr as shown in figure 2. The pressure is monitored using an ionization gauge for low pressure and a Pirani gauge for intermediate pressure. A Van de Graaff generator is used to provide the high voltage on which a battery floats to heat up the tungsten filament inside the high voltage terminal. Electrons are accelerated down the tube by the uniform electric field and strike a Faraday cup coated with phosphorescent powder to make the beam spot visible. An ammeter attached to the Faraday cup allows the beam current to be measured.

7 V. Construction The accelerating tube consists of 51 alternating high density polyethylene and 5052 aluminum rings, 0.6 cm and 0.3 cm thick respectively. Four 0.6 cm holes were symmetrically placed in the rings so delrin rods could be used to align the rings during the gluing process and strengthen the structure once the vacuum epoxy cured. Aluminum-bronze flanges were made to attach the accelerating tube to the vacuum system and the feed through flange for the cathode in the high voltage terminal. The flange attached to the HV side was cut at an angle to reduce the chance of accidental electric discharge.

8 VI. Conclusion The system should be completed and tested for leaks by Jan- Feb The bremsstrahlung spectrum will then be measured using a NaI scintillator. The beam current will be measured using the ammeter attached to the Faraday cup. The up charge of the Van de Graaff generator will be measured during testing. Future plans include adding an ion source to take advantage of the d-d reaction to produce neutrons in the range of 3 MeV. The polarity would be reversed to accelerate deuterons down the tube into a target impregnated with deuterium. To be used as a neutron source the deuterons need energies in the 150 kev 450 kev range.

9 Figure 1

10 Figure 2

14 Figure 1 Photograph of the initial design. The ionization gauge and Faraday cup are visible on the left. The high voltage terminal and Van de Graaff generator are at the end of the glass accelerating tube.

15 Figure 2 Photograph of the vacuum system. The diffusion pump and LN 2 cold-trap are in the foreground. The fore-pump can be seen in the background.

16 Simulation showing position of cathode that gives smallest beam spot. Simulation showing position of cathode that gives very wide beam spot. Simulation showing position of cathode that gives intermediate beam spot.

17 Ionization Vacuum Gauge High Voltage / Van de Graaff Terminal A Phosphorescent Powder Coated Glass Screen BATTERY Van de Graaff charge column Vacuum Pump System

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