12 INCH CYCLOTRON ELECTROSTATIC DEFLECTOR
The 2009 spring semester's cyclotron project was to design, construct, install, and operate an electrostatic deflector. The deflector 'peels' the ions out of their circular orbit to an orbit with a larger bend radius. Such a deflection channel is typically used to extract the cyclotron's beam from the chamber and send it down a transport beamline to an experiment station. It was not the intention of this project to extract the beam. Rather, we're using the electrostatic deflection channel as an EXB (wein) filter. Only ons of specific velocity will propagate through the channel, thus providing a method of measuring the beam's energy. Figure 1 shows the electrostatic deflector assembly. We have designed it to be modular, thus removable.
Fig.1 The electrostatic deflector assembly.
Our design intercepted the circulating beam at a radius of 4.0 inches, and deflected it to a radius of 4.5 inches in a 46 degree arc. The nominal radius of curvature of the deflector is 7.0 inches, thus the electrostatic field 'undoes' 3.0 inches of B-field curvature.
Fig.2 4.0 and 4.5 inch radii scribed in bottom chamber lid.
The following photos display the construction, installation, and operation of the electrostatic deflector. A superb job by both cyclotron student Tim P. and the Rutgers Physics and Chemistry Machine shop.! (Spring 2009)
Fig.3 The electrostatic deflector electrode during fabrication.
Fig.4 The shiny, well polished, deflector HV electrode.
Fig.5 The electrostatic deflector parts.
Fig.6 More of the electrostatic deflector parts.
Fig.7 The electrostatic deflector assembly shown edge on. The thin Stainless Steel (SS) vertical sheet is grounded and separates the deflection channel from the primary acceleration region.
Fig.8 The electrostatic deflector assembly.
Fig.9 The electrostatic deflector assembly, with the SS septum removed.
Fig.10 Relative placement of the electrostatic deflector assembly placement when placed in the chamber.
Fig.11 The electrical feed through for the 30kV deflector potential.
Fig.12 The beam is finally 'caught' on a phosphor screen that is located down stream of the deflection channel. Thus only ions with the correct velocity will hit and light up the screen.
Fig.13 The electrostatic deflector assembly finally installed in the chamber. The back of the phosphor screen is the smaller plate mounted on the left of the deflector assembly.
Fig.14 Tim Koeth installs the HV vacuum feedthrough for the HV deflection electrode.
Fig.15 Cyclotron student Tim P. works on the installation of the deflector.
Fig.16 Dan Hoffman is completing the deflector mounting.
Fig.17 The full crew.
Fig.18 This is the view port though which the phosphor screen and the beam spot will be observed.
Fig.19 A photo of the first beam to light up the phosphor screen.
Fig.20 500 keV protons hitting the screen. The HV power supply could only give us 28 kV on the electrode, we calculated that we needed 33 kV to center the spot on the screen. A 50 kV supply is on order.
Fig.21 At a reduced B-field (0.5 Tesla) but at the same frequency we should be able to detector molecular hydrogen (H2+). Indeed we saw that, and something else ! This figure is a series of photos of the screen, starting at a deflector voltage of 1.5 kV and ending at 8.0 kV in 0.5 kV incriments. The second peak, at 6.5 kV is molecular hydrogen. Any guesses as to the 3.5 kV peak ? We have a couple ideas, but we'd like to hear you thoughts.
A movie of the above sequence.
The next photos are of the present HV power supply. Just a negative 30 kV, constant voltage 500 uA CPS supply. Since it was only constant voltage, we had to put a 150MOhm current limiting resistor in series with the electrode. In the event of a breakdown or short, the 30 kV would drop across the resistor and limit the current to about 200 uA, thus protecting the supply.
Fig.22 The electrostatic deflector assembly.
.23 The 150 MOhm series resistor assembly.
Fig.24 The HV lead from the resistor (top of magnet) to the chamber, and the HV voltage monitoring probe. The phosphor screen view port is immediately to the right of the HV feedthroug.
Fig.25 The HV cable connecting to the HV vacuum feedthrough.
Click here for Cyclotron Student Tim P.'s final class presentation on the deflector project.
Many thanks go the folks in the Physics and Chemistry Machine shop, not just for the beautiful machining but for their patients in guiding the student in mechanical drawing as we as offering many suggestions that made the project functional. The cyclotron team is also very apreciative of Prof. Mohan Kalelkar for providing the needed support to make this project a reality.
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