Download Construction of a Small Cyclotron

Construction of a Small Cyclotron
Andrew Loucks and Mark Yuly. Department of Physics, Houghton College, One Willard Avenue, Houghton, New York
14744.
Abstract.
A cyclotron is being constructed at Houghton College. The cyclotron consists of a 17.2 cm diameter, 3.9 cm thick
evacuated chamber containing a hollow “dee” shaped electrode and a “dummy” electrode placed between the poles of a
1.1 T electromagnet. Low pressure gas will be released into the chamber where a filament will ionize the gas, and
magnetic and electric fields will force the ions into an accelerating spiral. Ideally, the final kinetic energies should be about
280 keV, 140 keV and 70 keV for protons, deuterons and helium nuclei, respectively. Currently, the vacuum system,
filament, gas handling system, and faraday cup are completed. The RF circuit is being built which should allow us to
begin testing.
Introduction. (add figure/diagram)
The cyclotron is built around the principle that, in a magnetic field, charged particles will move in a circle perpendicular to
the field. The magnetic field, however, does no work, so we need an electric field to accelerate the ions, provided via the
“dee” electrode. The electric field is not a constant, it oscillates. This way, as the ions leave the dee, the electric field
pushes them out, and as they re-enter the dee, the electric field pulls them in. On each cycle, the particle’s radius and
energy increases. In this way, high energies can be attained using relatively low voltages.
Cyclotron frequency.
The above section leaves one with the question, “How does one time when to oscillate the electric field?” To


F

m
a

solve, start with Newton,
. The magnitude of force due to a magnetic field is qvB , where q is the
charge on a particle, v is the velocity it is moving at, and B is the external magnetic field. The magnitude of
mv 2
v2
qvB 
r . The frequency of a
centripetal acceleration is r , where r is the radius of the particle. Thus,
v
v qB
qB

f 
2m . Fortunately, the frequency
particle moving in a circle is 2r , from above, r m , so frequency is
does not depend on how fast the particles are moving, or their distance from the center, it only depends on
constants, the charge and mass of the particle, and the magnetic field.
6
1
5
3
4
2
Design
1) Magnet
This is a GMW Associates 3473-70 Electromagnet. With our power supply, we can obtain fields of 1.1T.
2) Vacuum System
The vacuum system consists of a rotary forepump, diffusion pump, and LN2 cold trap, which can pump down to
pressures of magnitude 10-6 torr.
3) Gas Handling System
Consists of a Helium tank, copper pipe, and a vent, and connects into a needle valve on the chamber.
4) Power supply for magnet (capable of 50 amps), SRS IGC100 Ion Gauge Controller, HP 33120A 15 MHz
Function Generator (for RF circuit), and RF Amplifier.
5) SRS RGA 100 Gas Analyzer and Ion Gauge from Duniway Stockroom Corp.
6) Chamber
Faraday Cup
To Vacuum
View port
Filament
View port
Filament and
Dummy Dee
Feed through
Gas Flow
Needle Valve
Grounded Dummy
Dee Electrode
Dee Electrode
Connection from
Electrode to RF circuit
7) Faraday Cup (Picture and 2 diagrams)
A faraday cup is a device used to measure the beam current in a vacuum. Ours consists of a copper and
aluminum foil box connected through a shielded wire to an ammeter. It is held in place by a glass rod that also
acts as an insulator. A potential problem we are facing is trying to recapture ions that may ricochet off of the
conductor cup.
A solution that has been considered is placing a potential on the conducting cup, and placing an electrode inside
the cup. The potential would force any particles ricocheting off the electrode back onto it.
Conclusion
Currently, the cyclotron is nearing completion. An RF circuit is being devised. Also, there are some issues with
leaks that must be addressed. There is also an abundance of hydrogen that requires an explanation. With these
issues resolved, the cyclotron should be operational. An improved faraday cup will then allow us to use the
cyclotron for various experiments.