RUTGERS 12-INCH CYCLOTRON
For use in Modern Physics Laboratory
th By T. Koeth Rev. Feb 17, 2006
Rutgers 12 Inch, 1.2 MeV, Cyclotron Users Manual
II. Safety Concerns
III. Theory of the Cyclotron
IV. Theory of Operation of 12 Inch Cyclotron:
a. The LabView VI control system
b. Vacuum System
c. Magnet System (to be written)
d. The RF system (to be written)
e. The ion source
f. The beam detection system (to be written)
V. Procedures for operating the Cyclotron:
a. Pre-operational Check List
b. Starting Control Computer & VI
c. Turning on the Cooling Water
d. Turning on the Vacuum System
e. Turning on the Electromagnet
f. Turning on the RF System (50Watts)
g. Turning on the Ion Source VI. Maintenance & Trouble Shooting
a. Removing and Opening the Cyclotron Chamber
b. Replacing the Ion Source Filament (to be written)
c. Closing and (re-)installing the Cyclotron Chamber
a. Accel Physics: Experiencing Magnetic Resonance Acceleration (to be
b. Accel Physics: Magnetic Focusing (to be written)
c. Nuclear Physics: Measuring Q/m (to be written)
a. Operational Readiness Check List (to be written)
b. Note on Magnetic Field Measurements
c. Note of RF field measurements IX. References
a. Journal citations (to be written)
b. Text book references (to be written)
c. Expert Contact information
d. Equipment manuals
The Rutgers 12-Inch cyclotron is a particle accelerator. It utilizes a strong constant magnetic field and an oscillating high voltage to ―speed up‖ from rest – or accelerate - protons to a kinetic energy of 1,200,000
This cyclotron was initially designed and constructed by two Rutgers Undergraduate Students starting in 1995. Since that time, other Rutgers Students and Staff members as well as outside visitors have participated in the evolution of this machine. Although the 12 Inch cyclotron’s primary purpose is a teaching tool, it is built to research quality standards.
It is the goal of the original creators to inspire young scientists to enjoy building and working with apparatus. In a day, when most Physics labs are taught through computer simulation, the cyclotron offers real life experience: frustration and elation. The cyclotron project provides the student plenty of opportunities to make mistakes, to damage it, and then to repair it. Working with the cyclotron will gain the student exposure to the machine shop and electronics shop, experience in mechanical drafting, physical measurement, RF systems, high vacuum techniques, as well as insight into the physical world.
II. SAFETY CONCERNS
Interaction with Rutgers 12 Inch Cyclotron is intended to be a fun experience; however, there are plenty of
opportunities to seriously or fatally injure ones self if not cautious. There are five distinct hazards that
anyone operating or working with the cyclotron must be aware of.
1. Electrical Safety: High Voltage & High Currents
High voltages, i.e. voltages greater than 30 V, are found in many places in the cyclotron. As best as possible, care has been taken to shield and insulate all electrical connections. However, we will review of some of the more dangerous aspects of the cyclotron’s electrical systems. Starting with the mains power, 230 volts 60Hz AC is supplied to the control rack, and is internally distributed to the various components. Special attention should be made to the rear of the two Magnet Power Supplies located in the bottom of the control rack, as their input power connections are exposed terminal blocks. High voltages are necessary for the acceleration of the beam, one can reasonably expect voltages greater than 10 kV on the DEE inside the cyclotron chamber and RF matching box. AC mains for the RF is supplied through a 208 3φ power line. Similarly, inside the RF cabinet, an exposed terminal block has the live 208 V 3φ on it. The RF generator requires a DC high voltage, 5kV up to 1 ampere, to produce the needed RF power – this can be found inside the RF cabinet chassis. Even after the unit is unplugged, capacitors can store a lethal charge. Other units, such as the ion source’s filament bias
supply, -230V, and the ion gauge vacuum tube’s collector, have harmful voltages on them. It is important to know what you are working with before you begin any activity on the cyclotron.
The electromagnet coils require a significant amount of power to excite the magnetic field. The coils are wired in series, and at maximum excitation drop 80 Volts at a current of 50 amperes. Again, the conductors carrying this current are partially exposed inside the control rack.
2. Strong Magnetic Fields
Of course, during operation the cyclotron’s magnet produces a strong magnetic field. At full excititaion the field in the gap is approximately 1.2 Tesla. The field drops of very quickly, but not immediately. The cyclotron table is surrounded by slanted yellow and black warning tape installed on the floor. This boundary indicates the 5 gauss perimeter. When energized, individuals with pacemakers must not enter. Care should be taken to ensure that all iron and ferrous items are removed from the magnets area. Tools such as screwdrivers can become dangerous projectiles if they are pulled into the magnetic field.
3. High Temperatures
High temperatures are developed as a consequence of operation. Some are required, others are just a result. The most significant safety concern is the heated section of the oil diffusion pump. A 300 watt heater element is mounted at the very bottom of the diffusion pump stack. When at full temperature, the heated end of the diffusion pump can exceed 250 deg C ! Other sources of heat are mechanical pumps that have been running for some time, the ion source, the cyclotron chamber does become significantly warm after prolonged RF operation. Again, familiarize yourself with the equipment before operating or working on the cyclotron.
4. “High” Vacuum Pressures
The Cyclotron chamber is evacuated, placing 1 atmosphere of pressure on the chambers two view ports. If they are broken or cracked while under vacuum, it is likely that the pieces would be drawn into the chamber, however, it is possible that pieces of broken glass could bounce back and very high speeds. Use safety goggles when looking closely through the view ports.
5. Radiation Issues
This is the least of the safety concerns, but needs to be mentioned. 1MeV protons can potentially generate X-rays. The conversion efficiency and beam intensity are both very low. Further more, the thick stainless steel chamber shield to the side, while the exceptionally thick pole pieces shield top and bottom. To date, no measurable x-ray level above background has been made. Currently, with the materials inside, 1.2 MeV is too low for protons to generator neutrons. Never the less, REHS, feels it necessary for anyone operating or working on the cyclotron to ware a film badge dosimeter. REHS
will provide anyone intending to operate or work on the cyclotron with this badge, contact the lab’s instructor for details on how to obtain the dosimeter.
In general, be aware of the state of the machine, and your surroundings. Thoroughly communicate with others operating or working on the cyclotron. Use common sense, and if you are unsure, always ask first !
III. THEORY OF THE MAGNETIC RESONANCE ACCELERATOR or CYCLOTRON
The operation of a cyclotron is based on the fact that the period of the motion of a charged particle in a uniform magnetic field is independent of the velocity of the particle, as can be seen in the following derivation:
F = ma
2qvB = mv/r
Solve for r:
r = mv/qB
Now find the period, T:
T = 1/f = 2π/ω = 2πr/v = 2πmv/qBv
The v’s cancel:
T = 2πm/qB
The ―Cyclotron Frequency‖ f immediately follows:
f = qB/2πm
Fig. 1 is a schematic drawing of a cyclotron. The particles move in two semicircular metal containers called DEEs (because of their shape). The DEEs are housed in a vacuum chamber that is in a uniform magnetic field provided by an electromagnet. (The region in which the particles move must be evacuated so that the particles will not lose energy and be scattered in collisions with air molecules.) Between the DEEs there is maintained a potential difference ΔV that alternates in time with a period T, which is chosen to be equal to the cyclotron period that was found in the above derivation. This potential difference creates an electric field across the gap between the DEEs. At the same time, there is no electric field within each DEE because of the shielding of the metal DEEs.
The charged particles are initially injected into DEE 1 with a small velocity from an ion source near the center of the DEEs. They move in a semicircle in DEE 1 and arrive a the gap between DEE 1 and DEE 2 after time ? T, where T is the cyclotron period, and is also the period with which the potential across the DEEs is alternated. The alternation of the potential is adjusted so that DEE 1 is at a higher potential than EE 2 when the particles arrive at the gap between them. Each particle is therefore accelerated across the D
gap by the electric field across the gap and gains energy equal to q. Because it has more kinetic energy, the particle moves in a semicircle of larger radius in DEE 2, and again arrives at the gap after a time ? T. By this time the potential between the DEEs has been reversed so that DEE 2 is now at the higher potential. Once more the particle is accelerated across the gap and gains additional kinetic energy equal to q ΔV.
Each time the particle arrives at the gap, it is accelerated and gains kinetic energy q ΔV. Thus, it moves in larger and larger semicircular orbits until it eventually leaves the magnetic field. In the typical cyclotron, each particle may make up to 50 to 100 revolutions before reaching its final energy.
The kinetic energy of a particle leaving a cyclotron can be calculated by the following derivation: st1 set the following equal to the r of the DEEs and solve for the particle's velocity: max
r = mv/qB max
v = qBr/m
Next solve for the Kinetic Energy:
22222KE = ? mv = ? m (qBr/m)
Cancel the m’s and finally get:
222KE = qBr/2m
Now for an example with the Rutgers’ 12-Inch Cyclotron running protons, determine the cyclotron
frequency and maximum Kinetic Energy:
12-INCH CYCLOTRON PARAMETERS:
r5 inches = 0.127 meters max =
B = 1.2 Tesla max
q = -1.6E-19 Coulombs
m = 1.67E-27 kg
st 1 Determine the Cyclotron Frequency:
f = qB/2πm = (1.6E-19 C)(1.2 T)/( 2π)(1.67E-27 kg) = 18.3 MHz
nd 2 Find the maximum Kinetic Energy of the protons:
222 KE = (1.6E-19 C)(1.2 T)(0.127 m)/2(1.67E-27 kg)
= 1.78E-13 Joules or
= 1.1 MeV
IV. THEORY OF OPERATION OF THE CYCLOTRON
a. The LabView VI control system (to be written)
b. Vacuum System System The Vacuum System consists of mechanical ―fore‖ pump, a
―diffusion pump stack;‖ consisting of an oil diffusion pump, water cooled baffles, and a liquid nitrogen ―trap‖, and the system is supplemented with a ―roughing‖ pump. The fore pump can initially be used to rough the entire vacuum system when beginning from a cold start. The diffusion pump is protected from power failures by a fast acting ―safety‖ valve. The Fore pump brings the vacuum system down to level of molecular flow (~1E-3 Torr), allowing the diffusion pump to operate (reaching pressures of 1E-6 Torr or less). The diffusion pump transfers a downward momentum to randomly caught molecules, the molecules are then sent on their way to the fore pump. The momentum transfer comes about from boiling a high vapor pressure oil in the bottom of the diffusion pump. The boiling oil flows up a center stack and is sprayed downward through intervening chevrons. The oil jets are supersonic. The oil hits the outer wall of the diffusion pump and is cooled, drips down the wall and finally is returned to the boiling reservoir. A water cooled baffles is mounted on top of the diffusion pump to condense any ―back‖ streaming hot oil vapor prevent it from contaminating the cyclotron chamber. A liquid nitrogen trap (essentially a thermos) provides a similar function, but traps any molecules as light as Nitrogen, i.e. air. The ―Chamber Main Valve‖ (or Chamber MV) provides the means to isolate the cyclotron chamber from the diffusion ―pump stack.‖ Another
mechanical pump is teed into the ―high‖ vacuum line after the Chamber MV, as to allow the diffusion pump to remain on, while taking the cyclotron chamber out, servicing it, and returning it. The chamber must be at a pressure of 1E-2 Torr or less before re-introducing it to the diffusion pump. The rough pump’s ―rough valve‖ must be closed before opening the chambers main valve. The following is a block diagram outlining the cyclotrons vacuum system:
CHAMBER MV PIRANI GAUGE
ROUGH VALVE WATER COOLED BAFFELS
WATER COOLING SAFETY VALVE DIFF PUMP
FORE PUMP PIRANI GAUGE
CYCLOTRON VACUUM SYSTEM
c. Magnet System (to be written)
d. The RF system (to be written)
e. The ion source Ions are produced in the center of the 12-inch cyclotron by the simple
method employing a biased hot filament. The Thorium-Tungsten filament is held in place
by two connecting blocks inside a ceramic "boat." To generate ionization of hydrogen the
filament is held at a few hundred volts negative with respect to ground, heated to white
hot, and showered with hydrogen gas. Electrons emitted from the filament ionize the
hydrogen gas in the "boat." A thin ceramic cover with 1/8-inch aperture encapsulates the
volume of the boat. Gas is fed into the boat through a small pipe. This permits a region of
relatively high pressure near the filament while maintaining a good vacuum inside the
DEE and rest of the chamber.
The ceramic boat is mounted near the bottom of the cyclotron chamber, with its aperture located between the DEE and the Dummy DEE. Electrons and Ions emitted from the aperture are constrained to tightly spiral around the lines of magnetic field. This allows a column of ionization to occur in the center of the cyclotron, inside the gap between the DEE and Dummy DEE. Some of the ions created near the median plane are pulled into whichever DEE happens to be negative and are started on there way to 1 Million Volts.
The second possible path for the hydrogen to get into the ion source is through a Mass Flow Controller (MFC). Essentially this is a computer controlled leak, allowing the operator to control the flow of hydrogen into the ion source from the computer controls. Or, a feedback loop can be engaged to maintain the flow rate at a desired rate unattended. The addition of the MFC has greatly increased the time available to the operator to give attention to other components of the cyclotron while running.
A safety feature of the hydrogen flow system was the addition of a solenoid valve Upstream of the leak and MFC. Since the valve is controlled by the computer, all the operator must due is actuate the solenoid valve