The Magnetic Field Profile of New Pole Tips on the Rutgers 12-inch Cyclotron John McClain Robert Friedman

Cyclotron

What is a Cyclotron and How Does it Work? It’s a “table-top” particle accelerator! Accelerates charged particles in circular orbits of increasing radius. Essentially a “curled-up” linear accelerator. –The ion accelerates when it passes through an electric potential. –The increased ion velocity in the presence of the B-field, leads to an orbit of larger radius.

Basic Cyclotron Design

Rutgers 12-inch Cyclotron Instruments and Controls Cyclotron Chamber Pole Tip Hydrogen Solenoid Coils

Cyclotron Chamber The Dee Ion Collector Phosphorescent Screen Ion Source (cathode) “Dummy” Dee (grounded)

Chamber Reloaded

“Ion” Source Hot, negatively biased cathode thermoionically emits electrons. Electrons settle into tight orbits about field lines. As they travel along the lines, they collide with ambient hydrogen gas, ionizing the atoms Free protons are produced!

Beam Spreading Hydrogen ionized at all points in the electron beam –Initial axial (vertical) amplitude Protons have random initial velocities –Uniform magnetic field = Helical path –Small survival rate

Beam Focusing Small decrease in magnetic field with increasing radius Magnetic field lines are concave inward Off of the median plane B-field has a radial, as well as axial, component

Sloped Pole Piece

Beam Focusing Formalism Produces a restoring force on moving charged particles: AxialRadial

The n-value is the Thing! Can be used to find axial and radial oscillation damping and frequencies “The optimum shape of magnetic field in a cyclotron was found to be one in which the n-value rises approximately linearly with radius.” –Particle Accelerators by Blewett and Livingston

Measuring the Field Profile Measured field with a Hall Probe fixed to a motorized platform on a track. Probe was free to move in the radial direction. Reference position w.r.t. the cyclotron center identified by a “bullet” magnet with a sharply peaked profile. Made field measurements at stepped positions from the edge to the center of the magnet.

Bullet and Residual Fields

Residual-Removed Bullet Profile

Magnet Profile

n-value

The End