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Constance Walter

Researchers working on the Compact Accelerator System for Performing Astrophysical Research (CASPAR) will begin studying the processes in stars that create the heavier elements in the universe. Using a low-energy accelerator on the 4850 Level, they'll fire a beam of particles at various targets, including a particular type of neon gas (22Ne) as a way to better understand how all of that works. 

Sounds simple, but how do particle accelerators really work? Well, that depends on the type of accelerator. CASPAR's accelerator is modeled on the Van de Graaff accelerator, which is based on concepts developed in the early 1930s. It uses a motorized insulated rotating belt to transport a positive charge from ground to a high-voltage terminal to help accelerate charged particles up to 1 million Volts (for comparison, the LHC can accelerate particles up to almost 7 trillion Volts).

Accelerators rely on an ion or plasma source to produce charged particles. CASPAR uses radio-frequency energy to produce a beam of protons or alpha particles from hydrogen or helium gas.  

Once produced, ions enter the accelerating tube, which is kept at high vacuum. The tube, made up of insulating sections separated by metallic electrodes, must hold the entire high voltage between the terminal and ground. Connected to a resistor chain, the electrodes produce a nearly uniform voltage drop and ion acceleration, providing some focusing properties. 

The ion beam at the exit of the accelerator has a diameter of less than a few millimeters. Metallic circular rings enclose the belt and tubes, improving stability and keeping the electrical field as uniform as possible. 

The entire accelerator structure is placed in a high-pressure tank filled with electrically insulating gas (CO2/N2 mixture at 200 psi). To ensure that only particles with the right energy are directed to the target, a 25-degree bending magnet in CASPAR's beam line is utilized as an energy filter.

10 things you might not know about particle accelerators:…

  1. Turbo molecular pumping system: Used to evacuate the beamlines of air. Transporting particles within a vacuum tube reduces energy loss and scattering that can happen through collisions.
  2. Beam profile monitors: These intercept the beam periodically providing information on beam shape, size and position.
  3. Quadrupole magnet doublet: Electromagnetic focusing elements used to confine the beam and deliver a focused beam to the target.
  4. Faraday cup system: This can be inserted when required and used to intercept the beam and measure the amount of particles per second you are working with.  
  5. 4-jaw slits: Slit systems are used to define a region you wish to tune the beam of particles through. This can help define the beam shape and size.
  6. Accelerator tank: The accelerator is confined within a steel pressure vessel at ~ 200 psi of insulating gas. This helps maintain the voltage of the accelerator, independent of room conditions.
  7. Dipole analyzing magnet: This is an electromagnetic dipole magnet used to deflect ions (25 degrees). This is used to select an ion of interest based on its fundamental properties of mass, velocity and charge state.