Accelerator Physics

Accelerators are the workhorses of nuclear physics research — the ones with the muscle to smash atoms so the aftermath can be studied. Accelerators can be engineered to come in all sizes — from a tabletop to a small city. As a general rule, bigger accelerators pack more power, but new technologies developed at NSCL and other laboratories help to reduce their size and cost.


Ion Source Research: In order to accelerate a particle beam, one must use electromagnetic fields. Electromagnetic fields can accelerate only particles with an electric charge. Therefore, if the particles are neutral, like a typical atom balanced by its numbers of protons and electrons, one must first ionize them, converting them into charged particles (ions). This is done by removing electrons from the atoms in devices called ion sources.


Beam Dynamics: Most particle accelerators increase the energy of charged particles in several steps. In circular accelerators — such as cyclotrons— the particles are guided in circular orbits and forced to pass by the same radio frequency (RF) high voltage system many times. At every pass of the RF system, the energy of the particle is increased. In linear accelerators — which will be used in FRIB and ReA3 — multiple RF systems are lined up one behind the other to provide an energy boost to the particles that traverse them only once. In all cases, it is necessary to manipulate the beam to keep it from increasing in size and hitting the wall of the vacuum pipe in which it moves. Special care must be taken to keep the beam in perfect timing with the accelerating RF field. When intense beams are accelerated, the internal forces between the beam particles (space charge forces) can become important since particles of the same electric charge will repel each other and tend to increase the size of the beam. Beam dynamics is the study of all these effects.


Magnet Research: Various magnetic devices are needed to manipulate beam particles or analyze the outcome of experiments. As nuclear science continues to advance, increasingly powerful magnets are required. This demands new approaches to meet the challenges inherent in magnets. For one, material properties limit how high a magnetic field can be pushed before the magnet self destructs. Also, magnets are damaged by high levels of radiation, such as those that occur in advanced nuclear physics experiments. Thus, the science of magnetism is focused on extending present technology to new limits.

Superconducting RF: Radio frequency (RF) cavities are used to accelerate charged particles to high speeds. The electric field inside the cavity can deliver kinetic energy to a charged particle passing through it. A large speed is attained after the particle travels through many cavities lined up together in a series. A typical cavity generates a potential of over 1 million volts. The power being dissipated in the cavity walls makes the cavity very hot. As a result, copper cavities cannot be used; superconducting radio frequency (SRF) cavities must be used for this purpose. Special materials (niobium) and extremely cold temperatures (cryogenics) are needed to make a cavity superconducting.

Cryogenics R & D: The field of cryogenics – the branch of physics that studies the causes and effects of extremely low temperatures - is central to the operation of most modern accelerator facilities.

Control and Operations: In order to deliver the right beam for cutting edge experiments, computer software is heavily used for beam tuning and operation automation.