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Negatively charged ion beams ranging from hydrogen (Z=1) to uranium (Z=92) (excluding the noble gases) are available for injection into the tandem Van de Graaff accelerator from two ion sources. The diode ion source produces ions from gaseous materials while the sputter ion source is used primarily for producing ions from solid substances. Negative ions produced in the ion sources are accelerated to a few tenths of one percent of the speed of light and injected into the accelerator.
With the new Dowlish titanium spiral field tubes and new Oak Ridge-style resistors, the accelerator has a maximum accelerating voltage of 7.5 x 106 (7½ million) volts and produces typical beam currents of 1 x 10-6 (one millionth) amperes.
The tandem accelerator derives its name from the two stages of acceleration that particles undergo on their journey through the 35-foot-long evacuated accelerating tube. Midway along its length is the terminal, which is held at a positive potential up to the maximum voltage. The negatively charged ions injected into the accelerator from the ion source are attracted to the positive potential at the terminal and are accelerated through the first stage toward the terminal. At the terminal, the negatively charged ions pass through a region where electrons are stripped off the ions to leave them positively charged. These ions are now repelled by the positive potential at the terminal and are accelerated through the second stage, away from the terminal.
The positive potential at the terminal is maintained by dumping positive charge carried on a rotating, insulating belt onto the terminal. The potential at the terminal is varied by changing the amount of charge dumped onto the terminal from the belt. The belt, accelerating tube, terminal and their support structure are housed inside a large metal tank that is pressurized to 65 p.s.i. with an insulating gas called sulfur hexafluoride. This gas insulates the high voltage at the terminal from surrounding objects.
The ion beam emerging from the tandem accelerator has been accelerated to a velocity of up to 6% of the speed of light. If the ion beam is of sufficient energy for experimental application, it will be bent 90° and steered into one of the experimental areas for use in an experiment. If the beam is not of sufficient energy, it may be directed into the superconducting linear accelerator (LINAC) for further acceleration.
Go to the Tandem Bay for a virtual view of the tandem accelerator.
The tandem beams can be further boosted in energy by a superconducting LINAC "booster." The main body of the LINAC consists of 12 niobium split-ring resonators, with a total active accelerating length of 3 meters. Electromagnetic power is fed into the resonators at a frequency of 97 MHz which causes an alternating positive and negative accelerating field to build up in the structures of the resonators. The average electric field is about 2.5 MV/m resulting in an equivalent accelerating voltage of nine million volts.
Since the accelerating fields inside the resonators are changing as the resonator goes through its cycle, particles can be accelerated only at specific times. This requires that the normally continuous flow of positive particles from the tandem accelerator be grouped into equally spaced bunches. Two other resonators act as bunchers. The first compresses the tandem beam to fit the LINAC's acceptance. A typical time width for an injected beam is around 150 picoseconds. The other resonator is the last in the lattice and is used to tailor the beam characteristics for delivery to the target.
Because of the alternating positive and negative accelerating fields in the resonators, the LINAC can be used to both accelerate and decelerate particles. The "decel" mode of operation makes the LINAC a very versatile tool for atomic physics research. In "accel" mode, the LINAC can accelerate particles to velocities near 15% of the speed of light.
Go to the LINAC Area for a virtual view of the LINAC.
The Cryogenic Electron Beam Ion Source (CRYEBIS) is a plasma device designed to generate very highly charged ions at low velocity. This makes it an excellent complement to the larger high-velocity accelerators in the laboratory.
CRYEBIS consists of an electron gun that fires a space-charge-limited beam down the bore of a high-field superconducting solenoid. The magnetic field from the solenoid acts to compress the electrons into the flow regime known as "Brillouin flow." Arrayed around the beam, down the bore of the solenoid are sets of cylindrical electrodes. Collisionally ionized atoms are trapped radially by the space charge of the electron beam and axially by the fields put on the electrodes. Continuous bombardment of the trapped ions results in high charge states. When the ions have "cooked" enough, the fields on the electrodes can be ramped to expel them. The ions are magnetically analyzed and directed through a beamline switchyard to experiments. One of those beamlines now intersects with the ECR source and allows unique studies of ion-ion collisions.
Go to the CRYEBIS/Ion-Ion Area for a virtual view of the CRYEBIS and ECR Source.
The Ion-Ion Collision Facility links the CRYEBIS with an Electron Cyclotron Resonance (ECR) Source and a Penning Source to provide ion beams for ion-ion collision experiments. The ECR is a plasma device designed to provide highly charged ions at low velocities. The ECR source is smaller than the CRYEBIS, providing ions with lower mass and ultimate charge state, but with much greater beam current (up to one milliamp). A solenoidal magnetic field provides azimuthal confinement of electrons and ions while a hexapole array of permanent magnets gives radial confinement. The ions within the trap region are bombarded by electrons excited by 5 GHz microwaves into electron cyclotron resonance (hence the name). The Penning Source is a gas discharge source that produces ions of lower Z elements. Although much less versatile than CRYEBIS, the Penning Source can be coupled with the ECR for colliding ion beam studies while CRYEBIS is being used in "stand-alone" mode for experiments.
Go to the CRYEBIS/Ion-Ion Area for a virtual view of the Ion-Ion Collision Facility.
There are various target areas associated with the accelerators in the Macdonald Laboratory. Depending upon the type of atomic collision studies, different target apparatus may be used. Most of the target apparatus is custom built on site for specific experiments.
Go to the Long Room and Square Room for a virtual view of additional target areas.
Computing is of ever-growing importance to our scientific community. There are three families of computers available to users of the Macdonald Laboratory:
Finally, hordes of Intel-based PC microcomputers are available for process control, data reduction and presentation graphics work. These PCs are part of a Windows NT network domain that provides system flexibility and ease of maintenance.
All of these machine families are extensively networked together to provide easy transfer of data and maintain maximum user productivity and happiness.
Go to the Data Acquisition Room.
Liquid helium is used to cool both the CRYEBIS and LINAC to superconducting temperatures (-450° Fahrenheit). A CCI (now Cryo Technologies) helium refrigeration plant provides liquid helium to meet these cooling needs. This refrigerator gives about 300 watts of cooling without liquid nitrogen pre-cooling and about 600 watts with pre-cooling.
Go to the Helium Refrigerator Room and Helium Compressor Building for a virtual view of the helium refrigeration plant.
