|The birth of VENUS|
|Paul Preuss, [email protected]|
VENUS -- an acronym extracted with some ingenuity from the phrase "versatile ECR ion source for nuclear science" -- is a groundbreaking ion source gradually coming online at the Nuclear Science Division's 88-Inch Cyclotron.
VENUS is a superconducting electron-cyclotron resonance source (ECRIS) designed for optimum operation with high-frequency microwaves at 28 gigahertz (28 GHz, or billion cycles per second). Poised to set records for beam intensity and heavy-ion charge states, it has already set the record for the world's most powerful magnetic confinement system for an ECR plasma.
"The beauty of ECR ion sources is that they can make everything from hydrogen ions to uranium ions," says Daniela Leitner, head of ion-source development at the 88-Inch, "and they can produce them in very high charge states."
Ions are atomic nuclei stripped of one or more of the electrons that normally surround them: the more missing electrons, the higher the positive charge. When complete, VENUS will be able to routinely supply high currents of uranium ions charged up to 55-plus and higher -- uranium atoms stripped of over half their electrons.
VENUS joins two ECR ion sources already supplying the 88-Inch, one of them the previous charge-and-intensity record holder, the AECR-U. Developed under the leadership of the 88-Inch Cyclotron's director Claude Lyneis, these ECRISs have endowed Berkeley Lab with unique capabilities in heavy ion physics -- one reason the 88-Inch remains one of the most capable nuclear research facilities in the world.
"High charge means the ions can be accelerated to higher energies," says Leitner. "For accelerating ions in a linear accelerator, the higher the charge state, the shorter the LINAC. Or you can use an existing cyclotron to accelerate ions to higher energies without having to change the cyclotron itself."
The goal of VENUS is to deliver intense, very high charge-state beams, at least five times better than current high-performance ECRISs. VENUS also serves as the prototype source for the proposed Rare Isotope Accelerator (RIA) to be built by the Department of Energy and the National Science Foundation. RIA will need high-current, medium-charge-state beams, the most challenging being a beam of 5 to 10 particle microamperes of uranium 30-plus. (The current record for uranium beams is 1 particle microampere, held by the AECR-U.)
How an ECRIS works
"A plasma is an ionized gas, containing free electrons and ions," Leitner explains. "The number of electrons and ion charges is always balanced, so the plasma is overall neutral."
An ECRIS is a magnetic "bottle" that confines all these charged particles to a central region of the plasma chamber, where the magnetic field is weakest. Two donut-shaped solenoid magnets form the ends of the bottle, and a third, oppositely polarized, reduces the field in its center. Around the sides of the plasma chamber, six magnets (hence "sextupoles") are arranged like barrel staves. Whichever way a charge particle in the center of the bottle looks, it sees a rapidly increasing magnetic field.
To generate the plasma and transfer energy to the plasma electrons, microwaves are sent into the chamber. Charged particles travel curved paths in a magnetic field, and there is an magnetic surface in the plasma where electrons spiral in synchronization with the microwave frequency (thus "electron cyclotron"). An electron spiraling through this so-called heating surface gets an energetic kick from the microwaves (thus "resonance").
These energetic electrons zip back and forth in the magnetic bottle, colliding with ions and stripping off more and more electrons. "There are basically two ways to improve the performance of an ECR: you can increase the plasma density or increase the confinement time," says Leitner. "As a practical matter, you have to do both."
She explains that the way to make a denser plasma is to increase the microwave frequency, which requires a stronger magnetic field. Conversely, a higher field confines the plasma longer: therefore not only the overall density, but in particular the high-charge-state ion density will increase -- considerations that strongly influenced the design of VENUS.