VENUS gets under way

VENUS's planners recognized that improving upon the already high performance of the AECR-U would require the highest magnetic field strengths ever used in such an ion source. That, in turn, meant combining superconducting magnets in a way never done before. ZuQi "Dan" Xie, formerly of the Nuclear Science Division, developed the conceptual design; Clyde Taylor from the Superconducting Magnet Group of the Accelerator and Fusion Research Division was responsible for design and construction of the superconducting magnets and the cryostat.

The arrangement of the three solenoid and six sextupole magnets in VENUS

A fundamental problem was how to prevent movement of the close-packed sextupole magnets. In the very strong magnetic fields generated by these magnets and the ring-shaped solenoids surrounding them, the superconducting sextupoles literally try to blow themselves apart. Any movement at all can interrupt their superconducting state and lead to a "quench," a sudden return to normal conductivity, releasing the stored magnetic energy as heat.

A first set of sextupoles, wrapped with leftover superconducting wire made for the canceled Superconducting Supercollider accelerator, and formed around centers of solid iron, were the strongest ever produced for an ECR -- but when the field cranked up, they moved. So the team made an entirely new set of sextupoles, wrapped with new niobium-titanium superconducting wire and with iron and aluminum poles, so that the thermal expansion of wires and poles matched.

Sandwiched between an inner shell of stainless steel and an outer one of aluminum, the staves are separated by bladders filled with liquid metal under high pressure. Even at full field strength, the magnets remain frozen in place.

Working from Taylor's design, Wang NMR Inc. fabricated the successful superconducting magnet structure. The solenoid at the injection end of VENUS's plasma chamber, where the atoms to be ionized are introduced, achieves a 4-tesla magnetic field; at the end where the ions are extracted, the field is 3 tesla. The field strength at the surrounding chamber walls reaches 2.4 tesla, ten percent better than its design specified. Since Earth's magnetic field varies from a quarter to a half a gauss over the planet's surface, and a tesla is 10,000 gauss, VENUS's magnetic fields range from at least 48,000 up to 160,000 times Earth's field strength.

Powerful superconducting magnets form the heart of VENUS, but many other major components are needed to make it work. These were designed and constructed under the leadership of project manager Matthaeus Leitner (husband of Daniela), who now works on heavy-ion fusion in AFRD, and mechanical lead engineer Steve Abbott of the Engineering Division.

Of particular interest is VENUS's meter-long aluminum plasma chamber, water cooled to allow operation at 15 kilowatts of radio-frequency power. Plasma electrons touch the surface at six regions along the chamber walls; these strips are cooled by wide, shallow "flutes," and the cooling water is returned through 12 additional channels.

To gun-drill the 38-inch-long channels, followed by wire-cut electro-discharge machining (EDM) through the entire length of the chamber walls, "was a challenge at the cutting edge of fabrication capability," says Daniela Leitner; the Wisconsin machining facility that did the wire EDM cuts was one of only two in the country able to do the job.

The major components of VENUS

Other innovative features of VENUS include the high-temperature ovens that supply the gases and solids to be ionized, operating in the high vacuum system of the source, and a novel extraction system.

As they are extracted, the wanted ions must be formed into a beam. But highly charged ions, close together in a high-intensity beam, strongly repel one another, and these space-charge forces strongly affect beam transmission. Matthaeus Leitner was responsible for the low-energy beam transport system (LEBT) and the analyzing magnet, which extract and focus the beam and steer it through a 90-degree turn, spreading out the ions by mass and charge so that unwanted varieties can be stripped away.

The analyzing magnet has a unique design with specially shaped poles to correct the beam shape both horizontally and vertically. For the Rare Isotope Accelerator project, VENUS's LEBT system will serve as a test bed for high-current, heavy ion beam transport; it will provide an essential database for the design of future ECR high current injector systems.

During its first commissioning run last fall, operating with 18-GHz microwave power, VENUS produced a 30 percent higher current of oxygen plus-6 ions than the AECR-U's best performance. The challenge of high-intensity beams of highly charged ions remains. The next step, already in process, requires installing the 28-GHz microwave power supply for which VENUS was designed from its inception. By the end of this year, new records will fall to the power of VENUS.

Steve Abbot, main mechanical engineer; Daniela Leitner, head of ion-source development at the 88-Inch Cyclotron; Matthaeus Leitner, former VENUS project manager and project physicist; and Claude Lyneis, director of the 88-Inch Cyclotron, with VENUS

In addition to those named in the text, many others have contributed to the ongoing development of VENUS, including Roger Dwinell, Pat Casey, Dennis Collins, Jim Rice, Gudrun Kleist, and George Potter of the Engineering Division, Daniel Girlington of the Facilities Division, and Byron Nofrey (retired) of the Nuclear Sciences Division, with additional technical expertise supplied by Brian Bentley, Bob Connors, Bob Conroy, Al Harcourt, John Haugrud, Don Lester, Ron Oort, Bob Shannon, Jeff Trigg, Danny Williams, and Tim Williams of Engineering, and major subcomponents designed by Bob MacGill and Charlie Matuk (retired) of Engineering.

Additional information

• More about VENUS
• More about electron-cyclotron resonance ion sources (ECRISs)
• A nontechnical look at previous ion sources at the 88-Inch Cyclotron