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.”

An atom with an equal number of protons and electrons is electrically neutral. It becomes ionized when electrons are stripped away; the more missing electrons, the greater the positive charge.

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

Originally, plasmas were magnetically confined in the laboratory in an attempt to control nuclear fusion; heavy ions in these plasmas were a waste of energy. For nuclear researchers, however, such ions are valuable in themselves. In the mid-1970s, ECR ion sources were invented in France as a spin-off of controlled fusion research.

“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”).

Hot plasma — configured with six cusps — is shaped by the sextupole magnets surrounding the plasma chamber. Solenoid magnets form the ends of the magnetic bottle, confining the plasma to the central region of the chamber.

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.

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.

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