Advanced Ion Sources at 88-Inch Cyclotron Help Expand Universe of Nuclear ScienceFebruary 6, 1998By Paul Preuss, paul_preuss@lbl.gov
Peggy McMahan, research coordinator at the 88-Inch Cyclotron--a national user
facility--says that the ability to go to higher masses is essential to studies
of nuclear structure. "Certain experiments require heavy projectiles, and to
accelerate them, you need higher charge. With the AECR-U we can produce
energetic dysprosium, atomic mass 162.5, heavy enough for nuclear reactions."
And when the Berkeley Gas-filled Spectrometer experiment comes on-line in the
spring, there will be a need for high intensity beams of middle-mass particles,
as well heavy elements and elements far from stability. "We expect lots of
proposals from scientists all over the world," says McMahan.
Cyclotron ion sources were not always powerful or versatile. In the prehistoric
era (the early 1930s) Ernest Lawrence twisted a stopcock on a flask and let
hydrogen gas flow through a narrow tube to a hot filament at the center of his
cyclotron. Leaks were his worst problem--the Radiation Laboratory reeked of red
wax, which was kept constantly bubbling on the stove for painting cracks in the
cyclotron seals.
In the middle ages (the early 1980s) the way to get ions into a cyclotron was
to use a Penning ion gauge, or PIG--a device about as elegant as its acronym. A
PIG was a long pole that had to be manually shoved up into a cyclotron's
middle; it functioned erratically and produced few high-charge-state ions.
"The operation of these old sources had a lot of 'people-component,'" says
Claude Lyneis, the 88-Inch Cyclotron's director. "One crew would set up the
PIG, and by the time they got it working it would be time for another crew to
come on. And some operators just had a better feel for tweaking the knobs."
Modern times began in France with the invention of the electron-cyclotron
resonance ion source. "ECRs are basically magnetic bottles," Lyneis explains.
"They evolved from research into fusion devices, where the goal was to confine
a pure plasma of hydrogen ions and electrons. The fusion researchers didn't
want to see these high-charge-state heavy ions because they sucked the energy
out of the plasma; but Richard Geller, the father of this technology, said,
'let's use this, let's get those ions out.'"
The first ECR in the United States started operating at Berkeley Lab's 88-Inch
Cyclotron in Jan. 1985 and almost immediately revitalized nuclear research at
the Lab. Compared to PIGs, the ECR doubled the maximum energy of many less
massive ions and accelerated elements as heavy as krypton to 5 MeV per nucleon.
At the same time it was precise enough not to waste minute amounts of expensive
rare isotopes such as argon 36. And because the ECR is external to the
cyclotron itself, it's simpler to adjust from the control room--"much like a
video game," as Lyneis puts it.
A typical ECR is roughly the size and shape of an oil drum lying on its side.
Two or three donut-shaped solenoid magnets surround a cylindrical chamber with
six long magnets fitted like barrel staves around it. These sextupole magnets
repel charged particles from the inner walls of the chamber; the solenoid
magnets provide "mirrors" of high field strength at the ends of the chamber.
Because the field strength is lower between the mirrors, charged particles tend
to stay in the middle of the barrel. "Any way the plasma looks, it sees an
increasing magnetic field," says Lyneis.
An ECR produces ions by first accelerating electrons. Being 1800 times less
massive than protons, electrons are readily trapped by the plasma chamber's
magnetic fields. Their spiral "cyclotron" motion in the fields, plus the
heating mechanism, gives the electron-cyclotron resonance source its name.
In the AECR-U, the electrons are accelerated by two microwave frequencies. They
bounce between the magnetic mirrors, some in short patterns in the center of
the field, where they pick up energy (heat) by resonating with the lower
microwave frequency, while more energetic electrons move in longer spirals,
resonant with both frequencies.
Meanwhile, massive positive ions careen back and forth inside the plasma
chamber, passing through the dense cloud of cycling electrons. Collisions strip
more electrons away from the ions and increase their charge states. Eventually
they diffuse through a relatively weak "hole" in the mirror field at the
downstream end of the chamber, where they are attracted by lower-voltage
extraction electrodes.
The plasma--electrons and positive ions together--must remain in equilibrium,
which requires a constant source of cold electrons. One source is the wall of
the chamber itself; one of the AECR-U's major upgrades was a new chamber
machined from solid aluminum instead of copper. When the plasma reacts with the
aluminum, electrons are freed from the chamber walls, reducing the need for an
additional source.
The upgraded AECR-U is the workhorse at the 88-Inch; with powerful sextupole
magnets delivering a field strength at the chamber wall of 0.85 Tesla--some
47,000 times the strength of the Earth's magnetic field--not only has it
produced record currents of less massive ions, it has also produced multiply
charged massive ions, such as plus-38 xenon, plus-47 gold, plus-50 bismuth, and
low currents of plus-60 uranium.
"It's of interest to the ECR community that you can produce any of these ions,"
says Lyneis, and while he refuses to boast, in the case of plus-60 uranium no
one else has done it. Lyneis credits Z.Q. "Dan" Xie with the AECR-U's success.
"Dan has set the records. Now we're off to build the Third Generation."
The Third Generation ECR has a plasma chamber with ten times the volume of the
AECR-U, and will use three frequencies of microwaves to heat the plasma instead
of two. Its superconducting magnets are wound with 28 miles of copper-jacketed
niobium-tin wire left over when the Superconduct-ing Super Collider was
canceled.
The sextupole magnets are of a novel design, using 1600 turns of
superconducting wire around iron cores; all six performed better than their
design goals. Tested separately, two of the three solenoid magnets which shape
the mirror field also exceeded their goals (the central magnet achieved half
again its designed strength) but one of the solenoids achieved only 70 percent.
Even so, in a combined test the magnetic field at the wall of the chamber was
more than two and a half times as strong as the radial field of the AECR-U.
"We have good news and bad news," Lyneis says. "We have the strongest
sextupoles ever built for an ECR, but they're showing some movement; they may
not be clamped enough to reach their full field strength." The fields of the
Third Generation's superconducting magnets are so strong that, when assembled,
the sextupoles literally try to blow themselves apart. "Right now the technical
challenge is to build a magnet structure that will run at the designed fields."
Dan Xie, along with Clyde Taylor and Ron Scanlan of the Accelerator and Fusion
Research Division, who built the superconducting magnets, are at work on a new
clamping scheme. With its superconducting sextupoles firmly clamped, the Third
Generation ECR will soon become the world's strongest ion source for
cyclotrons, likely to expand the universe of nuclear science beyond anything
that could have been imagined in the "middle ages" just fifteen years ago.
Search | Home | Questions |
