Roger Bangerter (right)), head of LBL's Fusion Energy Research Program, joins Simon Yu, project leader for the injector which will be used for the Induction Linac Systems Experiments (ILSE). A Section of the multiple beam high-voltage injector looms behind them.
IN THE ENTERTAINMENT WORLD IT HAS BECOME AXIOMATIC that a great success merits a sequel. Thus did Star Trek lead to Star Trek: The Next Generation. In the world of science there is no rule that a great success be followed by a sequel, but a distinguished past often does give rise to a new generation of exciting opportunities.
At LBL, the past five years have seen the retirement of three particle accelerators -- the 184-Inch Cyclotron, the Super Heavy Ion Linear Accelerator or SuperHILAC, and the Bevatron whose distinguished past gave each a chapter in the history book of science. The closure of these old facilities marks a turning of the page for LBL as well. No longer the center of the universe for high energy particle physics (though still an important player), LBL has moved on to become a fully recognized multidisciplinary national laboratory with seven broad-based core competencies that, in addition to physics, encompass the biological, materials, and chemical sciences, plus environmental assessment and remediation.
This historic transformation does not mean, however, that LBL has abandoned the construction and operation of particle accelerators. Where once stood the 184-Inch Cyclotron, now stands the Advanced Light Source, an electron synchrotron that has been optimized for the production of laserlike beams of x-ray and ultraviolet light. Waiting in the wings are two more large accelerators that will be used to blaze new trails in areas of fusion research and nuclear science. These are ILSE (the Induction Linac Systems Experiments), and the IsoSpin Laboratory. A third accelerator, the Cyclotrino, which is no bigger than a microwave oven, could make radiocarbon dating a cottage-scale enterprise. Together, these three are the next generation of particle accelerators at LBL.
From the standpoint of potential value to human civilization, no accelerator at LBL may have ever been more important than ILSE.
Spearheaded by researchers in LBL's Accelerator and Fusion Research Division (AFRD), ILSE is a 50-meter long linear accelerator designed to answer some of the crucial questions that will help decide whether fusion becomes the energy source of the next century or remains an elusive dream. Fusion, the melding together of lighter atomic nuclei to form heavier nuclei, is the source of energy that lights up the sun and every other star in the universe. If it could be safely harnessed to generate electrical power here on earth, fusion would last forever and would not contribute to the greenhouse effect, acid rain, or the depletion of the ozone layer.
To achieve fusion on earth, the two isotopes of hydrogendeuterium and tritium are heated to 100 million degrees Celsius and confined long enough for their heated nuclei to interact. One approach to accomplishing this is called inertial fusion energy (IFE). A hollow shell of frozen deuterium and tritium is set inside a pea-sized sphere and surrounded by an "ablation" layer of solid material that is rapidly heated until it turns to plasma. The plasma flying out from the ablation layer implodes the fuel, compressing its density about a thousand times and causing it to burn. If the fuel burns rapidly enough, it is confined by its own inertia and requires no external confinement system. Hydrogen bombs are proof that inertial confinement works. But can the technology be scaled-down to useful size?
One of the chief IFE technical problems is the design of a "driver" that can rapidly heat the ablation layer. It takes up to 1,000 trillion watts of power to ignite a target of thermonuclear fuel. This power must be attained almost instantaneously and must be delivered in pulses about 10 billionths of a second (10 nanoseconds) in duration. Furthermore, a commercial-scale IFE driver must be able to deliver several shots of energy a second, and it must be reliable and durable enough to last about 30 years.
The U.S. Department of Energy, after much study, concluded that the most promising driver for a commercial-scale IFE reactor would be high-powered beams of heavy ions. Such ions deposit their energy at high energies over a short distance, which means that incoming beams can be set up to produce the conditions in a target needed to ignite the fuel. Although much of the technology pertaining to the particle accelerators that would create and deliver these beams had already been developed for high-energy physics research, there were still problems to be resolved, and so DOE established a Heavy Ion Fusion Accelerator Research (HIFAR) program.
High beam power is traditionally achieved with high currents. Obtaining adequate power requires total currents greater than 10,000 amperes. Induction linacs, linear accelerators that induce an electromotive force on ions by rapidly changing the strength of a magnetic field inside a cavity, have proven capable of producing the necessary current. Transporting and focusing such intense beams is another story. Problems arise as a result of "space-charge" forces -- the mutually repulsive forces between so many positively charged ions.
In the early 1980s, AFRD researchers at LBL demonstrated that heavy-ion beams could be transported and focused at currents several times higher than had been thought possible in beams dominated by space-charge forces. These currents, however, were still far short of what an IFE reactor needs. In 1985, AFRD's HIFAR group switched from working with a single high current beam to accelerating and transporting a number of independently focused, less intense beams. The idea was that the combined energies of these multiple beams, when overlapped on a target, would be much easier to control and could even be more effective than a single high-energy beam. This work culminated in MBE-4, the world's first induction linac capable of accelerating and focusing four parallel beams simultaneously to nearly one million electron volts (1 MeV) of energy at currents of up to 90 milliamps per beam. While impressive by the standards of what had been done in the past, this was still quite small by the standards of a commercial-scale driver.
To test whether four beams of heavy ions at the same current and space-charge as a commercial IFE driver can be controlled and manipulated, LBL's HIFAR group, which is now called the Fusion Energy Research Program, under the leadership of physicist Roger Bangerter, has proposed ILSE.
"ILSE will be a flexible experimental tool that should provide the data needed to determine the feasibility and cost of heavy ion fusion," says Bangerter.
ILSE starts with an ion source and injector that can generate four beams of heavy ions, probably neon or potassium, at energies of about 2 MeV and a full ampere of current. Beams emerging from the injector pass through a "matching section" where their profiles are matched to the fields used to focus and transport them through two acceleration sections. In the first and smaller section, the focusing force arises from electrical fields generated by a series of electrostatic quadrupoles. Once the four beams have been accelerated to 4.5 MeV, they are combined into a single beam and matched into a longer second acceleration section which focuses them through a series of magnetic quadrupoles. Magnetic focusing will be required for the highly energetic and intense beams of heavy ions used in an actual IFE driver. The choice of mid-sized ions like neon and potassium for ILSE provides useful data at a fraction of the cost of heavier ions by permitting the study of magnetic focusing at energies of only 5 to 10 MeV.
Initially, only one of the four electrostatically-focused beams will be ferried into the magnetically-focused section for acceleration to the full 10 MeV. A single beam is all that is needed to test the linac's ability to produce, accelerate, and focus the ions. The linac tests will also examine tradeoffs between actually steering the beam or simply aligning it through the focusing system, and will look at various techniques for controlling the shape of the beam which is important for achieving high energy gain at the target.
The combining of the multiple beams will take place as a part of ILSE's experimental program. This phase of the project will ultimately entail the adding on of beam transport sections that will give ILSE a J-shaped configuration. The bend is needed to determine how well space-charge dominated beams of heavy ions can be transported through large angles. Most IFE schemes call for the target to be heated from more than one side. This requires bending some of the multiple beams of heavy ions through angles as large as 270 degrees. While bending magnets are routinely used to handle heavy ion beams in particle accelerators, these beams have always been at low currents.
Another critical beam manipulation to be studied in ILSE's experimental program involves an "energy tilt" at the exit of the accelerator. This phenomenon of beam physics, in which the head of the beam moves slower than its tail, shortens the beam and increases its current, thereby boosting the power of the beam.
However, an energy tilt also makes the beam impossible to focus. ILSE will feature a drift-compression and power amplification section that will boost the power of the beam while removing an energy tilt of more than ten percent prior to its entering a final focusing section.
Although ILSE will not be used to ignite any thermonuclear fuel, it will give scientists their first opportunity to focus a high current heavy ion beam onto a target in a partial vacuum chamber.
If all goes well in ILSE's linac and experimental programs, the ends of the J could eventually be connected to form a complete ring, making ILSE the first high current heavy ion "recirculator" -- a rapid-cycling circular induction accelerator. A recirculator is like a synchrotron in that the ion beam is sent around through the acceleration sections a multiple number of times to further increase its energy. For example, a recirculator configuration might boost ILSE's total beam energy to 100 MeV.
Current plans call for ILSE to be located in a building adjacent to the Bevatron. The total projected cost of the project is approximately 50 million dollars, and full operation and experimentation could commence before the year 2000. In addition to Bangerter, other key LBL staff members in the ILSE project include Simon Yu, project leader for the ILSE injector, Andy Faltens, Edward Lee, Lou Reginato, Craig Peters, and John Pickrell.
Part Two of the Article: Accelerators: The Next Generation