Probing the Structure of Matter: a History of Accelerators at Los Alamos

Probing the Structure of Matter

a history of accelerators at Los Alamos

Adjusting the Van de Graaff Accelerator, 1946. Richard A. Reichelt

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he history of Los Alamos is target and interact with the target Lawrence in 1929. He called his in- intimately linked to machines. atoms. If the particles have a high vention the cyclotron. TMachines of all types—from enough energy, they interact with the reactors to computers to lasers— nuclei of the atoms, often yielding have been indispensable in advanc- “secondary” particles whose identi- World War II Accelerators ing scientific knowledge at the Lab- ties depend on the energy of the pri- oratory. Among all of those ma- mary particles and on the target ma- J. Robert Oppenheimer, director chines, accelerators occupy a special terial. Sometimes the accelerated of Project Y during World War II, position. During World War II ac- particles are only a means of produc- was responsible for bringing acceler- celerators provided vital information ing the secondary particles (neutrons ators to Los Alamos. He believed to scientists designing the first nu- and pions are examples of secondary that only by consolidating machines clear weapons, and after the war particles). Although the basic princi- and scientists in one location could a some of those machines were used ple of particle acceleration is simple, nuclear bomb be speedily built. And as tools of basic scientific research. its execution is not. Designing an speed was important—the Germans Then, in the early 1970s, a half- accelerator requires creativity and in- were thought to be well advanced in mile-long accelerator called LAMPF genuity, and operating an accelerator developing a nuclear bomb. Oppen- (formally named the Clinton P. An- can be an art in itself. heimer envisaged concentrating the derson Meson Physics Facility) was completed after a decade of planning and construction. Over the years LAMPF has been the workhorse for many Laboratory programs, including programs in nuclear physics, weapons research, neutron scattering, radioisotope production, and pion cancer therapy. Accelerators for more specialized purposes have also been developed or improved by Los Alamos scientists. The constant effort to upgrade and redesign accelerators has often led to unexpected results. New technological spin-offs, new vistas of scientific research, have opened up as one generation of machines replaced another. This photoessay briefly traces the evolution of accelerators at Los Alamos and examines Technical Building Z. This shack was built for the Cockcroft-Walton accelerator in 1943. their different applications. In the last section the promise of a future Most modern accelerators are work of designing and building the generation of machines is examined. classified as either linear accelera- the first nuclear bombs in one loca- First a few words of explanation. tors (linacs) or circular accelerators. tion so that ideas and findings could Most accelerators work according to The linac design, in which the accel- easily be exchanged. Accelerators, the same basic principle. An electric erated particles move in a straight as suppliers of nuclear data, were ur- field is applied to a stream of charged line, was first developed by Rolph gently needed to provide an experi- particles (typically electrons or pro- Wideröe in 1928 and later refined by mental foundation for the work. tons) and accelerates the particles to Luis Alvarez. The circular machine, Thus, in the spring of 1943, four greater and greater energies. Those in which the particles move in a cir- bulky machines were transported to accelerated particles can then strike a cular path, was conceived by Ernest a remote New Mexico location from

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predetonation, in the split and release energy and more plutonium bomb—a neutrons—had not been studied at problem arising from all the relevant neutron energies. the spontaneous fis- Several months earlier, in December, sion of fuel impuri- 1942, Enrico Fermi had induced the ties. Of the two prob- first fission chain reaction at the lems predetonation University of Chicago using a “pile” was the more in- consisting of enriched uranium in- tractable and the terleaved with graphite moderators. source of pessimism The moderators were designed to about the feasibility slow down neutrons and make the of a plutonium bomb. fission reaction more efficient. But The accelerators fission induced by fast neutrons was made it possible for expected to be less efficient than fis- The Wisconsin Short Tank. One of five accelerators comman- scientists to deter- sion induced by slow neutrons. In deered for use at Los Alamos during World War II, the Short Tank was an improved version of a Van de Graaff accelerator mine the critical an explosive chain reaction there and was designed mainly by Joseph McKibben at the University masses for each pro- could be no moderators; the neu- of Wisconsin. The lead shielding and concrete blocks surround- posed bomb design. trons would emerge from fission at ing the pressure vessel served as radiation protection. The machines sup- high energies—between 0.1 and 3 universities across the country. To plied neutrons for studying the neu- MeV (1 MeV = 1 million electron throw curious observers off the tron interactions involved in an ex- volts)—and travel unimpeded until track, the accelerators followed a plosive fission chain reaction. At they collided with other heavy nu- circuitous route. They were first di- that time neutron-induced fission— clei. How would the fast neutrons verted to a medical officer in St. the process by which a neutron caus- interact with heavy nuclei? What Louis and then shipped in boxcars to es the nucleus of a heavy atom to percentage of those neutrons would Santa Fe. Finally the accelerators were moved on flatbed trucks to Los Alamos, just as some of the world’s most eminent scientists were beginning to gather there. Massive steel pressure tanks for some of the machines arrived during technical con- ferences in mid April. The accelerators, it was hoped, would help scientists tackle two major challenges. The first was to determine the critical masses of the proposed nuclear fuels—plutonium (239Pu) and uranium highly enriched in the isotope 235U. (The critical mass is the smallest mass of nuclear fuel of a certain shape necessary to sustain a chain reaction.) Both fuels were so scarce that direct measurements of the critical mass were im- possible; production techniques for the fuels were only just being devel- The Illinois Cockcroft-Walton Accelerator. This accelerator was used by John Manley and his group to investigate the efficacy of different metals as a “tamper”—a liner surrounding the nuclear oped. The second challenge was to explosive that acts as a neutron reflector and makes the explosive chain reaction more efficient. find a way of preventing a “fizzle,” or Gold, uranium, platinum, tungsten, and other metals were investigated as possible tampers.

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fission of 240Pu were likely to initi- ate a chain reaction in the two chunks before the gun mechanism could bring them together into a supercritical mass. The result would be a fizzle. Therefore a gun-type plutonium weapon was out of the question. So in August the Labora- tory threw its resources into achiev- ing criticality in a plutonium weapon by implosion, that is, by detonation of a layer of conventional explosives surrounding a subcritical sphere of plutonium. The idea was that the inward force of the conventional explosion would compress the plutonium sphere and thereby create a supercritical mass, provided the implosion was sufficiently symmet- ric. The implosion option had been pursued previously—early experiments had involved detonating TNT wrapped around iron pipes—but im- plosive forces of the required symmetry had not been achieved. A Portion of the Harvard Cyclotron. A group led by R. R. Wilson investigated nuclear reac- To help in the design of an implo- tions induced by the low-energy neutrons provided by this 42-inch cyclotron. sion weapon, a betatron—a circular cause fission? How many neutrons MeV. And a cyclotron from Harvard electron accelerator— was procured would be released in a fission reac- University produced neutrons with in December, 1944. Pulses of x rays tion caused by a fast neutron? What even lower energies. Together the produced by the betatron were used percentage would be reflected back accelerators produced neutrons with to obtain a sequence of images of a into the fissile material from a metal energies spanning the pertinent sphere of mock fuel as it was being liner—or “tamper”—surrounding the energy spectrum. imploded by a particular configura- core? As sources of neutrons with The summer of 1944 signaled an tion of high explosive. This diag- various energies, accelerators were abrupt shift in the program at Los nostic technique, along with others, just the tools scientists needed in Alamos. In mid April Emilio Segré helped solve the problem of uneven order to study the nuclear reactions and his band of graduate students collapse in the implosion weapon. relevant to weapons physics. had discovered some bad news—that On July 16, 1945, a plutonium Within a few months of their ar- 240Pu (an isotopic impurity present weapon was tested near Alamogor- rival, all four accelerators were pro- in 239Pu) had a high cross section do, New Mexico. On August 6 a ducing neutrons in hastily erected for spontaneous fission. The origi- uranium weapon, Little Boy, was wooden buildings. A Cockcroft- nal plan for initiating an explosive dropped on Hiroshima, Japan, and Walton accelerator requisitioned fission chain reaction in either a plu- on August 9 a plutonium weapon, from the University of Illinois pro- tonium or a uranium bomb had been Fat Man, was dropped on Nagasaki. duced 2.5-MeV neutrons. Two Van to use a gun mechanism to fire one Soon after those bombing attacks de Graaff accelerators from the Uni- subcritical chunk of nuclear fuel Japan surrendered unconditionally. versity of Wisconsin produced neu- into another subcritical chunk. But Oppenheimer’s idea of consolidating trons with energies between a few if the fuel was plutonium, the neu- scientists and machines at Los hundredths of an MeV and several trons produced by the spontaneous Alamos had simultaneously un-

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leashed a horribly destructive force study neutron interactions relevant electron beam onto a tungsten target. and helped to bring about an end to to nuclear fusion. The old Harvard The x-ray bursts are sent through World War II. cyclotron was upgraded into a vari- model weapons at a remote blasting able-energy cyclotron that could ac- site and provide three-dimensional celerate different kinds of charged pictures of imploding spheres. Postwar Developments particles. Additionally, the cy- PHERMEX was also used to study clotron group developed a special fluid dynamics and the behavior of Norris Bradbury, who became di- camera that recorded the angular matter under extreme, shock-driven rector in October, 1945, had the job distribution of the accelerated parti- conditions. The origins of PHER- of charting the direction the Labora- cles after they had been scattered by MEX were in the pioneering World tory would take in the postwar the nuclei within a particular target War II work done with the betatron. decades. His primary concern was element. Such data provide infor- Still in operation, PHERMEX has to transform an institution that had mation about the energy levels of recently been used to study the been built for a short-term pur- the target nucleus. Scientists’ strength of ceramic tank armor. pose—designing and building nu- wives, many with university de- Ten years after the war Los Alam- clear bombs—into an institution grees, were enlisted to scan the pho- os was still the foremost nuclear- with long-range purposes and goals. tographs, becoming a team of first- physics laboratory in the world. Should Los Alamos become a nu- rank nuclear spectroscopists. Bradbury’s program of basic scien- clear-bomb factory? Or should it In August, 1949, the Soviets ex- tific research had fed into applied cease weapons work altogether? Or ploded their first nuclear bomb. fields in many ways—nuclear spec- should it build a foundation of basic President Truman subsequently an- troscopy, optical modeling, and the scientific research with weapons ap- nounced that the United States was thermonuclear weapon. But the plications? Recognizing the coming embarking on a program to build a world was catching up, and the competition with the Soviet Union variety of nuclear weapons, includ- wartime accelerators were not state- and wanting to avoid “technological ing a fusion, or thermonuclear, of-the-art for studying nuclei and surprise,” Bradbury (along with oth- bomb. Such a bomb utilizes a fis- nuclear forces. In 1946 the first ers) decided in favor of the last op- sion bomb to trigger the fusion of proton linac—built at the University tion. He also believed that such a deuterium and tritium nuclei; its ex- of California by Luis Alvarez, a program of basic scientific research plosive yield is many times higher physicist who had been involved in would help to retain the cadre of tal- than that of a fission bomb alone. the nuclear-bomb project—had be- ented individuals whose mentors had Actually, research on a fusion came operative. A linac accelerates been among the brightest scientists weapon had been pursued at Los charged particles with a series of of the twentieth century. As part of Alamos continuously since the war electrical “pushes,” each of which the experimental foundation for that years. Those early efforts were vital increases the energy of the particles new program, three wartime acceler- to the success of tests, called the by an amount that is small compared ators were purchased by the govern- Greenhouse series, that led to the to the total energy gain desired. Al- ment—the Short Tank, the Cock- first thermonuclear reaction—the varez used radar oscillators devel- croft-Walton, and the cyclotron. George shot—in 1951. oped during the war to produce the The Long Tank was returned to the Diagnostic tools for weapons accelerating electric field—a radio- University of Wisconsin. were also developed during the early frequency oscillating electric field— A high-energy Van de Graaff ac- Bradbury years. Two electron linacs in a single long resonant cavity. celerator, a vertical model designed were built to provide radiographs of Along the length of the cavity were by Joe McKibben, was built to re- the implosion process. That work forty-five “drift tubes” that prevent- place the Long Tank; it provided eventually led to the construction in ed the protons from being decelerat- monoenergetic neutrons with ener- 1963 of PHERMEX (pulsed high-en- ed during the negative phase of the gies up to approximately 8 MeV. ergy radiographic machine emitting electric field. The Alvarez design Those high-energy neutrons and the x rays), a huge electron accelerator, would have tremendous implications 14-MeV neutrons provided by the housed in a concrete bunker, that for accelerator development at Los Cockcroft-Walton were used to generates x rays by accelerating an Alamos.

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An Aerial View of LAMPF in 1983.

LAMPF LAMPF, described the original moti- still held today, that eventually this vation for building the massive ac- country and the entire industrialized LAMPF is a half-mile-long linear celerator as follows: world will be forced, whether they accelerator built atop a narrow mesa The most fundamental reason we like it or not, to a nuclear-energy not far from Los Alamos. In 1983 advanced [for requesting funds of economy. … We are simply running Louis Rosen, the chief architect of Congress] stemmed from a belief, out of conventional organic sources

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of energy. … If a nuclear-energy vantages and drawbacks, but the sci- they perfected a design, known as the economy is inevitable, how can we entists eventually decided in favor side-coupled cavity, that could accel- make it as efficient and safe as possi- of the linear accelerator. The cy- erate high-intensity beams of protons ble? With advanced nuclear technol- clotron couldn’t achieve the neces- to 800 MeV. ogy. The reason we are doing so sary proton energy without an unac- In 1967 a working prototype, badly now with power reactors is ceptable loss in beam intensity; a called the Electron Prototype Accel- that we didn’t start with enough tech- then unheard-of intensity of 1 mil- erator, demonstrated the viability of nology. And because technology is liamperes was desired. the new cavity design. The demon- the child of science, we need a stration had immediate strong science base. Without practical consequences. that base, technology cannot Several manufacturers of advance and will soon dry up. x-ray machines for the We felt that the need for sci- medical community began ence as the basis of technology incorporating the side-cou- was the most compelling rea- pled cavity design into son for Los Alamos to engage, their new models. The re- at a very high level, in basic sult was smaller, more ef- nuclear science. ficient machines. Today Rosen first proposed the the side-coupled cavity is idea of building the accelera- recognized as having revo- tor in 1962. In a memo to J. lutionized x-ray-therapy M. B. Kellogg, then leader of machines and other med- the Physics Division, he ical linear accelerators. sketched the scientific impor- Incidentally, the side-cou- tance of a “meson factory”—a pled cavity was never new, high-intensity proton ac- Victory Day: June 9, 1972. Louis Rosen and others watch control patented by its inventors. celerator that would supply an instruments as the LAMPF linac produces its first beam of 800-MeV The groundbreaking abundance of pi mesons to protons. ceremony for LAMPF study nuclear interactions and the The accelerator was envisaged in was held on February 15, 1968. structure of nuclei. Pi mesons, or three stages. In the first stage a Four years later, after numerous pions, are short-lived particles that Cockcroft-Walton would accelerate Congressional funding battles led by can be created by firing protons ac- the protons to a low energy (0.75 Rosen, the facility was completed. celerated to nearly the speed of light MeV). In the second stage an Al- LAMPF is operated as a national at light-element targets. Since most varez-type drift-tube linac would ac- user facility; that is, beam time is accelerators at that time were being celerate the protons further to a shared among many individuals designed to achieve higher energies medium energy (100 MeV). The from both the United States and per particle rather than higher beam third and final stage would acceler- abroad. An international committee intensities (or numbers of particles ate the protons to 800 MeV. But no of experts evaluates requests for per unit time), Rosen thought that a one yet knew how to accelerate pro- beam time and advises LAMPF’s di- meson factory could open an entire- tons to energies higher than 200 MeV. rector on their scientific merit. The ly new realm in nuclear physics. In the face of considerable skepti- accelerator has spawned a remark- The memo eventually reached cism from outside experts, a small able number of research programs. Bradbury and began to generate en- team of scientists led by Darragh In the area of pure science, LAMPF thusiasm among scientists at Los Nagle and Ed Knapp began the search acts as a unique bridge between the Alamos. A small group of experi- for a suitable acceleration scheme. particle-physics and nuclear-physics mental and theoretical physicists They investigated many different cav- communities, enabling each to un- began looking at two possible accel- ity designs, including pillbox struc- derstand the other’s methodologies erator designs—the cyclotron and tures, cloverleaf structures, and slow- and modes of thought (see “Medi- the linac. Both designs had disad- wave helices. Then, in early 1965 um-Energy Physics at LAMPF”).

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Medium-Energy Physics at LAMPF Mikkel B. Johnson

he LAMPF accelerator pro- helped to open up an entirely new For example, in a pion double- vides primary beams of pro- field of basic research—medium- charge-exchange experiment, nuclei Ttons and negatively charged energy nuclear physics. can be forced to accept two units of hydrogen ions as well as secondary By using the tools available at charge at one time. Measurements beams of neutrons, pions, muons, LAMPF, nuclei can be explored in of the dependence of the scattering and neutrinos. The uniqueness of new ways. Experiments with cross section on the final state of LAMPF as an experimental facility muons, pions, and nucleons have the nucleus have led to a strikingly derives from the high intensity of quantified the size, shape, and com- detailed picture of correlations in those beams and the consequent ca- position of nuclear states and mea- the motion of neutrons and protons. pability to exploit rare reactions to sured the response of nuclei to the In addition, double-charge-ex- answer specific questions about addition of charge, energy, and mo- change experiments have uncovered particles and nuclei. Additionally, mentum, as well as quanta of vibra- new modes of motion of nuclei in the high resolution of the spectrom- tional and rotational excitation. which two patterns of vibration co- eters and other detectors available Some of the most interesting results exist and have also led to the dis- at LAMPF make precision measure- have been obtained when the in- covery and study of new nuclear ments feasible. As a result, over coming particles cause the nuclei to species. In experiments with nucle- the last twenty years LAMPF has respond in certain extreme ways. on beams, nuclei have been forced to accept a small amount of energy and at the same time a large mo- mentum in various spin configura- tions hitherto incapable of being distinguished. For reasons that are not completely understood but are being actively sought, theories that work well in more normal situa- tions have been discovered to break down under those unusual conditions. And state-of-the-art studies in atomic physics have addressed previously inaccessible regions of the spectrum with a unique technique that combines a beam of laser light and a beam of negatively charged hydrogen ions. Additionally, various scattering experiments and measurements of the decay products of muons and pions have focused on the nature of The High-Resolution Proton Spectrometer at LAMPF. This instrument, known as the the underlying strong, weak, and HRS, is used to measure cross sections for elastic and inelastic scattering of protons from electromagnetic interactions. Pre- nuclei. Careful measurements with the HRS of the spin dependence of cross sections led to the widespread acceptance of relativistic descriptions of nuclear dynamics. HRS results also cision measurements with muons stimulated theoretical and further experimental investigations of how the nuclear medium af- have yielded new insights into fects the nucleon-nucleon interaction. quantum electrodynamics. A sys-

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tematic program extending over many years has completely mapped out the character of the nucleon- nucleon interaction over the entire energy range of LAMPF and has thus provided a bank of basic data for theorists and laid the foundation for interpreting nucleon-nucleus scattering experiments. An experiment using the world’s most intense source of very-low-energy neutrons has led to development of a completely new technique for detecting a signature of breakdown of a fundamental symmetry in nuclear forces. The technique, which uses properties of complex nuclei to magnify the signal for the breakdown of parity (the mirror symmetry between left and right) by a fac- The MEGA Detector at LAMPF. This detector will be used in a search, scheduled to begin tor of about 1 million, has uncov- this summer, for the decay of a muon into an electron and a gamma ray. The occurrence of ered unexpected results and opened that reaction would signify a breakdown of the standard model. The sensitivity of the MEGA detector to the decay is two orders of magnitude greater than that of detectors used in previ- a rich area of exploration. Other ous searches. investigations of fundamental interactions include the scattering of a possible explanation for the ob- ativistic motion and interacting neutrinos from various targets. served shortfall in electron neutri- through potentials. That picture is Neutrinos interact so weakly that nos coming from the sun. Yet other no longer adequate to describe what even experiments using the high-in- experiments at LAMPF hunt for the medium-energy beams “see” of tensity neutrino beam available at breakdowns of the standard model. nuclei. To understand the new LAMPF require several years to Although none has been detected, data, the catalogue of constituents complete. One such experiment the searches at LAMPF for the of nuclei has been enlarged to en- has provided the only available decay of a muon into an electron compass mesons and excited states measurement of electron and gamma rays, which would her- of nucleons themselves. Addition- neutrinoÐelectron scattering. The ald such a breakdown, have consis- ally, the picture of the dynamics of experimental results showed that tently led the world in sensitivity. their motion has changed. Relativi- the interplay predicted by the stan- Experiments such as those men- ty can no longer be ignored, and in- dard model between the charged tioned above constitute some of the teractions must be described in and neutral parts of the electroweak highlights of the contribution of terms of the coupling of mesons to interaction was indeed a reality. LAMPF to nuclear science. They nucleons. Even today the picture is Therefore, since only the neutral have provided answers to many continuing to evolve as particle and part of the weak force is involved specific questions and at the same nuclear physicists realize deeper in the interaction of electrons with time have paved the way for a slow connections between their once the muon neutrino or the tau neutri- but very important transformation quite distinct fields. no, the interaction of the electron in the way nuclear physicists think neutrino with electrons is funda- about their subject. Before the era mentally different from the interac- of the meson factory, nuclei could Mikkel B. Johnson, a Laboratory Fellow, has pursued research in theoretical nuclear physics tion of the other neutrinos with be largely understood as a collec- at the Medium Energy Physics Division since electrons. That difference provides tion of nucleons undergoing nonrel- 1972.

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From the beginning it was recog- But other uses also were planned that research on accelerator technol- nized that LAMPF could be a source for the neutron pulses. Neutrons, un- ogy should receive special emphasis. of particles other than pions, in par- like x rays, are scattered hardly at all A new division, called the Accelera- ticular, of neutrons. Neutrons are by the electrons of atoms, but they tor Technology Division, was produced at LAMPF through “spal- are scattered by atomic nuclei. And formed in 1978. AT Division was lation”—a nuclear reaction in which those scatterings provide information tasked with developing new acceler- neutrons are knocked loose as a about the structures of solid materials ator designs, and the older Medium heavy nucleus breaks apart, or and of large molecules, including bio- Energy Physics Division continued spalls, after being impacted by ener- logical molecules, in solution. For operating the LAMPF accelerator, getic protons. Consequently, in the that reason the neutron is an invalu- developing user programs, and pur- early 1970s the Weapons Neutron able tool in condensed-matter suing nuclear- and particle-physics Research Facility was built as an ad- physics, materials science, and bio- research. junct to LAMPF. A unique feature physics. In 1986 the Department of Under the leadership of Ed of the facility is a 30-meter-diameter Energy, acting in concert with various Knapp, AT Division soon built pro- ring of magnets called the Proton national committees, designated the totypes of two innovative linear ac- Storage Ring, a device that com- WNR’s neutron source as a national celerators with medical and industri- bines a long train of short proton user facility for neutron scattering. al applications. One, called PIGMI pulses into a single equally short but The new facility is known informally (pion generator for medical irradia- much more intense proton pulse. as LANSCE and formally as the tions), had the potential of leading Any 800-MeV proton that enters the Manuel Lujan, Jr. Neutron Scattering to a small accelerator—about 500 magnetic field created by the mag- Center. Interestingly, it is now the re- feet long—for use in cancer-treat- nets is forced to travel around and search at LANSCE that might be the ment programs. The other was built around the same circular path. Fur- key to LAMPF’s future, as will be for the Fusion Materials Irradiation thermore, the time required for an discussed below. Test Facility at the Hanford Site in 800-MeV proton to travel once Richland, Washington. That proto- around the circular path is the same type was designed to produce neu- as the time between the short pulses trons with which to test different that make up the long train of pulses wall materials of planned fusion re- from LAMPF. Therefore, at the in- actors. Both the PIGMI and FMIT stant that one proton pulse has trav- accelerators employed a new accel- eled once around the circular path, a eration device called a radio-fre- second pulse enters the magnetic quency quadrupole cavity (RFQ). field and melds with the first. Simi- The history of the RFQ is instruc- larly, at the instant that the two tive because it demonstrates the way melded pulses have traveled once attempts to upgrade a technology around the circular path together, a can lead to new and unforeseen sci- third pulse enters the magnetic field entific developments. The RFQ was and melds with the other two. The originally conceived by the Soviet process is allowed to continue until physicists Kapchinskii and all the short pulses in a pulse train Teplyakov in 1969. It remained un- have been combined into a single in- known in the West until 1977, when tense pulse. The intense pulse is a Russian-educated Czech refugee then “kicked” out of the magnetic The PIGMI accelerator. named Joe Manca began working at field and aimed at a tungsten target. the Laboratory. Both he and Don Reactions between the protons and New Accelerator Technologies Swenson, who had learned about the nuclei within the target create an in- RFQ at a Russian international con- tense burst of neutrons with a wide Harold Agnew became Laboratory ference, kept telling colleagues range of energies—valuable tools director in 1970. As a result of about the new, very efficient device for studying weapons physics. LAMPF’s successes, Agnew decided for accelerating charged particles to

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Soviet arsenal in the 1970s, space affect the performance of such a weapons were contemplated as a weapon. Consequently, on July 13, means of non-nuclear defense. Could 1989, an Aries rocket with a special a beam of laser light or a beam of neu- experimental payload called BEAR tral hydrogen atoms be fired thousands (beam experiments aboard a rocket) of miles through space to destroy an was launched at White Sands Mis- enemy ballistic missile? The Labora- sile Range. When the rocket tory, working on an Army project reached its apogee, the accelerator called Whitehorse, considered the pos- fired a neutral-particle beam at dif- sibility of neutral-particle beams as ferent orientations to the earth’s defensive weapons. Then, in the mid magnetic field. The BEAR experi- 1970s, using back-of-the-envelope cal- ment, which was a collaboration be- culations, some physicists at Los tween Los Alamos scientists and in- Alamos began examining the theoreti- dustry, demonstrated that a neutral- cal feasibility of laser-beam and neu- particle beam is unaffected by the tral-particle-beam weaponry. When in space environment and that a com- 1975 John Madey and coworkers at pact accelerator can survive launch. Stanford University built a device A key component of the payload was coined the free-electron laser, its im- an RFQ. plications for non-nuclear defense The Ground Test Accelerator is were immediately recognized. the next step in developing neutral- Madey’s device used an accelerator to particle-beam technology. In an in- create a laser beam by moving elec- An RFQ Cavity. The accelerator is shown here in front of the accelerator it may some- trons past an array of magnets called a day replace—the LAMPF Cockcroft-Walton. wiggler. The resulting laser beam could be tuned to any desired frequen- low energies. At first few people cy. Interested in Madey’s discovery, a took the pair seriously, but develop- small group of Los Alamos scientists ment work at the Laboratory by designed a similar device and built a Dick Stokes and others proved them prototype of a small free-electron laser to be correct. The first RFQ outside in the basement of the Laboratory’s of the Soviet Union was first operat- Physics Building. ed as part of the PIGMI prototype in In a televised speech in 1983, February, 1979. President Reagan called on the na- The RFQ marked an abrupt depar- tion’s scientific community to begin ture from previous low-energy ac- a program that would enable the celeration devices. It could acceler- U.S. to “intercept and destroy strate- ate almost 100 percent of the beam gic ballistic missiles before they from an ion source. And unlike the reached our own soil or that of our drift-tube linac, which employs a allies.” The new program, called magnetic field to focus a beam, the the Strategic Defense Initiative, was RFQ employs rapidly alternating to employ two technologies that electrical fields to both focus and were already under development at “bunch” the beam. The well-defined the Laboratory: the free-electron bunches are completely matched for laser and the neutral-particle beam. follow-on acceleration devices. At Los Alamos the first challenge Soon the RFQ played a role in a in building a neutral-particle-beam Aries Rocket at White Sands Missile Range. This type of rocket carried into space an ac- growing technological field—space weapon was to determine how celerator for producing a neutral-particle weaponry. During the buildup of the launch and space environment would beam.

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Ground-penetrating Radar Units. These units are effective at testing the integrity of shallowly buried fuel-storage tanks. Depth penetration is up to tens of meters in lossy soils and up to a kilometer under ice. dustrial partnership with Grumman since 1986, has begun to reshape the AT Division, under Stanley Corporation, the Laboratory is direction of the Laboratory. Al- Schriber, has begun a gradual shift building a large facility to test though the stewardship of nuclear from nuclear science toward materi- whether such a device can do its in- weapons and other defense interests als science. Accelerators that were tended job in space. And Los Alam- remain the highest priorities, many developed for SDI turn out to have os and Livermore national laborato- of the new programs are aimed at many applications in materials re- ries are contributing to development enhancing the United States’s eco- search. For example, efforts to up- of free-electron-laser weapons by nomic and industrial competitive- grade the present generation of free- providing technical expertise for the ness. Efforts to apply accelerator electron lasers have great potential Ground-based Free-Electron Laser technology to production of silicon for processing of materials, surfaces, Project being undertaken at White chips, transmutation of nuclear and chemicals. One example is sili- Sands Missile Range. waste, developing sources for neu- con-chip production. The so-called tron scattering, and cleanup of haz- Advanced Free-Electron Laser is a ardous waste are being pursued. compact, portable free-electron laser Into the Future Also being investigated are ad- that can be tuned to very small vances in accelerator technology wavelengths—in the extreme ultra- Prompted by the end of the Cold through the use of superconducting violet—to etch silicon chips. The War, Siegfried Hecker, director materials and microwaves. eventual goal of the program, led by

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Rich Sheffield, is to produce a gi- bium, a material that, when cooled Acknowledgments gascale chip, one including about a to temperatures below 9 kelvins, be- billion components. A free-electron comes superconducting. The cost of I wish to thank Joe McKibben, Louis laser might be used also to liquefy niobium itself and of developing the and Mary Rosen, Ed Knapp, Stanley methane gas at remote refineries in fabrication technology might be off- Schriber, Dave Schneider, Tom Wan- Alaska or Australia to enable safe set by savings in radio-frequency- gler, Joe Di Marco, Mike Lynch, Bob transport to stateside refineries. In power costs. Another group, led by Hoeberling, Rich Sheffield, Mikkel October, 1992, the world’s first Bob Hoeberling, is developing a Johnson, Ed Arthur, T. J. Trapp, portable free-electron laser—not ground-penetrating radar that is ca- Rulon Linford, Lew Agnew, Bob Sei- much larger than the bed of a pickup pable of testing the integrity of del, Jim Stovall, Nancy Shera, and truck—produced its first beam at buried gasoline and waste tanks—a Walt Reichelt for their insights about Los Alamos. Other SDI spin-offs potentially invaluable tool in haz- accelerators and their help in writing include focusing and alignment de- ardous-waste cleanup. this article. vices for accelerators, new telescope The trend toward materials sci- technologies, and technologies that ence may also affect LAMPF. As support the Superconducting Super discussed in “Neutrons in Our Fu- Collider planned to be built near ture: A Proposed High-Flux Spalla- Dallas, Texas. tion Neutron Source,” the Laboratory SDI accelerator developments hopes to upgrade LAMPF so that it have also made possible a variety of could produce more intense neutron future applications that require aver- pulses. The Laboratory’s neutron age powers much higher than the scattering center would then be power of LAMPF, presently the competitive with the most advanced highest-power accelerator in the neutron-scattering facilities in the world. A conceptual design for the world and would have significant Accelerator Production of Tritium implications for the United States’s (APT) system is being developed by ability to develop new technologies a team involving Los Alamos, San- in materials science. dia, and Brookhaven National Labo- ratories and six companies. Tritium is an essential component of U.S. nuclear weapons; since it decays it must be regularly replaced. The APT system could provide a non-re- actor source of tritium for the future. Related applications of high-power accelerators could include destruction of excess plutonium, nuclear- waste transmutation, and electric- power production; these applications are being evaluated by the National Academies of Science and Engineer- ing. (See “Acceleratorbased Conver- sion of Surplus Plutonim.”) Other efforts by AT Division exploit new developments in materials science. One group, led by Joe Di- Marco, is investigating the fabrica- The Clean Room at Accelerator Technology Division. Superconducting accelerator cavities tion of accelerator cavities from nio- of niobium are fabricated under conditions of extreme cleanliness.

1993 Number 21 Los Alamos Science 105

Probing the Structure of Matter

sent in spent fuel from nuclear reactors. Ac- for burning plutonium and long-lived wastes. Accelerator celerator-based conversion systems would Further advantages include smaller end-of- transmute these long-lived radioactive mate- life inventories and potential safety enhance- rials into stable or short-lived fission prod- ments. The fast burn-up of material in the Based ucts. The controlled consumption of plutoni- ABC method requires frequent chemical pro- um afforded by ABC technology could thus cessing to remove the stable and short-lived Conversion provide an international method to reduce products for disposal. The unfissioned ac- opportunities for proliferation. tinides, including plutonium, and dominant As shown in the figure, the ABC system long-lived fission products are returned to the of Surplus uses a proton beam from the accelerator to blanket for further exposure to the high neu- produce an intense neutron source at the tar- tron flux. The addition of accelerator-pro- get. The blanket surrounding the target con- duced neutrons to the blanket not only en- Plutonium tains plutonium and other actinides that are sures that adequate numbers of neutrons are to be destroyed. The neutrons from the tar- available to transmute all of the unwanted get are moderated, or slowed down, in the materials but also provides for subcritical op- blanket, where they induce fission of the un- eration in the blanket and therefore prompt eductions in the number of nuclear wanted materials, which, in turn, releases control of fission reactivity. This type of con- Rweapons in the United States and the more neutrons. Some of these neutrons are trol may prove to be particularly advanta- former Soviet Union have resulted in tons of captured in the nuclei of long-lived fission geous in designs involving very high neutron weapons-grade plutonium that need to be products and thereby transmute those nuclei fluxes and continuous flow of material disposed of safely. Storage facilities are to short-lived or stable products. The intense through the blanket. The heat generated by needed for the near term, but the ultimate flux of thermal neutrons allows the ABC sys-Accelfission fig1 in the blanket is converted to electric disposition is also important. Some solutions tem to have lower inventories of actinides 4/8/93power. Some of this electric power can be involve converting the plutonium to a form and fission products for a given burn rate of used to run the accelerator, and the rest can similar to other high-level wastes destined those materials than other proposed systems be made available to the electric-power grid. for geologic repositories, such as spent reac- tor fuel and glassified wastes. Those forms would be substantially more proliferation-re- INTEGRATED FACILITY sistant than the present concentrated form. Power Even so, plutonium originating from weapons Power recycled Power conversion programs and the larger, growing quantities to accelerator and distribution to grid of plutonium from commercial spent fuel would continue to present a proliferation Accelerator Beam nuisance. transport The Accelerator Based Conversion Target (ABC) technology under investigation at the Laboratory and illustrated in the figure could Blanket be used to destroy plutonium from both Heat weapons and commercial reactors. The exchanger/ steam technology is being designed to transmute generator the “dominant” long-lived radioactive prod- Residue Recycling of recovery plutonium, actinides, ucts generated during plutonium consump- and long-lived fission tion (those that are most difficult to dispose products of safely) and to generate electric power from the heat released by the various con- Low-level On-site version processes. Initially, ABC systems waste disposal could destroy the plutonium returned from ABC Site for the weapons program. They could also re- Plutonium Short-lived Engineered duce the long-term toxicity of existing de- Destruction Chemical processing fission storage fense wastes destined for a geologic reposi- products tory. In the longer term ABC plants could consume plutonium, other actinides, and dominant long-lived radioactive waste pre- Surplus plutonium inventory

106 Los Alamos Science Number 21 1993