Story of Particle Accelerators

Progress in science is rarely sudden or dramatic. More often it proceeds slowly and even imperceptibly. This is certainly the case with the history of the particle accelerator. From a crude arrangement of glass tubing and static electricity, physicists have created a machine in which protons whiz around in a closed path 17 miles in length, buried deep beneath the French-Swiss border. What makes this story remarkable is that it was not an isolated episode confined to a few decades or a handful of individuals. It is a history of innovation and invention, of physicists working at the very edges of their knowledge, of a process that continues to this day and may well proceed long after we are gone.

Physicists in the nineteenth century did not work in the white-coated, security-intensive environments familiar to us from modern television programs and films. They experimented with Crookes tubes: simple glass vessels from which most of the air had been evacuated, leaving a near-perfect vacuum, with metal electrodes at either end. A high-voltage power supply was used to create an electrical potential difference between the two electrodes.

They did not set out to revolutionize physics when they did this sort of thing, and indeed, those who worked with such devices had no idea what they were actually dealing with. In 1895, Wilhelm Röntgen was investigating one such tube when he noticed that a screen of barium platinocyanide placed near the tube glowed when the tube was charged, even when it was wrapped in black paper. Röntgen correctly surmised that the tube emitted some invisible rays that could penetrate the paper and excite the screen. Within a year, these rays - subsequently dubbed X-rays for lack of a better name - were transforming medicine, as their ability to penetrate tissue and reveal denser structures such as bone proved invaluable for diagnostic purposes. In 1897, at the Cavendish Laboratory in Cambridge, J. J. Thomson was also investigating Crookes tubes when he discovered that the mysterious rays produced by them consisted of charged, subatomic particles (which we now call electrons) that behaved in a manner wholly different from anything known before. Among other things, this finding represented a major blow to the early nineteenth-century view of the atom as an indivisible unit.

It is perhaps unfair to use the term acceleration at this point, since the electrons produced in these experiments were byproducts of another process, occurring in the vacuum within the Crookes tubes. However, at about this time, physicists began to realize that electrons could be utilized as tools. By subjecting them to an electric field, they could be accelerated to velocities never before seen.

If you look at early twentieth-century writings on physics, you will find a recurring theme: that natural radioactivity is not a sufficiently powerful or versatile probe of the nucleus to be useful in the investigation of atomic structure. What was needed, many physicists felt, including the great Ernest Rutherford himself, was a source of accelerated particles, a device capable of boosting the energy of charged particles to much higher levels than any natural source could manage on its own. Rutherford’s own words provide the best illustration of the challenge he set for his contemporaries: “What the physicist needs is a million volts in a soapbox.”

Physicists jumped at the challenge. Some pursued the idea of using high voltages to accelerate charged particles directly. Early efforts were limited to voltages on the order of tens or even hundreds of thousands of volts. Higher voltages caused unwanted side effects such as insulation breakdown and disruptive sparking. Nevertheless, a number of early pioneers, among them John Cockcroft and Ernest Walton at the Cavendish Laboratory, succeeded in creating a voltage multiplier, a device that used series of capacitors and rectifiers to produce a steady direct current. And in April 1932, Walton went to his laboratory in a small wooden shack and accelerated protons to strike a target of lithium-7 nuclei. The collision produced alpha particles, which in turn caused the chamber to glow, confirming that the lithium nucleus had split into two helium nuclei. Cockcroft and Walton had produced the first human-made nuclear reaction in history, and in doing so, they had provided a dramatic confirmation of Einstein’s mass-energy equivalence, E = mc 2 . The voltage multiplier used to accelerate the protons was subsequently termed the Cockcroft-Walton generator, and while it would soon be replaced by more powerful devices, it nonetheless marked a major step forward in humanity’s ability to probe matter at the deepest levels.

The limitations imposed on early accelerators by high voltages and insulation were overcome through the development of a fundamentally new method of accelerating charged particles. Instead of subjecting a particle to one huge voltage boost, a particle could be accelerated in a series of small steps by looping it around a magnetic field, over and over again. This development led to the creation of the circular accelerator, also called the synchrotron. The device consists of a pair of superconducting magnets that are used to form a closed loop, and an oscillating electric field that accelerates the particle as it travels around the loop. Ernest Lawrence built the first circular accelerator in 1930, after being inspired by a lecture on linear accelerators that he had attended the previous year. The tiny device, no larger than a hand, consisted of a vacuum chamber nearly 4 inches in diameter and was constructed primarily of sealing wax, glass, and bronze. This delicate arrangement, costing a mere $25 to build, was nonetheless capable of delivering energies comparable to those of much larger devices.

Lawrence’s cyclotron represented a paradigm shift in the way that physicists thought about accelerating machines and their place within the broader context of scientific inquiry. Physicists began to think more and more in terms of efficiency, and the concept of reusing equipment and infrastructure became increasingly appealing. Lawrence’s cyclotron is notable not only for the paradigm shift it embodied but also for the way in which its influence has continued to be felt in the present day.

During World War II, many physicists found themselves engaged in research that had little direct connection to the development of new types of accelerators. The study of radar played an important role in this period, as it led to the development of new microwave technologies that would prove invaluable in the construction of linear accelerators. These accelerators, which used carefully calibrated electromagnetic waves to propel charged particles along a linear path, represent an entirely different design philosophy from the circular accelerators pioneered by Lawrence.

One of the most significant discoveries to come out of the Second World War was a potential solution to the limitation imposed by special relativity on early accelerators. As a given particle approaches the speed of light, an additional push will not accelerate it further as expected, since relativistic effects cause its mass to increase. In effect, a particle accelerator built using a constant frequency alternating current (AC) would only be able to push a particle to a certain maximum energy before encountering this barrier imposed by relativity. This limitation was circumvented by Edwin McMillan in the United States and independently by Vladimir Veksler in the Soviet Union, when both realized that the frequency of the accelerating voltage could be varied to make up for the fact that the particle was approaching the speed of light. The result was the synchrotron, which was able to achieve much higher energies than the older cyclotron. McMillan and Veksler both received the Atoms for Peace Award in 1963, for their contributions to the field.

Accelerators continued to grow in complexity and capability throughout the 1950s and 1960s. Accelerator physics was becoming too difficult and expensive to be done by single institutions. CERN (the European Organization for Nuclear Research) was founded in 1954, with its main facility located near Geneva, Switzerland. Twelve European nations participated in its founding, and it survives to this day as a symbol of international scientific cooperation. In the United States, Fermilab was founded in 1967, near Chicago, Illinois. The trend was clearly away from purely national efforts at big science; the entire endeavor was too great for any one country to handle alone.

Storage rings and colliders also made their appearance during this period. These represented a new way of thinking, not only about accelerators but about physics itself. Colliders work by accelerating two beams of particles toward one another, allowing a much greater portion of the kinetic energy to be transferred to the collision products, compared to a scenario in which one beam is fixed and the other is accelerated to meet it. Physicists have uncovered an amazing amount of information by studying the collisions of various subatomic particles, and this sort of research has provided much insight into the fundamental structure of matter.

At the core of this latest development was a machine that was, in many ways, the culmination of decades of effort. The Large Hadron Collider at CERN, which first began operating in 2008, is a tremendously powerful particle accelerator. Located several miles below the French-Swiss border, it consists of a ring 17 miles in circumference. Inside the ring, protons are accelerated to nearly 7 Tera-electron volts per beam and made to collide with one another up to millions of times per second, creating conditions similar to those shortly after the Big Bang. These collisions have resulted in the creation of previously unknown particles and have contributed to physicists’ understanding of the fundamental forces in nature. Some 10,000 scientists from more than 100 countries have contributed to experiments conducted at the LHC, an incredible display of international scientific cooperation. The LHC represents the fulfillment of a dream first advanced by Rutherford more than a century earlier, who sought to create a machine capable of producing a copious supply of high-energy particles.

Perhaps most notably, the LHC has played an integral role in allowing physicists to confirm the long-suspected existence of the Higgs boson. First postulated in 1964, the Higgs boson was associated with the Higgs field, which is responsible for giving other particles their mass. On July 4, 2012, two major LHC experiments, ATLAS and CMS, announced that they had observed the boson. The discovery of the Higgs represented a major milestone for physicists everywhere, and it also marked the end of a long journey for those who had been involved in this particular line of research.

While high-energy physics has long dominated popular imagination, it has only ever been a small part of the story. There are approximately 30,000 accelerators currently in existence, and the vast majority of them are used for completely different purposes. For example, physicists have long been aware of the value of ionizing radiation for medical purposes. Accelerators are used extensively in radiation therapy to treat a variety of diseases, from cancer to multiple sclerosis. They are also used for diagnosis, allowing doctors to see inside the human body and obtain images of soft tissues that would otherwise be difficult to observe. These accelerators are much simpler than the mighty LHC, but they nonetheless play a major role in a host of medical procedures every day.

Accelerators have also found their way into a number of industrial applications, from sterilization to material analysis. Many everyday objects, including microprocessors used in personal computers and mobile phones, were made possible by accelerators. The design of such things as microchips and microcircuits often makes use of non-destructive testing procedures that rely on accelerators. Even something as common as a microwave oven makes use of a principle first discovered by physicists using an accelerator.

An interesting development in the field of accelerators is that they have provided new ways for physicists to harness light. The Advanced Photon Source near Chicago and the European Synchrotron Radiation Facility in Grenoble, France, are home to electron storage rings that produce X-rays of extraordinary brightness and precision. These tools have allowed physicists to probe the properties of matter at the atomic level, obtaining information that has proved invaluable for a number of different scientific disciplines. Light produced using an accelerator makes it possible to observe the properties of individual atoms and molecules, providing insight into fields as diverse as biochemistry and condensed matter physics.

None of this happened because someone decided that we needed a particle accelerator that would stretch for 17 miles beneath the French-Swiss border. It happened because a series of physicists asked themselves difficult questions about the nature of matter, and in response to these questions, they built remarkable machines. In doing so, they did not work in a vacuum - their efforts were aided by a spirit of innovation and an openness to new ideas. We are today continuing this tradition, seeking answers to questions not asked yet, building devices that may well endure long after we are gone.