Alvarez proton linear accelerator

Alvarez proton linear accelerator

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Object 1978.1073.01.1 is the first seven feet of the 40-ft. long Alvarez proton linear accelerator (linac), with two of the total of 9 oscillators. The Alvarez linac became operational in 1947-48.
The accelerator assembly.01.1 consists of the following major components: horizontal cylindrical vacuum tank enclosing a cylindrical copper cavity and central beam tube, (.01.1.01), connected externally to two vertical cylindrical oscillators (.01.1.02 and 01.1.09) and their associated power systems. The vacuum chamber is open at one end to show the internal cavity and linear array of drift tubes at the center. From its open end, five feet (approx.) of top half of the vacuum chamber has been cutaway to show the outer surface of the copper cavity. As displayed in Atom Smashers exhibition, a flange-type support attached under the vacuum tank is mounted on two pyramidal-shaped, vertical supports, which rest on a low platform surface. With the exception of oscillator.01.1.09, the other components numbered from.01.1.03 to.01.1.17 are minor (e.g., bolts, pipe, cables, electrical fittings, etc. - see curator's notes for details).
History and basic principles
The linear resonance accelerator, developed by E.O. Lawrence and colleagues during the 1930’s, was one of the earliest designs for attaining high particle energy without the requirement of correspondingly high voltage. Numerous attempts to realize it were made before World War II, but all proved disappointing, largely due to the limitations of technology for generating high-frequency electric fields.
As the name implies, linear resonance accelerators are straight machines in which, as in circular cyclotrons, the accelerated motion of charged particles is synchronized with an oscillating electric field. They were first made practical by the radar technology developed during World War II. Pulsed radio transmitters of extremely high peak power at ultra-high frequency made it possible to think about adding a million volts to the energy of protons and electrons for each linear foot of the accelerator. The opportunity was recognized by many physicists developing radar in Britain and the U.S. The boldest in exploiting it was Luis W. Alvarez, one of the scientists in Lawrence’s cyclotron group at University of California, Berkeley. In the Alvarez accelerator, an intense 200 MHz electric field is produced within the copper cavity by powerful oscillators - - originally surplus radar transmitters - - for 400 microseconds, 15 times per second. Protons injected into the end of the cavity from a Van de Graaff (electrostatic) accelerator with an energy of 4 MeV are accelerated further by the oscillating electric field when crossing the gaps between the drift tubes, but are shielded by the tubes when the electric field is in the opposite, retarding, phase. Along the length of the cavity, the individual drift tubes (see object ID no. 1978.1073.01.4-.5) lengthen in proportion to the increasing particle velocity, so that the protons always take the same time to travel from gap to gap, thus remaining in step with the oscillating electric field. The proton bunches are longitudinally stable as in a synchrotron, and are stabilized transversely by the action of converging fields produced by focusing grids (see object ID no. 1978.1073.01.3.01-.03). In 1947, the linac’s 40-ft. long cavity accelerated protons to 31.5 MeV, which, until that time was the highest energy to which protons had ever been accelerated. (Berkeley’s synchrocyclotron leap-frogged the energy to 350 MeV the next year.)
By 1947 the synchrotron emerged as the most practical concept for a high-energy particle accelerator, and the linac was subsequently used as the proton injector into the synchrotron. A smaller version of the Alvarez linac was used to inject 10 MeV protons into Berkeley’s “Bevatron”, a billion electron volt (BeV) synchrotron. Present injector linacs are hundreds of feet in length and produce particle beams of hundreds of MeV. For basic principles and history of synchrotrons, see McMillian synchrotron, Object ID no. N-09621.01 in the Modern Physics Collection.
Principle of strong focusing
Until 1952 designers of circular accelerators - - cyclotrons, betatrons, synchrotrons - - relied on the relatively “weak” focusing action of the magnetic field guiding ion or electron beams to hold these charged particles in stable circular orbits during acceleration. In 1952 the principle of alternating, or “strong”, focusing of particle beams was discovered at Brookhaven National Laboratory, and immediately found wide application in all types of accelerators.
Focusing forces on a particle deviating vertically are stronger the more the lines of magnetic force bulged outward, and focusing forces on a particle deviating from the orbit circle are much stronger if the magnet gap narrowed, so that the lines of magnetic force bulged inward. These two conditions, incompatible simultaneously, do not completely cancel their effects if applied successively. Thus, a sequence of magnets whose radial field gradients are directed alternatively outward and inward can have a powerful net focusing action on the particles passing through them. This approach is known as alternating-gradient focusing. The same principle applies to the action of electric fields on charged particles. After learning of the possibility and value of applying alternating-gradient focusing to proton linear accelerators, Alvarez’s group demonstrated strong radial focusing of the proton beam with electrostatic quadrupole lenses. In autumn of 1952, they immediately removed the original focusing grids and installed one such electrostatic quadrupole lens inside each drift tube (see object ID no. 1978.1073.01.5.2). The intensity of the proton beam doubled, and would have quadrupled had the insulation of the electrical leads held the required voltage. Thus the first proton linac was also the first strong-focusing accelerator. (The first accelerator to use magnetic alternating-gradient focusing was the 1 GeV synchrotron at Cornell University in 1953.) Before the prototype 32-MeV linac was shipped from Berkeley to its final location for operations at the University of Southern California, all of its drift tubes had been fitted with electrostatic strong-focusing electrodes.
Currently not on view
Object Name
Linear Accelerator, Alvarez
date made
Physical Description
steel; copper (overall material)
vacuum chamber (approx.): 7 ft x 40 in; 2.1336 m x 101.6 cm
ID Number
catalog number
accession number
Credit Line
University of Southern California
Science & Scientific Instruments
See more items in
Medicine and Science: Modern Physics
Energy & Power
Science & Mathematics
Modern Physics
Data Source
National Museum of American History
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