The simplest type of particle accelerator is constructed by mounting a particle source on one end of an insulated, evacuated tube and creating a high voltage between the ends, with the polarity such that the particles are impelled from the source toward the far end of the tube. Such an accelerator is necessarily linear, and the electrostatic field can be applied to a given particle only once (unless, as in the tandem accelerator described below, the charge of the particle undergoes a change in sign). The simplicity of concept becomes complex in execution when the electric potential exceeds one million volts (1 megavolt, or 1 MV); these high voltages produce corona discharges and lightninglike sparks outside the accelerator, which dissipate the potential needed to accelerate the particles. Even more difficult to control are sparks within the equipment and, in positive-ion accelerators, unwanted secondary beams produced when the accelerated ions strike the end of the tube.
Voltage multipliers (cascade generators)
The source of the high voltage for Cockcroft and Walton’s pioneering experiments was a four-stage voltage multiplier assembled from four large rectifiers and high-voltage capacitors. Their circuit in effect combined four rectifier-type direct-voltage power supplies in series. The alternating voltage supplied by a high-voltage transformer was transmitted to the higher stages through an array of capacitors; a second group of capacitors kept the direct voltage constant. The final direct voltage would have been four times the peak voltage available from the transformer (200,000 volts) if corona discharge had not drained away considerable power. Nevertheless, the apparatus did accelerate protons to energies of 710 keV, sufficient to bring about the hoped-for result, a reaction with lithium nuclei. This achievement, the first nuclear reaction effected by artificially accelerated particles, was recognized by the award of the Nobel Prize for Physics in 1951.
Cockcroft and Walton’s system for building up high direct voltages can be extended to multiplication factors many times that originally demonstrated. Commercially available accelerators that reach 4 MeV are based on this circuitry.
Van de Graaff generators
In Van de Graaff generators, electric charge is transported to the high-voltage terminal on a rapidly moving belt of insulating material driven by a pulley mounted on the grounded end of the structure; a second pulley is enclosed within a large, spherical high-voltage terminal. The belt is charged by a comb of sharp needles with the points close to the belt a short distance from the place at which it moves clear of the grounded pulley. The comb is connected to a power supply that raises its potential to a few tens of kilovolts. The gas near the needle points is ionized by the intense electric field, and in the resulting corona discharge the ions are driven to the surface of the belt. The motion of the belt carries the charge into the high-voltage terminal and transfers it to another comb of needles, from which it passes to the outer surface of the terminal. A carefully designed Van de Graaff generator insulated by pressurized gas can be charged to a potential of about 20 megavolts. An ion source within the terminal then produces positive particles that are accelerated as they move downward to ground potential through an evacuated tube.
In most constant-voltage accelerators, Van de Graaff generators are the source of high voltage, and most of the electrostatic proton accelerators still in use are two-stage tandem accelerators. These devices provide a beam with twice the energy that could be achieved by one application of the high voltage. For the first stage of a tandem accelerator, an ion source yields a beam of protons, which are accelerated to a low energy by an auxiliary high-voltage supply. This beam passes through a region containing a gas at low pressure, where some of the protons are converted to negative hydrogen ions by the addition of two electrons. As the mixture of charged particles moves through a magnetic field, those with positive charge are deflected away. Those with negative charge are deflected into the accelerator tube, and the beam of negative ions is then accelerated toward a positive high-voltage terminal. In this terminal the particles pass through a thin carbon foil that strips off the two electrons, changing many of the negative ions back into positive ions (protons). These, now repelled by the positive terminal, are further accelerated through the second part of the tube. At the output end of the accelerator, the protons are magnetically separated, as before, from other particles in the beam and directed to the target. In three- or four-stage tandem accelerators, two Van de Graaff generators are combined with the necessary additional provisions for changing the charge of the ions.
Van de Graaff and Cockcroft-Walton generators also are utilized for accelerating electrons. The rates at which charge is transported in electron beams correspond to currents of several milliamperes; the beams deliver energy at rates best expressed in terms of kilowatts. These intense beams are used for sterilization, industrial radiography, cancer therapy, and processing of plastics.
Betatrons
A betatron is a type of accelerator that is useful only for electrons, which sometimes are called beta particles—hence the name. The electrons in a betatron move in a circle under the influence of a magnetic field that increases in strength as the energy of the electrons is increased. The magnet that produces the field on the electron orbit also produces a field in the interior of the orbit. The increase in the strength of this field with time produces an electric field that accelerates the electrons. If the average magnetic field inside the orbit is always twice as strong as the magnetic field on the orbit, the radius of the orbit remains constant, so that the acceleration chamber can be made in the shape of a torus, or doughnut. The poles of the magnet are tapered to cause the field near the orbit to weaken with increasing radius. This focuses the beam by causing any particle that strays from the orbit to be subjected to forces that restore it toward its proper path. The theory of this focusing was first worked out for the betatron; by analogy, the oscillations of particles about their equilibrium orbits in all cyclic accelerators are called betatron oscillations.
Just after the sinusoidally varying strength of the magnetic field has passed through zero and starts increasing in the direction proper to guide the electrons in their circular orbit, a burst of electrons is sent into the doughnut, where—in a 20-MeV betatron—they gain about 100 eV per revolution and traverse the orbit about 200,000 times during the acceleration. The acceleration lasts for one-quarter of the magnet cycle until the magnetic field has reached its greatest strength, whereupon the orbit is caused to shrink, deflecting the electrons onto a target—for example, to produce a beam of intense X-rays.
The practical limit on the energy imparted by a betatron is set by the emission of electromagnetic energy from electrons moving in curved paths. The intensity of this radiation, commonly called synchrotron radiation (see below Synchrotrons: Electron synchrotrons), rises rapidly as the speed of the electrons increases. The largest betatron accelerates electrons to 300 MeV, sufficient to produce pi-mesons in its target; the energy loss by its electrons through radiation (a few percent) is compensated by changing the relation between the field on the orbit and the average field inside the orbit. At higher energies this compensation would not be feasible.
Betatrons are now commercially manufactured, principally for use as sources of X-rays for industrial radiography and for radiation therapy in medicine. X-ray beams are produced when an electron beam is directed onto a target material with a heavy atomic nucleus, such as platinum.