November 1957 Popular Electronics[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from Popular Electronics.
By 1957, betatrons, cyclotrons, cosmotrons, synchrocyclotron, bevatrons, and other forms of "trons" had the physics world all agog with anticipation of the next big discovery. Quarks were still a decade away from being discovered and something as exotic of the Higgs boson (aka god particle) hadn't entered anyone's mind. The news media was agog with reports of the world possibly coming to the end as a result of those experiments sparking a nuclear reaction chain that would cause the whole world to explode. Today, the news media is no smarter, because nowadays they fret over the Large Hadron Collider (LHC) generating a black hole that will implode the whole world. What a ship of fools.
The Cyclotron. Dr. E. O. Lawrence and Dr. M. S. Livingstone built the first cyclotron (Fig. 1) in 1931. Picture a flat circular pill-box cut vertically down the middle - separated into two "D"-shaped halves. The hollow half-boxes or dees are connected across a source of high-frequency voltage and enclosed in a gas-filled chamber. The whole assembly is then mounted between the poles of a powerful magnet. An electrically heated filament in the position shown provides an abundance of electrons which serve to ionize the gas in the chamber.
Suppose that at a given instant A1 is made positive with respect to A2 by the high-frequency voltage; a positive ion from the source F will be attracted toward A2, but instead of moving directly toward A2, it will move in a circular path due to the action of the uniform magnetic field produced by the pole pieces. As it reaches the gap between the two dees, it is accelerated by the strong electric field at this point and proceeds into the opposite box with increased velocity.
Synchrocyclotron. To compensate for the increasing mass of the whirling protons or alpha particles, the frequency of the accelerator voltage can slowly be changed so that it keeps step with the diminishing speed increment of the ions. In 1950, the huge Columbia University synchrocyclotron went into operation using this synchronization principle. The most significant departure in construction from prior cyclotrons was the use of a single rather than a double dee.
In the synchrocyclotron, a grounded deflecting electrode of simpler construction serves as the second side of the applied high-frequency voltage. Recent reports indicate that the Columbia synchrocyclotron produces alpha particle energies in excess of 400 m.e.v. An ion having this energy travels at approximately 93,000 miles per second or at a speed high enough to carry it to the sun in about 10 seconds flat!
Thus far, no mention has been made of electrons, since the cyclotron and synchrocyclotron are positive-charge accelerators. Why can't they be used for accelerating electrons? If we remember that an electron has a rest mass of about 1/1800th that of a proton, it is evident that we would have to speed it up far more than a proton to obtain the same kinetic energy from it. (Kinetic energy depends upon both mass and velocity. If we want a large k.e. in a body of small mass, the velocity must be made very much greater.)
As illustrated in Fig. 3, the major features of the betatron include a large electromagnet whose pole pieces protrude into the center of an evacuated glass-tube "doughnut." Electrons are produced outside the betatron by thermionic methods and are given a preliminary acceleration by an external electric field of about 50,000 volts potential. They are then fed or injected into the doughnut. The current through the electromagnet coils is alternating so that the magnetic field is a varying one; the injection process occurs at the precise time when the magnetic field is building up in the right direction. The effect of the field is to induce a voltage within the doughnut which increases the energy of the electrons.
Although the electrons tend to spiral outward as their velocity increases, the magnetic field grows at exactly the right rate to counterbalance this tendency. It will be remembered that the field of the cyclotron is steady and that the spiral path is characteristic of such an unchanging magnetic flux. Thus, the electrons in the betatron follow essentially circular paths. The magnetic field around the doughnut is allowed to grow for only one-quarter of the a.c. cycle, but during this brief interval each injected electron makes over 275,000 turns around the circle!
Protons are first accelerated by a cyclotron and are then injected at A in Fig. 4. They are further accelerated by a varying magnetic field as in the betatron. As soon as the magnet becomes saturated, the betatron part of the operation ceases, and further acceleration is then produced by a high-frequency voltage applied to electrode C. This electrode, known as a cavity resonator, therefore performs a function similar to the dees in the cyclotron.
The output of the Berkeley bevatron consists of a series of pulses at intervals of about six seconds from the 10,000-ton magnet. During each pulse a proton travels 270,000 miles - a distance greater than that to the moon! The average energy output of the Berkeley bevatron is on the order of six billion electron-volts!
Where will it all end? The scientist has an insatiable thirst for knowledge and the boundless energy to see it through. Man, in his quest for the keys to the universe, has already duplicated the energy of cosmic rays in his new atom-smashers and has surpassed the heat of the sun in his nuclear bombs. Can we doubt that next will come the "trevatron," and then the "quadrevatron," and then - who knows?
Posted June 20, 2012