作者：Lawrence Radiation Laboratory
格式: AZW3, DOCX, EPUB, MOBI, PDF, TXT
LRL Accelerators The 184-Inch Synchrocyclotron试读：
THE 184-INCH SYNCHROCYCLOTRON
His success with the 60-inch cyclotron in 1939 led Dr. E. O. Lawrence to propose a much more powerful accelerator, one which could produce new types of nuclear rearrangements and even create particles. Grants totaling $1,225,000 permitted work to start on the 184-inch cyclotron in August 1940.It was designed to accelerate atomic particles to an energy of 100 million electron volts (Mev), five times that possible with the 60-inch machine.Fig. 1. The electromagnet under construction during the period 1940 to 1942.
Before the new cyclotron could be finished World War II began. Construction on the cyclotron was therefore halted. However, because of interest in separating the isotopes of uranium by the electromagnetic method, work on the giant magnet continued at an even faster pace. This magnet would contain 3700 tons of steel in its yoke and pole pieces, and 300 tons of copper in its exciting coils (Fig. 1). By May 1942 the magnet was completed. During that summer it was used in a 235pilot plant to separate the first significant amounts of U ever obtained. The 184-inch magnet remained in use in a research and development program at Berkeley until the end of the war, supplying information to Oak Ridge, Tennessee, where a large separation plant had been erected.
Construction on the rest of the cyclotron was resumed in 1945. By that time a new principle had been discovered which made it possible to obtain ion beams of much higher energy than originally hoped for. Yet a considerably lower accelerating voltage could be used. This important discovery was made independently by Dr. V. Veksler in Russia and by Dr. Edwin M. McMillan, present Director of the Lawrence Radiation Laboratory. Before attempting to discuss this principle, we should first review the operation of a conventional cyclotron.
PRINCIPLE OF OPERATION OF A CONVENTIONAL CYCLOTRON
Fig. 2. Basic parts of a cyclotron.The main parts of a cyclotron are represented in Fig. 2. Charged particles (ions) are accelerated inside an evacuated tank. This is to prevent the beam from colliding with air molecules and being scattered. The vacuum tank is placed between the poles of an electromagnet, whose field bends the ion beam into a circular orbit.
The operation begins when the ions are introduced into the region between two accelerating electrodes, or "dees."Because the ions carry a positive electric charge, they are attracted toward that dee which is electrically negative at the moment. Were it not for the magnetic field, the ions would be accelerated in a straight line; instead they are deflected into a circular path back toward the dee gap. By the time the ions again reach the dee gap, the sign of the electric potential on the dees is reversed, so that now the ions are attracted toward the opposite dee.
As this process of alternating the electric potential is repeated, the ions gain speed and energy with each revolution. This causes them to spiral outward. Finally they strike a target inserted into their path or are extracted from the cyclotron for use as an external beam.
The time required for an ion to complete one loop remains constant as it spirals outward. This is because its velocity increases sufficiently to make up for the increased distance it travels during each turn. This means that the electric potential applied to the dees must alternate at a constant frequency, called the "resonant frequency."
The resonant frequency f is given by the relationship
where H, e, π, c, and m are constants. H is the strength of the magnetic field of the cyclotron, e is the electric charge carried by the ion, π equals 3.14, c is a conversion factor, and m is the mass of the ion. For example, the resonant frequency for protons accelerated in a 15,000-gauss magnetic field is 23.7 megacycles (Mc).We call such a rapidly alternating potential a "radiofrequency voltage" and the electronic circuit for producing it a "radiofrequency oscillator."
The energy E of an ion emerging from the cyclotron is given by
where H, e, and m are as defined above, and R is the radius at which the beam is extracted. From this equation we see that for a given type of ion (where e and m are constant), the energy depends on the diameter and strength of the magnet, but not directly upon the voltage applied to the dees.
The number of revolutions that an ion can make in a conventional cyclotron is limited to about 70 to 100. This is due to a very curious effect: as an ion is accelerated, its mass increases！[This phenomenon is explained by Einstein's special theory of relativity (see Fig. 3).] Referring back to Eq. (1), we see that if the ion mass (m) does not remain constant, but rather increases, then the resonant frequency (f) decreases. But since the dee potential continues alternating at a constant frequency, an ion soon begins to arrive "late" at the dee gap. By the time it has made about 70 to 100 turns an ion is so badly out of phase that it is no longer accelerated.
Suppose now that we want to obtain an energy of 10 Mev. Because an ion can make a maximum of about 100 turns, the accelerating potential would have to be about 100,000 volts. However, Professor Lawrence hoped to reach 100 Mev with the new 184-inch cyclotron. This meant that the accelerating voltage would have to be about 1,000,000 volts. Preventing such a high voltage from sparking promised to be one of many formidable engineering problems.Fig. 3. Graph showing how the mass of an object increases as its velocity approaches that of light.
THE PRINCIPLE OF PHASE STABILITY
Fortunately, Drs. Veksler and McMillan showed that relatively low dee voltages can be used to accelerate ions to very high energies. This is possible if the oscillator frequency is continuously decreased to keep it in synchronism with the decreasing rotational frequency of the ions. This would allow an ion to make many revolutions without becoming out of phase. This principle of phase stability was experimentally verified with the 37-inch cyclotron before being incorporated into the design of the 184-inch machine. Because it utilizes this principle, this machine has usually been referred to as a "synchrocyclotron" or "frequency-modulated cyclotron." However, it is sometimes called simply a "cyclotron."
The 184-inch synchrocyclotron was first operated in November 1946. With a maximum dee voltage of only 20,000 volts, it accelerated deuterons to 190 Mev and alpha particles to 380 Mev.In 1949 it was modified to permit production of 350-Mev protons also.
Between 1955 and 1957 the synchrocyclotron was rebuilt so that now the following energies can be obtained:
Protons Deuterons Alpha Particles Helium-3 nuclei
730 Mev 460 Mev 910 Mev 1140 Mev
In reaching an energy of 730 Mev a proton, for example, makes 75,000 revolutions in just 6 milliseconds (msec). It travels a distance of 450 miles and attains a velocity of 152,000 miles per second, or 82％ of the speed of light！During this brief journey its mass increases 75％, giving very convincing evidence for the validity of Einstein's theory. Similar data for other ions may be found in the appendix.
试读结束[说明：试读内容隐藏了图片]