Lawrence Berkeley National Laboratory Recent Work Title The Guided Tour of UCRL Accelerators Permalink https://escholarship.org/uc/item/9sr7r4f2 Author Lawrence Berkeley National Laboratory Publication Date 1956-05-01 eScholarship.org Powered by the California Digital Library University of California THE GUIDED TOUR OF ACCELERATORS RAD/AT/ON LABORATORY UN/VERS/TY OF CA L/FORN/A THE GUIDED TOUR THE GUIDED TOUR 0 f UCRL ACCELERATORS Employees and associates who participate as guides in the Radiation Laboratory visitor program are hosts representing the Laboratory. In this role their help in Contents enlightening the public on atomic research and on its value to the country's welfare is inestimable. Since the advent of the Atomic Era and since the shortage of sci- THE BEVATRON entists and engineers has become acute, the Laboratory recognizes the great importance of encouraging and in- General Principles of Operation. 4 spiring young people to pursue scientific and technical The Bevatron Magnet. 7 studies. Our program of formal tours attended primarily The Electric Field . 8 by high school and college students is therefore of par- Phase Stability. -10 ticular significance. Not only students but also, occa- Need for an Injector . -11 sionally. professional scientific and technical organi- History of a Proton During Acceleration zations make such tours. Ion Source . 12 Cockcroft-Walton . -12 Graduate students in physics and chemistry who are Steering Magnet. .13 on the Laboratory payroll act as guldes on most of these Buncher and Linac. , . 14 tours, usually scheduled for Saturday mornings. The infor- Focusing Magnet and Inflector. .14 mation in this booklet is prepared as an aid to Laboratory Acceleration. -15 guides in effectively describing accelerators to students Bringing the Protons to a Target . .19 and other guests who do not have a thorough knowledge of (-1 Types of Experiments . , . 19 nuclear science. Experimental Equipment . -21 Counters . -23 The two areas visited at the present time are the Bevatron Statistics. -24 Bevatron and the linear accelerator. Guides whose work Bevatron Lecture Stations. .30 has not acquainted them with these machines will be taken on orientation tours by representatives of the groups THE LINEAR ACCELERATOR working with them. Such arrangements, when requested, will be made by the Visit Office, Building 65. Extension 461. Principles of Linac Operation. .32 Linac Statistics. , . -34 I THE BEVATRON THE BEVATRON 1. The string constrains the ball so that it moves in a circle of constant radius and continually re- circles past the boy. GENERAL PRINCIPLES OF OPERATION r) 2. Even if the boy only hits the ball weakly, he The Bevatron is a machine that accelerates protons can impart a great deal of energy to it if he hits it to relativistic energies--that is, energies at which the often enough. velocity of the particles approaches the velocity of light. It is a member of a class of accelerators called 3. As the ball gains energy and, therefore, ve- proton synchrotrons. Its operation is analogous to the locity, the boy's "timing" changes; that is, the fre- behavior of a system consisting of a baseball tied to a quency with which he hits the ball must increase. string and revolving about a post, and a boy striking the ball with a bat each time it passes him (see Fig. 1). 4. As the ball gains energy and, therefore, momentum, The important features of this analogy are: the force the string must exert to keep the ball revolv- ing in a circle increases. A limit to the amount of energy that can be imparted to the ball will be reached I when its momentum is so high that the string breaks. i In the Bevatron: 1. The magnetic field constrains protons to move in nearly circular orbits of constant radius so that they DUSEDULL ANALOGY continually recircle through a region containing an ac- celerating electric field. (This "constant-radius" fea- ture is important for economic reasons. If the protons spiraled out from the center, the whole circle w~uld have to be covered with magnet iron. Here the magnet covers only the outer edge.) 2. The electric accelerating field imparts relatively little energy to the proton during a single traversal. ~ighenergies are obtained by having many traversals. 3. The electric accelerating field is produced by a radio-frequency oscillator. This field obviously must be baseball = proton in the right direction when the protons pass through it, and considerations relating to stability further string = magnetic Field I/ require that the protons enter the field within a limited phase interval. This is the problem of "timing." boy with bat = dectric Cield E AS the proton gains energy and, therefore, velocity. I UCRL ACCELERATORS THE BEVATRON THE BEVATRON MAGNET the time it requires to circle the Bevatron decreases. Therefore, to keep the timing correct, the frequency of the rf oscillator must increase during acceleration. > The weight and massive bulk of the Bevatron are k, 4. As the proton gains energy and. therefore. momen- 1 argely associated with producing the magnetic field. tum, a stronger magnetic field is required to constrain The magnet iron weighs 10,000 tons, or the equivalent of it to move in the same orbit. Thus the magnetic field a medium-sized cruiser. The motor generator., which sup- plies current for the magnet coil, must deliver a peak must increase during acceleration. There is a limit to the strength of field the Bevatron magnet can produce, power of 100.000 kw and an average power of 7,000 kw. and it is this which limits the energy that can be im- (A town of 30.000 population consumes about 7.000 kw.) parted to the protons. To achieve the rising magnetic field, the coil is The baseball analogy breaks down in one respect, pulsed with an essentially dc voltage at the beginning of which is discussed later. the accelerating cycle. Owing to the self-inductance of the system, the current requires time to build up and the There is a unique relation between the value of the result is a gradually rising magnetic field. magnetic field, the frequency of the rf oscillator, and the energy of the proton being accelerated, which must be satisfied at every instant. For this reason a proton syn- It is necessary to have access to the region where chrotron is not a continuous machine, but must be pulsed. the protons circulate to insert the accelerating elec- A group of protons starts off together, and the energies trode, targets, vacuum pumps, etc. Therefore, while in principle the Bevatron could have the shape of a ring, of the protons, the value of the magnetic field, and the actually there must be access regions where there is no rf oscillator frequency all increase simultaneously, keep- ing in step until the end of the accelerating cycle. At magnet yoke and, therefore. no magnetic field. In these that time this group of high-ener'gy protons is available regions the particles travel in straight lines. At the to an experimenter. Then the magnetic field and the rf os- Bevatron there are four of these straight sections. The cillator frequency return to their initial values and the protons are injected into the Bevatron at the east straight section; they are accelerated by an electric cycle can begin again. In the Bevatron, acceleration takes- 1.8 seconds. field in the north straight section; and some of the targets are located in the west straight section. Since for successful acceleration the frequency of the rf oscillator and the strength of the magnetic field must satisfy a certain relationship, either the frequency must be made to track the magnetic field or the strength of the field must be made to track the frequency. In the Bevatron the magnetic field rises according to the char- acteristics of the magnet excitation equipment and of the magnet itself; the rf oscillator has been designed so that its frequency stays precisely in step. UCRL ACCELERATORS I THE BEVATRON THE ELECTRIC FIELD rectangular drift tube inside north straight saction I Energy can be imparted to the protons when they pass c?-2 through the north straight section. In the Bevatron the Protons first actually lose some energy and then gain back a greater amount. This happens 9s follows: In the north straight section there is a rectangular tube through which the protons pass. The rf oscillator makes the potential of this tube, with respect to the rest of the Bevatron, vary sinusoidally in time. The protons enter the tube when its potential is positive and rising (see Fig.2). If the protons get to Position 2 at Time t2. they will have lost energy, because they had to climb a potential hill of 14 kv. Once they get to Position 2, potentia\ on t they can coast to Position 3, because the region inside tube with respect to the tube is all at the same potential, and therefore Bevatron grqund there are no fields to exert forces. The crucial point is that during the time it takes the protons to coast inside the tube from Position 2 to Position 3, the poten- tial of the entire tube rises from 14 kv to 15.7 kv. When the protons come out of the tube, they gain 15.7 kv in going from Position 3 to Position 4. Thus. they fall down a potential hill that is 1.7 kv higher than the one they had to climb, so the net result.' is a gain in energy of 1.7 kev. The protons then circulate through the en- tire Bevatron without changing their energy until the ACCELLRAT ING ELECTRIC rlfLD magnetic field brings them back to the north straight section, where the process is repeated.