American physicist and inventor of the Van de Graaff generator, a type of high-voltage electrostatic generator that serves as a type of particle accelerator. This device has found widespread use not only in atomic research but also in medicine and industry.
Robert Jemison Van de Graaff was born on December 20, 1901 in Tuscaloosa, Alabama. His mother was Minnie Cherokee Hargrove and his father was Adrian Sebastian Van de Graaff. Robert attended the Tuscaloosa public schools and then attended the University of Alabama where he received a BS degree in 1922 and an MS degree in 1923. Both degrees were in mechanical engineering.
Van de Graaff
After graduating from college he worked for the Alabama Power Company for a year as a research assistant. He studied at the Sorbonne in Paris from 1924 to 1925 and while there, attended lectures by Marie Curie on radiation. In 1925 he went to Oxford University in England as a Rhodes Scholar. At Oxford he received a BS in physics in 1926 and a Ph.D. in physics in 1928.
While at Oxford, he became aware of the hope of nuclear experimenters such as Ernest Rutherford, that particles could someday be accelerated to speeds sufficient to disintegrate nuclei. By disintegrating atomic nuclei much could be learned about the nature of individual atoms. It is from these ideas that Robert Van de Graaff saw the need for a particle accelerator.
In 1929 Van de Graaff returned to the United States to join the Palmer Physics Laboratory at Princeton University as a National Research Fellow. In the fall of that year he constructed the first working model of his electrostatic accelerator which developed 80,000 volts. Improvements were made to the basic design and in November, 1931 at the inaugural dinner of the American Institute of Physics, a demonstration model was exhibited that produced over 1,000,000 volts. The invention was reported at a meeting of the American Physical Society in 1931.
In 1931, when Karl T. Compton became president of Massachusetts Institute of Technology, Van de Graaff was invited to come to MIT as a research associate. In 1931 Van de Graaff constructed his first large machine in an unused aircraft hangar at Round Hill, the estate of Colonel E.H.R. Green, in South Dartmouth, Massachusetts. The machine used two polished aluminum spheres, each 15 feet in diameter mounted on 25 foot high insulating columns, which were 6 feet in diameter.
The columns were mounted on railway trucks that boosted the spheres to 43 feet above ground level. The machine had its debut on November 28, 1933 and was able to produce 7,000,000 volts. This accomplishment was reported in the New York Times for November 29, 1933 in a story titled “Man Hurls Bolt of 7,000,000 Volts”. In 1937 the machine was moved to a pressurized enclosure at MIT.
John D. Cockcroft and Ernest Walton of the Cavendish Laboratory in England had built a successful particle accelerator in 1932. This machine used voltage-multiplier circuits to produce the required high voltages for particle acceleration. It was bulky and complicated and limited in its voltage capability. In contrast to the Cockcroft-Walton machine, the Van de Graaff machine was simple and compact and was easier to regulate and capable of producing higher voltages and therefore higher accelerations.
Van de Graaff generators
The Van de Graaff generator is an impressive electrostatic generator that is capable of producing enormously large static electric potentials. In fact, giant Van de Graaff generators can produce miliions of volts leading to awesome displays of corona and lightning. More modest “class room” sized Van de Graaff generators typically produce 100,000 V to 500,000 V. Originally the generator was built using a pure silk ribbon driven by a small motor. The charges on the ribbon were collected on a terminal that actually was a tin can. His early devices were capable of developing 80,000 volts. Of course, this device was limited by the edge effects of the tin can.
However, his theory lead to Van de Graaff generators that have produced upwards to 10 million volts. Interestingly, Van de Graaff may not have been the first to develop a belt driven electrostatic generator. In 1893, Von Busch presented a device having two pulleys and a horizontal belt with a charge collector comb and an insulated sphere. Even earlier than this, Rouland invented an electrostatic generator in 1785 using a continuous silk ribbon running between two horizontal pulleys with a collector tube at the center. Regardless of its “true” inventor, Van de Graaff generators have been instrumental in a number of fields, including particle acceleration, electrostatics, ESD, as well as providing an awesome display of the power of electricity.
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 the large, spherical high- voltage terminal, as shown at left. 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.
Two-stage tandem accelerator
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. Figure at left is a diagram 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 negative charge are deflected into the accelerator tube, and those with positive charge are deflected away.
The beam of negative ions is then accelerated toward the 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 polymerization of plastics.
In 1935 Van de Graaff received a patent for his invention (Patent US 1,991,236). He was guided in the preparation of the patent application by Karl T. Compton and Vannevar Bush who was vice president of MIT. Van de Graaff also worked with John G. Trump, a professor of electrical engineering at MIT and with William W. Buechner, a professor of physics at MIT in an effort to achieve higher voltages, more homogeneous particle beams, and more compact designs.
A medical Van de Graaff used to produce X rays for treating cancerous tumors with precisely penetrating radiation was first used clinically in 1937 at Harvard Medical School. In 1935 he was elected a Fellow of the American Academy of Arts and Sciences. In 1936 Van de Graaff married Catherine Boyden. They had two sons, John and William.
During the 1937 Paris Universal Exhibition, an impressive Van de Graaff was installed in the newly opened Palais de la Decouverte, which was (and still is) located in the Grand Palais. This apparatus, built by A. Lazard under the direction of the famous French physicist Frederic Joliot (1900-1958), was supposed to be used atter the exhibition as a powerful source of radiolelements.
This machine was composed of two Van de Graaff generators accumulating charges of different polarity at a total tension of 5 Mvolt. The generators were 14 meters high and mounted on rails. The spheres at the top of them had a diameter of 3 metres. Each generator had three independent endless-belts driven by separate motors and charged by a 10,000 volts direct current source. The system was entirelv enclosed in a gigantic Faradav’s cage. This machine, which amazed visitors to the fair with its spark several metres long was on the front-page of many magazines, but had unfortunately a sad fate.
Because of World War Il it was forgotten in the Palais de la Decouverte and only in 1942 was it possible to undertake its removal to the Joliot’s laboratorv in Yvry near Paris. The machine had to be overhauled and a few mechanical pieces had to be substituted, but, due to the shortage and the critical wartime situation, nothing could be done. Therefore, this spectacular Van de Graaff was never used for any scientific research and it finally scraped.
During WW II Van de Graaff was director of the High Voltage Radiographic Project, sponsored by the Office of Scientific Research and Development. Along with William W. Buechner he directed the adaptation of the electrostatic generator to precision radiographic examination of U.S. Navy ordnance. After the war, in 1945, Van de Graaff received a Rockefeller Foundation grant for the development of an improved accelerator at MIT. On December 19, 1946 Van de Graaff and Trump formed the High Voltage Engineering Corporation (HVEC) in Burlington, Massachusetts.
HVEC was formed for the commercial production of particle accelerators. Denis M. Robinson, a professor of electrical engineering from England, became president of the new corporation. John G. Trump became technical director and Van de Graaff became chief physicist and a board member. HVEC became the leading supplier of electrostatic generators that were used in cancer therapy, industrial radiography and in the study of nuclear structure. In 1947 Van de Graaff received the Duddel Medal of the Physical Society of Great Britain.
In 1951 Luis W. Alvarez of the University of California at Berkeley rediscovered the tandem principle first developed by Willard Bennett in 1937. A tandem Van de Graaff machine accelerates a negatively charged particle (typically having a charge of -1 or having 1 extra electron) toward a positively charged terminal. As the particle passes through the terminal, electrons are removed from the particle.
This causes the particle to become positively charged and therefore accelerated away from the positively charged terminal. When heavy ions such as gold or uranium are used as many as 25 to 30 electrons may be removed. The high voltage terminal is thus used to accelerate the ion twice (tandem).
In the late 1950s Van de Graaff invented the insulating core transformer. The insulating core transformer generated high voltage direct current using magnetic flux rather than electrostatic charging. Van de Graaff also devised many methods of controlling particle beams during and after acceleration so they could be adapted to individual research requirements. Using Van de Graaff accelerators physicists accumulated vast quantities of information on nuclear disintegrations and reactions. This data led directly to sophisticated theories of nuclear structure.
Van de Graaff published many scientific papers and received numerous patents, including those of the high-voltage electrostatic generator and the insulating-core transformer. His work on electrostatic generators was widely recognized within the scientific community. Van de Graaff received several honorary degrees and awards,
Van de Graaff remained an associate professor of physics at MIT until 1960 when he resigned to devote himself to his increasing involvement with HVEC. In 1966 he was awarded the Tom W. Bonner Prize for his contribution and continued development of the electrostatic accelerator, “a device that has immeasurably advanced nuclear physics” by the American Physical Society. The prize was named for a scientist who had used the Van de Graaff particle accelerator to achieve the results of his fundamental research.
Robert J. Van de Graaff died on the morning of January 16, 1967 in Boston at the age of 65. At the time of his death there were over 500 Van de Graaff particle accelerators in use in more than 30 countries.