Editing Van de Graaff generator (section)

== History ==
{{multiple image
| align = right
| direction = horizontal
| header =
| image1 = WestinghouseAtomSmasher.jpg
| width1 = 200
| image2 = Westinghouse Van de Graaff atom smasher - cutaway.png
| width2 = 168
| footer = The [[Westinghouse Atom Smasher]], the {{val|5|ul=MeV}} Van de Graaff generator built in 1937 by the [[Westinghouse Electric (1886)|Westinghouse Electric]] company in [[Forest Hills, Pennsylvania]]
}}
[[File:Az első magyar gyorsító Van de Graaff-generátora(2).jpg|thumb|240px|right|This Van de Graaff generator of the first Hungarian linear particle accelerator achieved 700&nbsp;kV in 1951 and 1000&nbsp;kV in 1952.]]
[[File:Van den Graaff DSC09091.JPG|thumb|right|240px|A Van de Graaff particle accelerator in a pressurized tank at [[Pierre and Marie Curie University]], Paris]]

=== Background ===
The concept of an electrostatic generator in which charge is mechanically transported in small amounts into the interior of a high-voltage electrode originated with the [[Kelvin water dropper]], invented in 1867 by [[William Thomson, 1st Baron Kelvin|William Thomson]] (Lord Kelvin),<ref name="Thomson">
{{cite journal
 | last1 = Thomson
 | first1 = William
 | title = On a self-acting apparatus for multiplying and maintaining electric charges, with applications to the Voltaic Theory
 | journal = The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science
 | series = Series 4
 | volume = 34
 | issue = 231
 | pages = 391–396
 | date = November 1867
 | url = https://books.google.com/books?id=2lgwAAAAIAAJ&pg=PA391
 | access-date = September 1, 2015}}</ref> in which charged drops of water fall into a bucket with the same polarity charge, adding to the charge.<ref name="Gray">{{cite book
 | last1  = Gray
 | first1 = John
 | title  = Electrical Influence Machines
 | publisher = Whittaker and Co.
 | date   = 1890
 | location = London
 | pages  = 187–190
 | url    = https://archive.org/stream/electricalinflu00graygoog#page/n210/mode/1up
}}</ref>
In a machine of this type, the [[gravitational force]] moves the drops against the opposing electrostatic field of the bucket. Kelvin himself first suggested using a belt to carry the charge instead of water. The first electrostatic machine that used an endless belt to transport charge was constructed in 1872 by [[Augusto Righi]].<ref name="Van de Graaff" /><ref name="Gray" /> It used an [[india rubber]] belt with wire rings along its length as charge carriers, which passed into a spherical metal electrode. The charge was applied to the belt from the grounded lower roller by [[electrostatic induction]] using a charged plate. John Gray also invented a belt machine about 1890.<ref name="Gray" /> Another more complicated belt machine was invented in 1903 by Juan Burboa.<ref name="Van de Graaff" /><ref name="Burboa">[https://patents.google.com/patent/US776997 US patent no. 776997, Juan G. H. Burboa ''Static electric machine'', filed: August 13, 1903, granted: December 6, 1904]</ref> A more immediate inspiration for Van de Graaff was a generator [[W. F. G. Swann]] was developing in the 1920s in which charge was transported to an electrode by falling metal balls, thus returning to the principle of the Kelvin water dropper.<ref name="Van de Graaff" /><ref name="Swann">
{{cite journal
 | last1 = Swann | first1 = W. F. G.
 | title = A device for obtaining high potentials
 | journal = Journal of the Franklin Institute
 | volume = 205
 | pages = 828
 | date = 1928
}}</ref>

=== Initial development ===

The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de Graaff at [[Princeton University]], with help from colleague Nicholas Burke. The first model was demonstrated in October 1929.<ref>{{cite web |url=http://chem.ch.huji.ac.il/~eugeniik/history/graaff.html |title=Robert Jemison Van de Graaff |website=The Institute of Chemistry – The Hebrew University of Jerusalem |access-date=2006-08-31 |url-status=dead |archive-url=https://web.archive.org/web/20060904024639/http://chem.ch.huji.ac.il/~eugeniik/history/graaff.html |archive-date=2006-09-04}}</ref> The first machine used an ordinary tin can, a small motor, and a silk ribbon bought at a [[variety store#North America|five-and-dime store]]. After that, he went to the chairman of the physics department requesting $100 to make an improved version. He did get the money, with some difficulty. By 1931, he could report achieving 1.5 million volts, saying "The machine is simple, inexpensive, and portable. An ordinary lamp socket provides the only power needed."<ref>{{cite journal |last=van de Graaff |first=R. J. |title=Minutes of the Schenectady Meeting September 10, 11 and 12, 1931: A 1,500,000 volt electrostatic generator. |journal=Physical Review |publisher=American Physical Society (APS) |volume=38 |issue=10 |date=1931-11-15 |issn=0031-899X |doi=10.1103/physrev.38.1915 |pages=1919–1920}}</ref><ref>''Niels Bohr's Times'', Abraham Pais, Oxford University Press, 1991, pp.378-379</ref> According to a patent application, it had two 60-cm-diameter charge-accumulation spheres mounted on [[Pyrex|borosilicate glass]] columns 180&nbsp;cm high; the apparatus cost $90 in 1931.<ref>"Van de Graaff's Generator", in "Electrical Engineering Handbook", (ed), CRC Press, Boca Raton, Florida USA, 1993 {{ISBN|0-8493-0185-8}}</ref><ref>{{cite journal |last1=Wolff |first1=M.F. |title=Van de Graaff's generator |journal=IEEE Spectrum |date=July 1990 |volume=27 |issue=7 |pages=46 |doi=10.1109/6.58426|s2cid=43715110 }}</ref>

Van de Graaff applied for a second patent in December 1931, which was assigned to [[Massachusetts Institute of Technology]] in exchange for a share of net income; the patent was later granted.<ref>{{cite journal |title=This Month in Physics History: February 12, 1935: Patent granted for Van de Graaff generator |journal=APS News |date=February 2011 |volume=20 |issue=2 |url=https://www.aps.org/publications/apsnews/201102/physicshistory.cfm |access-date=10 May 2022}}</ref>

In 1933, Van de Graaff built a 40&nbsp;ft (12&nbsp;m) model at MIT's [[Round Hill, Massachusetts|Round Hill]] facility, the use of which was donated by [[Edward Howland Robinson Green|Colonel Edward H. R. Green]].<ref>{{cite journal |last1=Thomas |first1=William |title=A profile of John Trump, Donald's accomplished scientist uncle |journal=Physics Today |date=7 September 2016 |issue=9 |page=22954 |doi=10.1063/PT.5.9068 |bibcode=2016PhT..2016i2954T |url=https://physicstoday.scitation.org/do/10.1063/pt.5.9068/full/ |access-date=10 May 2022}}</ref> One consequence of the location of this generator in an aircraft hangar was the "pigeon effect": arcing from accumulated droppings on the outer surface of the spheres.<ref>{{cite web |last1=Wilson |first1=E.J.N. |title=Overview of Accelerators |url=https://acceleratorinstitute.web.cern.ch/ACINST1112/Lecture1(OverviewI).pdf |website=Accelerator Institute |publisher=CERN |access-date=10 May 2022}}</ref>

=== Higher energy machines ===

In 1937, the [[Westinghouse Electric (1886)|Westinghouse Electric]] company built a {{convert|65|ft|m|abbr=on}} machine, the [[Westinghouse Atom Smasher]] capable of generating 5&nbsp;MeV in [[Forest Hills, Pennsylvania]]. It marked the beginning of nuclear research for civilian applications.<ref>{{cite book |first=Franklin |last=Toker |title=Pittsburgh: A New Portrait |year=2009 |page=470 |publisher=University of Pittsburgh Press |url=https://books.google.com/books?id=nJM6AQAAIAAJ |isbn=9780822943716}}</ref><ref>{{cite web |title=Van de Graaff particle accelerator, Westinghouse Electric and Manufacturing Co., Pittsburgh, PA, August 7, 1945 |website=Explore PA History |publisher=[[WITF-TV]] |url=http://explorepahistory.com/displayimage.php?imgId=1-2-151C |access-date=February 19, 2015}}</ref> It was decommissioned in 1958 and was partially demolished in 2015.<ref name="bo">{{cite news|last=O'Neill|first=Brian|date=January 25, 2015|title=Brian O'Neill: With Forest Hills atom smasher's fall, part of history tumbles|newspaper=[[Pittsburgh Post-Gazette]]|url=https://www.post-gazette.com/opinion/brian-oneill/2015/01/25/Brian-O-Neill-With-Forest-Hills-atom-smasher-s-fall-part-of-history-tumbles/stories/201501250109}}</ref> (The enclosure was laid on its side for safety reasons.)<ref>{{Cite web|title=Atom smasher in Forest Hills torn down; restoration promised|url=https://www.post-gazette.com/news/science/2015/01/21/Forrest-Hill-nuclear-relic-waits-in-limbo/stories/201501200209|access-date=2022-01-17|website=Pittsburgh Post-Gazette|language=en}}</ref>

A more recent development is the tandem Van de Graaff accelerator, containing one or more Van de Graaff generators, in which negatively charged [[ion]]s are accelerated through one [[potential difference]] before being stripped of two or more electrons, inside a high-voltage terminal, and accelerated again. An example of a three-stage operation has been built in Oxford Nuclear Laboratory in 1964 of a 10&nbsp;MV single-ended "injector" and a 6&nbsp;MV EN tandem.<ref>J. Takacs, ''Energy Stabilization of Electrostatic Accelerators'', John Wiley and Sons, Chichester, 1996</ref>{{page needed|date=April 2016}}

By the 1970s, as much as 14 MV could be achieved at the terminal of a tandem that used a tank of high-pressure [[SF6|sulfur hexafluoride]] (SF<sub>6</sub>) gas to prevent sparking by trapping electrons. This allowed the generation of heavy ion beams of several tens of MeV, sufficient to study light-ion direct nuclear reactions. The greatest potential sustained by a Van de Graaff accelerator is 25.5&nbsp;MV, achieved by the tandem in the Holifield Radioactive Ion Beam Facility in [[Oak Ridge National Laboratory]].<ref>{{cite web |url=https://www.ornl.gov/news/american-physical-society-names-ornls-holifield-facility-historic-physics-site |title=American Physical Society names ORNL's Holifield Facility historic physics site |date=25 July 2016 |publisher=Oak Ridge National Laboratory}}</ref>

A further development is the [[pelletron]], where the rubber or fabric belt is replaced by a chain of short conductive rods connected by insulating links, and the air-ionizing electrodes are replaced by a grounded roller and inductive charging electrode. The chain can be operated at a much greater velocity than a belt, and both the voltage and currents attainable are much greater than with a conventional Van de Graaff generator. The 14&nbsp;UD Heavy Ion Accelerator at [[the Australian National University]] houses a 15&nbsp;MV pelletron. Its chains are more than 20&nbsp;m long and can travel faster than {{convert|50|km/h|mph|abbr=on}}.<ref>{{cite web |title=Particle Accelerator |url=http://www.anu.edu.au/CSEM/machines/Accelerator.htm |date=November 2002 |url-status=dead |archive-url=https://web.archive.org/web/20190608111732/http://www.anu.edu.au/CSEM/machines/Accelerator.htm |archive-date=2019-06-08}}</ref>

The Nuclear Structure Facility (NSF) at [[Daresbury Laboratory]] was proposed in the 1970s, commissioned in 1981, and opened for experiments in 1983. It consisted of a tandem Van de Graaff generator operating routinely at 20 MV, housed in a distinctive building 70&nbsp;m high. During its lifetime, it accelerated 80 different ion beams for experimental use, ranging from protons to uranium. A particular feature was the ability to accelerate rare isotopic and radioactive beams. Perhaps the most important discovery made using the NSF was that of super-deformed nuclei. These nuclei, when formed from the fusion of lighter elements, rotate very rapidly. The pattern of gamma rays emitted as they slow down provided detailed information about the inner structure of the nucleus.<ref>J S Lilley 1982 Phys. Scr. 25 435-442 {{doi|10.1088/0031-8949/25/3/001}})</ref> Following financial cutbacks, the NSF closed in 1993.<ref>{{cite journal |author1=David Dickson |title=Curtain falls on Britain's nuclear structure facility |journal=Nature |date=March 1993 |volume=362 |issue=6418 |pages=278 |url=https://www.nature.com/articles/362278b0.pdf |publisher=Nature Publishing Group |doi=10.1038/362278b0 |access-date=6 February 2024}}</ref>