Question: What was the contribution made by de Graaff to the technology John Trump and Robert Van de Graaff developed together?

Context paragraphs:
William T. Van de Graaff: William Travis "Bully" Van de Graaff (October 25, 1895 – April 26, 1977) was an American football player, coach, and college athletics administrator.  He attended Tuscaloosa High School.  He played college football at the University of Alabama, where he was selected as an All-American in 1915, Alabama's first.  He was 6'1" 187 pounds.  "Bully" was placed on an "Associated Press" Southeast Area All-Time football team 1869-1919 era.  Van de Graaff served as the head football coach at Colorado College from 1926 to 1939, compiling a record of 49–47–6.  He coached hall of famer Dutch Clark.  He died in Colorado Springs, Colorado on April 26, 1977 at the age of 81.  He was the older brother of physicist Robert J. Van de Graaff, the designer of the Van de Graaff generator which produces high voltages. Bully's two older brothers, Hargrove and Adrian, were also Alabama football players.

Jan van de Graaff: Jan van de Graaff (born 24 September 1944) is a retired Dutch rower.  He won the world title in the coxed pair at the 1966 World Rowing Championships and a bronze medal in the coxed fours at the 1964 Summer Olympics.  At the 1964 Olympics his team mates were Marius Klumperbeek (cox), Lex Mullink, Bobbie van de Graaf and Freek van de Graaff.  The three "van de Graaf(f)s" were all born in 1944.

1915 College Football All-Southern Team: The 1915 College Football All-Southern Team consists of American football players selected to the College Football All-Southern Teams selected by various organizations in 1915.  Josh Cody and Baby Taylor were selected third-team All-Americans by Walter Camp, and Bully Van de Graaff was selected for his second-team.  Van de Graaff was Alabama's first ever All-American.  Buck Mayer of the 8–1 Virginia Cavaliers was the south's first consensus All-American, selected first-team All-American by Frank G. Menke and Parke H. Davis.  The "point-a-minute" Vanderbilt Commodores won the SIAA.

John G. Trump: John George Trump (August 21, 1907 – February 21, 1985) was an American electrical engineer, inventor, and physicist.  He was a recipient of U.S. President Ronald Reagan's National Medal of Science, and a member of the National Academy of Engineering.  John Trump was noted for developing rotational radiation therapy.  Together with Robert J. Van de Graaff, he developed one of the first million-volt X-ray generators.  He was the paternal uncle of U.S. President Donald Trump.

Van de Graaff generator: A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials.  It produces very high voltage direct current (DC) electricity at low current levels.  It was invented by American physicist Robert J. Van de Graaff during 1929.

Robert J. Van de Graaff: Robert Jemison Van de Graaff (December 20, 1901 – January 16, 1967) was an American engineer, physicist, and noted for his design and construction of high-voltage Van de Graaff generators.  He taught at Princeton University and Massachusetts Institute of Technology.

Freek van de Graaff: Frederik Robbert "Freek" van de Graaff (20 February 1944 – 24 June 2009) was a Dutch rower who won a bronze medal in the coxed fours at the 1964 Summer Olympics.  His team mates were Marius Klumperbeek (cox), Lex Mullink, Bobbie van de Graaf and Jan van de Graaff.  The three "van de Graaf(f)s" were all born in 1944.

Bobbie van de Graaf: Robert "Bobbie" van de Graaf (born 17 March 1944) is a retired Dutch rower who won a bronze medal in the coxed fours at the 1964 Summer Olympics.  His team mates were Marius Klumperbeek (cox), Lex Mullink, Freek van de Graaff and Jan van de Graaff.  The latter two were also born in 1944 and are unrelated to Bobbie.

Carnivàle Lune Bleue: Inspired by traveling carnivals of the 1930s, Carnivàle Lune Bleue is a live entertainment event created in Ottawa, Canada by Executive Producer and Founder Wayne Van De Graaff.  Van De Graaff has stated that he was inspired to pursue his lifelong dream and recreate a vintage Depression-era carnival after watching the HBO series Carnivàle.

Westinghouse Atom Smasher: Westinghouse Atom Smasher was a 5 MeV Van de Graaff electrostatic nuclear accelerator operated by the Westinghouse Electric company at their research center in Forest Hills, Pennsylvania.  It was instrumental in the development in practical applications of nuclear science for energy production.  It was the first industrial Van de Graaff generator in the world, and marked the beginning of nuclear research for civilian applications.  Built in 1937, it was a 65 ft tall pear-shaped tower.  It went dormant in 1958.

Supporting paragraphs: John G. Trump Robert J. Van de Graaff
Answer: high-voltage Van de Graaff generators