This video and article is about the 4 MV (that is 4.000.000 Volt !) Van De Graaff generator (VDG) that was installed at the Copenhagen University Department of Theoretical Physics (UITF). UITF was later in 1969 renamed to Niels Bohr Institute, after its founder, Nobel Laurate and famous Danish scientist. The 4 MV VDG was however built later and used by Aage Bohr, a son of Niels Bohr.
A Van De Graaff generator can generate very high voltages, just like a Tesla coil, but they are fundamentally different in the constructions and method of obtaining high voltages.
The 4MV Van De Graaff generator
This system is a 4 MeV high-voltage particle accelerator based on the Van de Graaff generator design and is the same Van de Graaff unit used by Aage Bohr in his research work. It converts mechanical energy into a stable, high-energy proton beam for nuclear physics experiments and related applications.
The accelerator is installed inside a large pressurized tank filled with insulating gas. The pressurized environment increases the electrical breakdown strength, allowing the system to safely operate at potentials up to 4 million volts. This design reduces the risk of arcing and improves overall voltage stability at high energy levels.
An 11 kW motor drives a continuous insulating belt that transports electric charge to the high-voltage terminal. The motor provides steady torque and speed to maintain consistent charging current, even when the accelerator is delivering beam. A precisely controlled corona spray system applies charge to the belt. By adjusting the spray current, operators can regulate the terminal voltage accurately, keeping the output stable and minimizing ripple or unwanted discharges.
As charge accumulates on the isolated metal terminal, it builds up to the full operating voltage of up to 4 MeV. This creates a strong electric field between the terminal and ground. Inside the terminal, an RF-excited hydrogen ion source generates a stable plasma without filaments, improving reliability and reducing maintenance. The source produces hydrogen ions that are extracted into the acceleration tube.
The beam column is kept under high vacuum using a diffusion pump to prevent ion collisions and electrical breakdown. Ions accelerate smoothly through the tube toward ground potential, reaching energies directly determined by the terminal voltage. The result is a stable, well-defined proton beam suitable for precision nuclear research.
4MV VDG Control and measurements
The top of the control panel features two motor control panels, suggesting that this panel has been used to control both the Van De Graaff generators at the UITF. There is a central meter for motor current.
A unmarked plate underneath the motor controls has a uA meter, but no markings for the potentiometer and selector switch.
A cutout plate has clearly been used for a speaker installation, the speaker is however removed.
Four 0 to 100 % analog meters with each their potentiometer is used for measurement and control of vacuum, gas, focus and anode voltage of the ion source.
The panel with two uA meters and a variac in the middle is control for the belt high voltage spray and the beam current in the column going downwards to the deflecting magnet.
The data acquisition system is a Nuclear Data model ND60. It is not possible to locate information about this particular model. Old advertisement from 1975 shows similar named, later models, ND100 as a multichannel analyzer system. This would correspond with the energy spectrum graphs produced by Aage Bohr in the experiments.
Images of a Nuclear Data ND600 can be found at https://computergraphicsmuseum.org/images/nuclear_data/ND-812/ and shows the exact same keyboard layout, but without the advanced settings to the right of the monitor.
Aage Bohr
Aage Bohr (1922–2009) was one of Denmark’s leading nuclear physicists and a central figure in advancing our understanding of atomic structure. Building on the scientific foundation established by his father, Niels Bohr, he combined theoretical insight with precise experimental work to explore the behavior of atomic nuclei.
At the University Institute for Theoretical Physics (UITF), Aage Bohr carried out key nuclear physics experiments using a 4 MV Van de Graaff generator. This high-voltage accelerator enabled controlled nuclear reactions, allowing detailed measurements of energy levels and nuclear excitations. The experimental data obtained formed a crucial foundation for the development and validation of the collective model of the nucleus, describing coordinated motion of protons and neutrons within deformed nuclei.
His work exemplified the close integration of advanced instrumentation and theoretical physics. In recognition of his decisive contributions to nuclear structure theory, Aage Bohr was awarded the Nobel Prize in Physics in 1975, together with Ben Roy Mottelson and James Rainwater. His legacy remains a benchmark in the precise interplay between high-voltage technology and fundamental research.
