Let us briefly consider a scenario in the possible future of medicine: An endoscope detects a tumour. The doctor guides the instrument precisely up to the growth and presses a button. Nothing appears to happen, however fact electrons are being fired like bullets from the tip of the endoscope on the tumour cells to destroy them. This would be a very targeted cancer therapy that does not damage any of the healthy tissue around the tumour.
However, this is not possible currently because electron accelerators fill an entire room with their magnets and microwave resonators. from the Uwe Niedermayer at TU Darmstadt would like to change this situation. In cooperation with his colleagues Thilo Egenolf and Professor Oliver Boine-Frankenheim and the members of the Department of Electrical Engineering and Information Technology, the engineer has presented a concept for an electron accelerator with a length of less than one millimetre. It can be fabricated in a similar way as computer chips so that large amounts could be produced at low prices. “This would mean that every university laboratory could afford their own electron accelerator”, says Niedermayer. Accelerator on a Chip International Program (ACHIP)
“This would mean that every university laboratory could afford their own electron accelerator.”
A laser accelerates the electrons
Despite its small size, the device would be able to accelerate electrons to the same speed as a standard accelerator. In physics, the energy supplied to an electron is given in electron volts, since the electron is driven by the force of an electrical field. The mini accelerator from TU Darmstadt could be extended to any size and could therefore theoretically achieve unlimited energy. 70,000 volts have been achieved in less than a millimetre in the latest model. You would need around 50,000 standard batteries like those sold in the supermarket to reach this level of driving force.
However, a laser drives the particles here. Light also produces an electrical field but because it is a wave, this electrical field constantly changes direction. An electron would be initially accelerated but then in the next instant slowed down again by the same amount. The research team has discovered a trick that allows them to weaken the decelerating part of the field. A first design was presented in 2018. This has now been improved so that the accelerator chip can work without additional external equipment.
Interaction between silicon and light waves
The principle behind it can be explained using the first, simpler design: Two parallel rows of tiny cylindrical pillars made of silicon – similar to a colonnade – are attached to the base. The channel between the cube-shaped blocks is only about 200 nanometres wide (a nanometre is one millionth of a millimetre), 25 times thinner than a human hair. The pillars themselves are also only about this size. The distance between the pillars corresponds approximately to one tenth of the wavelength of the laser. The laser beam is set up perpendicular to the channel and enters the “colonnade” from both sides. The acceleration process works roughly as follows: The silicon in the pillars interacts with the electrical field produced by the light waves and strengthens the field between the two pillars.
A wave forms along the channel that oscillates more strongly between the pillars than in the free gaps. If an electron with a suitable initial velocity is sent into the channel so that it is always located between two pillars when the electrical field is pointing in its direction of travel, it will be accelerated. As the electron travels forward, it passes through the free gaps between each pair of pillars where the electrical field is weaker. This means that it is decelerated much less than it was previously accelerated. The electron travels increasingly faster as a result. The colonnade and the laser wave interact in a complex way so that the changeover from acceleration to deceleration adapts to the increasing speed of the electrons.
Keeping the particles “straight”
One huge challenge that faced Niedermayer and his team was how to ensure that the electrons travel in a straight line through the channel. This is a problem because the strength of the electrical field changes at the edge of the channel, meaning that the electrons are driven away from their straight path.
In a standard electron accelerator, special magnets force the particles to travel in a straight line. The force that these quadrupole magnets apply to an electron can be described by comparing the electron with a ball on a saddle. The saddle is curved along its long side so that the ball will roll from the edge to the middle of the saddle where it should remain. However, the saddle curves downwards from the middle on its short sides so that the ball rolls away and falls from the saddle. But if the saddle is rotated quickly, after initially starting to roll away the ball would then be caught again quickly by the rotating, upwardly curved surface of the saddle. As a result, the ball remains in the middle of the saddle. This principle is recreated in particle accelerators by switching the field alignment of the quadrupole magnets along the path travelled by the electrons.
The scientists have designed their colonnade analogue to this principle. There are a series of large gaps between the otherwise regularly ordered columns. These create a similar effect to the rotation of a quadrupole magnet. If the electrons have been pulled away before a gap, the electrical field behind the gap changes so that the electrons are steered back into the middle.
“Our design has shortened the path to commercial application”
The first version of their design still suffered from the drawback that the electrons were still diverging vertically. This focussing problem has now also been solved in the current design by making the lower section of each pillar out of glass and the upper section out of silicon. The refractive index contrast of these materials produces a similar focussing effect to that in the horizontal direction.
Niedermayer, who received the “Young Scientist Award for Accelerator Physics 2020” from the German Physical Society, is delighted with the simple design. “Our design has shortened the path to commercial application”, says Niedermayer. This is because there are already low-cost chip wafers available on the market that provide a double layer of glass and silicon. The Darmstadt design could be produced from these wafers using established processes in the semiconductor industry. Moreover, the team has now had a new idea that will reduce the technical complexity and costs even further, for which it has just submitted a patent application. This will take the researchers a step closer to achieving one goal of the ACHIP project: A particle accelerator, including its electron source and laser optics, which fits into a shoe box and could in the far future even be integrated into an endoscope.