01 March 2022

Calculations by scientists of the Moscow Polytechnic University helped to determine the properties of proton beams accelerated by a laser pulse. This knowledge is necessary for the development of proton therapy.

Proton beam therapy allows you to destroy cancerous tumors, practically without damaging neighboring healthy tissues. But to do this, you need to generate a powerful and precisely directed beam of protons: for example, so that it goes through the skull and releases the maximum radiation dose at a certain depth — directly into the tumor, killing cancer cells there and not penetrating deeper into healthy brain tissue. "Today, to organize proton therapy, it is necessary to fill a room the size of a gym with expensive equipment," says Stepan Andreev, head of the Mathematics department at Moscow Polytechnic University. — Physicists are working on making this room smaller to the size of a laboratory table. And this can be done with a laser."

This is not about an ordinary laser beam, but about ultrashort and super—powerful laser pulses. The shorter the pulse, the more powerful the radiation is. So, modern lasers generate pulses lasting several femtoseconds (1/1015 fraction of a second). The power of one such pulse can be greater than that of all power plants on the planet.

"When a super—intense laser pulse affects a target, extremely interesting physical processes occur in it, including nuclear reactions," explains Stepan Andreev. — I am engaged in modeling these processes, and as a result of our work, a relatively small and inexpensive device for proton therapy may appear: a laser on a table sends pulses to a certain target, on which a proton beam is formed, with the energy necessary for therapy. If such technology appears, proton therapy centers can be organized in every hospital."

In a new paper, Stepan Andreev calculated what happens when a super-intense laser pulse interacts with an aluminum target, on the back surface of which there is a layer of protons. Calculations have shown that proton acceleration occurs not only in the area of the "spot" of the laser pulse, but also on the entire back surface, the length of which in the model is six times larger than the diameter of the laser "spot".

"This is due to the fact that the "hot" electrons that create an accelerating electrostatic field on the back surface of the target, performing oscillatory movements and repeatedly passing through the target and back, cover with their trajectories almost the entire volume of the target, and not just the area of laser exposure," explains the professor.

Most protons move perpendicular to the target and form a beam whose angle of expansion does not exceed 15-20 degrees. Such a proton beam has a high degree of laminarity, that is, it moves in space without mixing. It spreads over relatively long distances without changing the transverse size. According to Andreev, the reason for this behavior may be the quasi-neutrality of the beam, in which the Coulomb repulsive forces of positively charged protons are compensated by the attractive forces of electrons moving with protons.

"The issue of quasi—neutrality has not been sufficiently investigated, and this state of proton beams is of interest not only for fundamental science, but also for materials sciences and laser medicine," Andreev notes.

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