2023-08-02 17:30:00
Accelerating protons with strong laser pulses – this still young concept promises many advantages over conventional accelerators. For example, it seems possible to build much more compact installations.
However, current prototypes, in which laser pulses are fired at ultra-thin metal foils, have weaknesses, particularly in the frequency at which they can accelerate protons.
At Helmholtz-Zentrum Dresden-Rossendorf (HZDR), an international working group has tested a novel technique that uses frozen hydrogen as a “target” for laser pulses, a method that might eventually form the basis of advanced tumor therapy concepts.
A new angle of approach for proton acceleration
Conventional proton accelerators, such as the Large Hadron Collider at CERN in Geneva, use particle acceleration via powerful radiofrequency waves. Laser acceleration, on the other hand, relies on pulsing particles with ultra-bright pulses of light. The extremely short and powerful laser pulses are directed towards ultra-thin sheets of metal. The heat generated by the light is such that electrons are ejected in large numbers, while the heavy atomic nuclei remain in place.
The absence of electrons, negatively charged, facing the atomic nuclei, positively charged, creates a strong electric field. This field can then propel a pulse of protons with tremendous force over a distance of only a few micrometres, allowing them to reach energies for which much longer systems would be required with conventional acceleration technology.
Limits to traditional methods
However, the traditional method of directing laser pulses at sheets of metal has drawbacks. First of all, it is difficult to generate several proton pulses per second because the sheet is destroyed by a single laser shot and must therefore be constantly replaced. In addition, the acceleration process is quite complex and relatively difficult to control.
The protons to be accelerated come in fact from hydrocarbons which have accumulated on the metal sheets in the form of a layer of contaminants, which is not ideal for perfect control of the experiment.
Diagram of the experimental device: By sending an intense pulse (red) of the HZDR DRACO high-power laser onto a constantly renewing jet of frozen hydrogen (blue), the researchers were able to massively accelerate protons over an extremely short distance (orange ). They recorded the process using a synchronized optical laser pulse (green). SLAC National Accelerator Laboratory / G. Stewart
A new method: filament rather than foil
Faced with these constraints, the German-American research team led by Dr. Karl Zeil, physicist at HZDR, proposed an alternative. Instead of a sheet of metal, they use a fine jet of strongly cooled hydrogen. This jet serves as a target for high intensity laser pulses. Hydrogen, initially in gaseous form, is cooled in a copper block until it becomes liquid. It is then directed through a nozzle into a vacuum chamber, where it continues to cool and solidifies into a filament a few micrometers thick.
This renewable filament offers a new, undamaged target with each laser shot. This configuration allows a more favorable acceleration mechanism: the laser pulses not only heat the material, but use the radiation pressure to eject the electrons from the hydrogen and create the extreme electric fields necessary for the acceleration of the protons. .
Promising implications for tumor therapy
With this new approach, the team was able to accelerate protons to an energy of 80 MeV, an achievement close to the previous record for proton acceleration by laser. However, unlike previous installations, this technique has the potential to generate multiple bursts of protons per second.
Furthermore, the acceleration process is relatively simple to simulate for hydrogen targets using high-performance computing, a task that also involved the Center for Understanding Advanced Systems (CASUS) at HZDR. This makes it possible to better understand and optimize the interaction between the laser and the material.
In the future, this technology might prove interesting for a new form of radiotherapy. Today, certain types of tumors are already successfully treated with proton irradiation. Laser acceleration might increase the dose, and therefore shorten the irradiation time, while better protecting the surrounding healthy tissue, as a study by HZDR suggests.
Synthetic
The exploration of new approaches to proton acceleration offers prospects, both in terms of the miniaturization of installations and of medical implications, in particular for the treatment of tumours. The method tested by the international HZDR working group, which uses frozen hydrogen as the target for the laser pulses, seems particularly promising. It offers the possibility of more controlled, more repeatable and potentially more efficient acceleration, paving the way for advanced tumor therapy concepts.
Article “Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density”, in Nature Communications, 2023 (DOI: 10.1038/s41467-023-39739-0) DOI : 10.1038/s41467-023-39739-0
Photo: Hydrogen jet snapshots © : HZDR
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