The application of particle accelerators is not just restricted to fundamental research in high-energy physics. Massive accelerators and huge devices (for example, the Large Hadron Collider (LHC)) are used for basic studies. However, for industry (electronic engineering, cargo scanning, food sterilization), medicine (cancer treatment, diagnostic imaging), and different types of analyses (analysis of artworks, archeological surveying, oil prospecting), comparatively smaller accelerators are used.
Regardless of the use, improving particle flow efficiency and controlling chaos are the objectives of the researchers in this field.
A paper by Meirielen Caetano de Sousa, a postdoctoral student with a scholarship from FAPESP working at the University of São Paulo’s Physics Institute (IF-USP) in Brazil, and her supervisor Iberê Luiz Caldas, Full Professor at IF-USP, reports an innovative contribution in this area of study and has recently been published in the Physics of Plasmas journal.
“We performed a theoretical study with modeling and numerical simulation to investigate ways of controlling chaos inside accelerators and increasing the maximum velocity of accelerated particles,” Sousa told Agência FAPESP.
The researchers developed a mechanism that is based on deploying a transport barrier to restrict the particles and arrest their movement from one region of the accelerator to the other. Until now, this process has not been used in ordinary accelerators; however, it is implemented in tokamaks (experimental toroidal reactors used for the study of nuclear fusion), where particle confinement is employed to prevent superheated plasma from interacting with the walls of the device.
In tokamaks, the transport barrier is obtained by means of electrodes inserted into the plasma edge to alter the electric field. This hasn’t yet been done in accelerators, where the usual solution is to add an electrostatic wave with well-defined parameters to the system.
When the wave interacts with the particles, it controls chaos in the system but creates multiple barriers that don’t seal the region as precisely. This is a less robust solution. In our study, we modeled a system with a single barrier along similar lines to what happens in tokamaks.
Meirielen Caetano de Sousa
This single powerful barrier would be created by a resonant magnetic perturbation. As a response to the RMP, the plasma is restricted to a single region.
“We created the model and described it mathematically. The numerical simulations showed that it works. The next step is to take the proposal to experimental physicists who can test it in practice,” stated Sousa.
An electron gun is used for producing the particles by virtue of the difference in potential between the cathode and the anode or by applying a laser pulse to the plasma. Successive bouts of energy applied from electromagnetic waves are used to accelerate the particles. Interaction between the particles and the electromagnetic waves results in disorder. One solution experimentally tested in accelerators involves the addition of another wave with parameters that are tweaked to compensate for the chaotic process.
This was discussed in a previous article published in 2012 in Physical Review E. The method works, but as noted, it creates multiple transport barriers that are susceptible to perturbation, making particle confinement less effective. In this latest study, we modeled a solution based on a single robust barrier, which continues to exist even in the presence of high perturbations.
Meirielen Caetano de Sousa
Substitution of Radioisotopes
The transport barrier controls the disorder, thereby increasing the maximum particle velocity and reducing the required initial velocity. In the case of a low-amplitude wave, the simulated final velocity increased by 7%, and the initial velocity decreased by 73%.
In the case of a wave of higher amplitude, although the system was found to be disordered in the absence of the barrier, it was regularized once the barrier was deployed. The final velocity was found to be increased 3%, and the initial velocity decreased by about 98%. This indicates that the main contribution of the transport barrier is to reduce the initial velocity needed for the particles while injecting them into the accelerator.
“What’s expected of an accelerator is that all particles arrive together at the end without going astray en route, and with more or less the same energy and velocity. If they behave chaotically, that doesn’t happen, and the beam is of no use for any application,” stated Caldas.
“Particle emission for medical or industrial use is still based mostly on the use of radioactive materials. This causes a number of problems, such as pollution, decay of the emitter material requiring replenishment, and high cost. Accelerators avoid these problems and are a partial substitute for radioisotopes. Hence the strong interest in optimization of accelerator functioning.”
The article “Improving particle beam acceleration in plasmas” (https://doi.org/10.1063/1.5017508) by Meirielen Caetano de Sousa and Iberê Luiz Caldas can be retrieved from aip.scitation.org/doi/10.1063/1.5017508.