# Distortion of Thomson Parabolic-Like Proton Patterns Due to Electromagnetic Interference
## 1 Introduction  
A traditional Thomson parabola ion spectrometer (TP) is a device that can distinguish ions propagating through it according to their charge-to-mass ratio and their kinetic energy [1]. The spectrometer employs parallel magnetic and electric fields that are perpendicularly arranged with respect to the ion propagation direction. Particles entering a pinhole in the front are deflected and they then interact with an image plane where a recording system is installed (e.g., plastic nuclear track detector, photostimulable image plate, or microchannel plate coupled to a phosphor screen and a charge-coupled device camera). Typically, the magnetic field is provided by two parallel magnets, and the electric field is created by applying a voltage to electrical plates through high-voltage cables. Theoretically, perfect parabolic tracks of ions are drawn in the spectrometer detector plane. Nevertheless, the tracks can be perturbed by a high-energy laser pulse under real experimental conditions.  
A major source of ion-track distortions is EMP, which is generated during laser-target interaction [2; 3; 4]. High-voltage cables connected to the TP can pick up EMP noise during a laser shot, which results in the distortion of the spectrometer electric field. Ions with different energies thus experience varying electric-field strength, and their tracks on the detector plane are distorted. The high-pass filter on high-voltage cables can be incorporatedin order to reduce this effect [5]. Alternatively, Faraday cage shielding around cables and the spectrometer can be used to mitigate this effect.  
Another source of deviation from perfect tracks is attributed to the emission direction of the laser-accelerated proton beam itself at the the target surface (which is usually called \"pointing\"). A high-spatial-resolution Thomson spectrometer was employed to measure the pointing of proton beams generated from the rear side of plane target foils. Small bumps and deviations from the perfect parabolic tracks of ions were observed and identified as a feature of the emitted proton beam that occurred due to small fluctuations in the acceleration sheath [6].  
Track oscillations and distortions complicate the analysis and interpretation of ion numbers, especially when many different kinds of ion species are detected by the spectrometer. The overlapping of proton tracks and fast ion tracks can appear (as demonstrated in Figure 1) and lead to an incorrect number of detected particles. In addition, fitting and extracting the parabolic trajectories becomes difficult, as they do not follow smooth analytically derived curves. Here, we report the observation of distorted parabolic tracks at the PALS laser facility when a 600 J, 350 ps (FWHM) laser pulse was focused on solid targets reaching an intensity of \(3\times 10^{16}\) W/cm\({}^{2}\). Since the EMP affecting plasma diagnostics can be effectively detected by a simple antenna, and the recorded signal can be processed in the frequency domain [7,8], we compared the measured EMP signal with distorted parabolic tracks in order to investigate the effect of the EMP on the spectrometer, and to prove that the cause of track distortions at the PALS laser facility is the strong EMP generated in the target chamber.