###### Abstract  
In-situ measurements of space plasma throughout the solar system require high time resolution to understand the plasma's kinetic fine structure and evolution. In this context, research is conducted to design instruments with the capability to acquire the plasma velocity distribution and its moments with high cadence. We study a new instrument design, using a constant magnetic field generated by two permanent magnets, to analyze solar wind protons and \(\alpha\)-particles with high time resolution. We determine the optimal configuration of the instrument in terms of aperture size, sensor position, pixel size and magnetic field strength. We conduct this analysis based on analytical calculations and SMION simulations of the particle trajectories in our instrument. We evaluate the velocity resolution of the instrument as well as Poisson errors associated with finite counting statistics. Our instrument is able to resolve Maxwellian and \(\kappa\)-distributions for both protons and \(\alpha\)-particles. This method retrieves measurements of the moments (density, bulk speed and temperature) with a relative error below 1%. Our instrument design achieves these results with an acquisition time of only 5 ms, significantly faster than state-of-the-art electrostatic analyzers. Although the instrument only acquires one-dimensional cuts of the distribution function in velocity space, the simplicity and reliability of the presented instrument concept are two key advantages of our new design.  
solar wind; space plasma; in-situ plasma analyzer; high time resolution; small-scale processes; magnetic analyzer +
Footnote †: journal: Applied Physics Letters  
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Footnote 19: Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Dorking, Surrey RH5 6 NT, UK; [EMAIL_ADDRESS] (G.N.); [EMAIL_ADDRESS] (Gany space-plasma instrument must resolve non-thermal properties of the particle VDFs. Moreover, as the solar wind passes over the measurement point at a supersonic speed (approximately in the range from 300 to 800 km/s) in the spacecraft frame, the acquisition of VDF variations must be achieved with high cadence. Assuming average solar-wind conditions, we translate this cadence requirement into a sampling time requirement for the instrument. Since many important kinetic processes occur on scales of the order of the proton gyro-radius \(\rho_{p}\), we require that a suitable instrument for the study of ion-kinetic processes resolves spatial scales of size \(\rho_{p}\). For a typical solar wind speed of \(u=500\) km/s, the resolution of a structure of size the proton gyro-radius at a heliocentric distance of 1 au (\(\rho_{p}=80\) km) [8; 9] requires a sampling time of \(T_{s}=2\pi\,\rho_{p}/2u\approx 500\) ms according to the Nyquist criterion. Modern space plasma instrumentation aims at sampling the VDF with a high enough cadence to study sub-ion-scale variations in the VDF. This is the direction taken by, for example, the Debye mission [10] and the THOR mission [11]. The goal of our instrument design is a cadence of 5 ms, which is two orders of magnitude faster than the requirement to resolve structures with a size of the average \(\rho_{p}\) at a heliocentric distance of 1 au.