Generation of polarized proton beams with gaseous targets from CO2-laser-driven collisionless shock acceleration

Abstract

We propose obtaining polarized proton beams based on CO2-laser-driven collisionless shock acceleration (CSA) of the pre-polarized HCl gas. By tailoring the density profile of the pre-polarized HCl gas, the intense CO2 laser pulse heats the plasma target and forms a strong shock that reflects the polarized protons to high energy. According to particle-in-cell simulations implemented with the spin dynamics, directional proton beams of several MeV were generated at a total beam polarization of over 80%. Simulations showed that proton spin precession occurred in the azimuthal magnetic fields generated by the Biermann effect and plasma currents. The latter was the main depolarization mechanism in the early stage of shock wave formation. For CSA at CO2 laser intensities around 1017–1018 W/cm2, the proton depolarization was insignificant and the beam polarization purity was preserved. As pre-polarized hydrogen targets were available at gaseous densities in-state-of-art facilities, CSA driven by relatively long wavelength lasers provided a feasible solution for obtaining ultra-fast polarized proton sources.

Enhanced polarized proton acceleration driven by femtosecond laser pulses irradiating a micro-structured solid–gas target

“…This can be attributed to the significantly larger magnetic field B z in the bipolar distribution corresponding to an increase in a 0 that induces strong depolarization of the protons away from the central axis around the shock front region. The results depicted in figure 5(a) are consistent with the fact that the polarization rate decreases with increasing a 0 [42,51]. As depicted in figure 5(d), protons depolarize faster at 10 n c than at 50 n c .…”

“…Therefore, reflected protons with high velocities undergo a relatively weak spin precession. Notably, the polarization stability of such CSA-driven highenergy protons has been discussed in a previous study [51].…”

Spin-polarized proton acceleration by an intense laser pulse with a foil-gas target

“…Moreover, as shown in figure 4(a), s x decreases rapidly within t = 300 fs and then decreases slowly. The decreasing trend of s x slows down, proving that the depolarization process occurs mainly at the early stage of shock wave formation [20]. It is because, in the late stage of shock wave formation, the large number of reflected protons would move to the weak region of magnetic field B z .…”

“…Combining the rotation intrinsic of equation ( 1) and the relationship between the initial spin direction e x and precession direction e ϕ , the time evolution of the spin vector can be expressed as [20]: (3) where e ϕ × e x = e r . According to equations ( 2) and ( 3), the spin component can be expressed as:…”

Polarimetry for ³He Ion Beams From Laser-Plasma Interactions

“…Various theoretical works have developed strategies to realize plasma-based acceleration of polarized electron, proton and ion beams, most of them making use of pre-polarized gaseous targets, see e.g., Refs. [7][8][9]. However, we are still awaiting experimental proofs and only first steps in this direction have been taken, employing unpolarized targets where no beam polarization is expected [10,11].…”