Session CO2 - Laser Accelerators; Ultraintense Laser Applications.
ORAL session, Monday afternoon, November 15
Room 203, SCC
One key property of laser-accelerated MeV ion beams is their ultra-low transverse emittance. In recent experiments, it was found to be <0.004 mm mrad [Cowan et al., Phys.Rev.Lett.92, 204801 (2004)], at least 10^4 times smaller than the emittance of conventional thermal ion sources. We investigate the reasons for the low emittance of laser-accelerated proton beams by checking several candidates for emittance-generation mechanisms, apart from the geometrical effect that is caused by the roughness of the target surface; it is not considered here.
As our main tool we use 1D particle-in-cell simulations that have been modified to include binary collisions between particles [see Kemp et al., Phys. Plasmas, Sept 2004], and analytical work. Our restriction to 1D detains us from looking at geometrical effects, but it does allow to study the transverse width of the proton beam in momentum space, which is the relevant information for the beam emittance. We find that the lower emittance limit is mainly determined by electron-ion collisions during the initial acceleration phase from the cold rear target surface. Another possible mechanism responsible for the experimentally determined emittance is an ion-acoustic beam plasma instability. We find that it is not important for the fastest ions, while it could play a role for the lower energy ions.
This work was supported by DOE/NNSA under UNR grant DE-FC52-01NV14050.