Misjudging negative ions for electrons in intense laser plasma diagnostics

Intense laser produced plasmas are known to be novel compact sources of high energy electrons. Electrons deflected by magnetic fields and imaged on phosphorescent plates is a well used diagnostic for electron spectrum measurements. As only negatively charged particles are filtered by the magnetic field, it is a common practise to interpret the measured scintillation entirely as a signal due to the electrons. We show here that such an interpretation can be erroneous. Low energy H− ions arrive concurrently at the same position as high energy electrons and can lead to a wrong assessment of the maximum electron energy measured, which is often used in understanding the underlying physics. Conventional accelerator based experiments prove that the H− ions are indeed detected even when the image plate is covered with a metal foil.