Abstract
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.
Highlights
Laser matter interactions of intense (>1016 Wcm−2), ultrashort (10’s of fs) lasers are known to accelerate various charged particles like electrons and ions.1–3 Ability to produce compact acceleration schemes with field strengths orders of magnitude larger than conventional methods has been a major attraction for studies in this field
While LANEX screens are better for online, shot to shot data acquisition, image plates (IP) are advantageous for capturing data over multiple shots, especially when the event probability is low and long time acquisition is required for better statistics
We have demonstrated in this paper that image plates can detect low energy negative ions either by direct incidence or by detection of secondary radiations
Summary
Laser matter interactions of intense (>1016 Wcm−2), ultrashort (10’s of fs) lasers are known to accelerate various charged particles like electrons and ions. Ability to produce compact acceleration schemes with field strengths orders of magnitude larger than conventional methods has been a major attraction for studies in this field. Ability to produce compact acceleration schemes with field strengths orders of magnitude larger than conventional methods has been a major attraction for studies in this field Among these electron acceleration have been studied intensively to understand the underlying interaction dynamics. It is presumed that electrons are the only negatively charged particles detected by the IP This assumption is based on the fact that in laser plasma experiments high field, high temperature and density conditions make generation of negative ions unlikely. The other reason why the presence of negative ions in IP signals is considered non existent is because of a standard protocol of encasing the (image plate) IPs in thin metal foils like Aluminium, that act as filters for low energy electrons and negative ions, ensuring that only high energy electrons be incident on the detector. In order to reaffirm that negative ion signals can be obtained even in the presence of a metal foil, we perform a signal calibration experiment with a standard accelerator based negative ion source at different energies
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