Abstract
Our understanding of the dynamics of ion collisional energy loss in a plasma is still not complete, in part due to the difficulty and lack of high-quality experimental measurements. These measurements are crucial to benchmark existing models. Here, we show that such a measurement is possible using high-flux proton beams accelerated by high intensity short pulse lasers, where there is a high number of particles in a picosecond pulse, which is ideal for measurements in quickly expanding plasmas. By reducing the energy bandwidth of the protons using a passive selector, we have made proton stopping measurements in partially ionized Argon and fully ionized Hydrogen plasmas with electron temperatures of hundreds of eV and densities in the range 1020–1021 cm−3. In the first case, we have observed, consistently with previous reports, enhanced stopping of protons when compared to stopping power in non-ionized gas. In the second case, we have observed for the first time the regime of reduced stopping, which is theoretically predicted in such hot and fully ionized plasma. The versatility of these tunable short-pulse laser based ion sources, where the ion type and energy can be changed at will, could open up the possibility for a variety of ion stopping power measurements in plasmas so long as they are well characterized in terms of temperature and density. In turn, these measurements will allow tests of the validity of existing theoretical models.
Highlights
The physics of fast-ion slowing-down in ionized matter is a topic relevant to diverse areas of research and applications including solid-state physics[1,2,3], astrophysics[2], plasma strippers, plasma ion heating and fusion sciences[4,5]
The intrinsic difficulty in realizing such experiments is to couple a well-characterized ion beam to an well-known plasma with linear dimensions adequate to allow for a detectable energy loss
Provided by accelerators[16] and diodes[25]. This implies that usually the ion beam propagates through plasmas of varying temperature and density, an effect which must be deconvolved in order to retrieve the varying ion stopping power
Summary
The spectrum selected for analysis was centered on where the proton beam had passed through the center of the target jet and the analysis lineout width was 500 μm at the target jet plane, as shown, i.e. compensated for the magnification effect inside the spectrometer For this experiment, an X-ray spectrometer[56], utilizing a flat field grating with a bandwidth of 200–2000 eV, was setup to look at Argon L-shell lines to determine the plasma temperature of the heated target plasma jet. The spectra are presented with normalized units for ease of comparison; normalization was done at the very end, i.e. after conversion into proton number and after background subtraction Argon was selected such that the gas was only partially ionized with the employed laser heating parameters, in order to be able to observe the effects of the so-called “enhanced stopping”, as was already highlighted in all previous experimental investigations of ion stopping in plasmas.
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