Abstract
The irradiation of a target with high laser intensity can lead to self-generation of an intense magnetic field (B-field) on the target surface. It has therefore been suggested that the sheath-driven acceleration of high-energy protons would be significantly hampered by the magnetization effect of this self-generated B-field at high enough laser intensities. In this paper, particle-in-cell simulations are used to study this magnetization effect on sheath-driven proton acceleration. It is shown that the inhibitory effect of the B-field on ion acceleration is not as significant as previously thought. Moreover, it is shown that the magnetization effect plays a relatively limited role in high-energy proton acceleration, even at high laser intensities when the mutual coupling and competition between self-generated electric (E-) and B-fields are considered in a realistic sheath acceleration scenario. A theoretical model including the v × B force is presented and confirms that the rate of reduction in proton energy depends on the strength ratio between B- and E-fields rather than on the strength of the B-field alone, and that only a small percentage of the proton energy is affected by the self-generated B-field. Finally, it is shown that the degraded scaling of proton energy at high laser intensities can be explained by the decrease in acceleration time caused by the increased sheath fields at high laser intensities rather than by the magnetic inhibitory effect, because of the longer growth time scale of the latter. This understanding of the magnetization effect may pave the way to the generation of high-energy protons by sheath-driven acceleration at high laser intensities.
Highlights
Laser-driven ion acceleration has been the focus of much research activity for several decades because of its potential to provide compact energetic ion sources with unique beam properties, including short duration, high brilliance, and low emittance
A theoretical model including the v 3 B force is presented and confirms that the rate of reduction in proton energy depends on the strength ratio between B- and electric field (E-field) rather than on the strength of the B-field alone, and that only a small percentage of the proton energy is affected by the self-generated B-field
It is shown that the degraded scaling of proton energy at high laser intensities can be explained by the decrease in acceleration time caused by the increased sheath fields at high laser intensities rather than by the magnetic inhibitory effect, because of the longer growth time scale of the latter
Summary
Laser-driven ion acceleration has been the focus of much research activity for several decades because of its potential to provide compact energetic ion sources with unique beam properties, including short duration, high brilliance, and low emittance. Especially for realistic TNSA scenarios, there is a lack of any quantitative estimates of magnetization effects on sheath-driven ion acceleration, for example, estimates of how much the ion energy can be reduced by a self-generated B-field. As well as the PIC simulations, a simplified TNSA model including the v 3 B force is presented in order to provide a relatively quantitative estimation of the magnetization effect on sheath-driven ion acceleration. The degraded scaling of proton energy at high laser intensities is shown to be the result of a decrease in acceleration time due to the increased sheath field, rather than to the inhibitory effect of the intense self-generated B-field
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