Fig. 1. Tape target system (left) and cut of the target charging monitor (TCM; right). The TCM has two opposite miniature high voltage Bayonet Neill-Concelman radio-frequency (MHV-BNC) connectors with soldered pins to pass through the pulsed current issued by relativistic laser interaction in the top to the application side in the bottom. The TCM comprises a solid copper body forming a cup with a cylindrical top; both of which are later separated by dielectric material polyoxymethylene (POM). The through current induces a magnetic field enclosed in the cylinder, which causes an induced current to flow in a small squared loop formed by the core of an RG142 coaxial cable connected to an output SMA connector. The current pulse itself is issued by the discharge of the solid tape target and coupled into one of the insulated support rods of the tape, which are connected to an RG142 coaxial cable leading to the TCM via an MHV-BNC connector on the system’s chassis. The other support rod is isolated from the ground.

Fig. 2. The circuit-corrected signal of the TCM for an aluminium target exhibits a clear positive peak for the rising edge of the positive current pulse. It is preceded by a low-noise pedestal and followed by pulses streaming from the grounding to the target: first the EMP-induced noise and second the reflection of the current pulse at the impedance mis-matched the ground. The time-base at the TCM relative to laser arrival is approximately equal to 0 ns.

Fig. 3. Current pulse (blue line) from an aluminium target retrieved by numerical integration from the derivative measured with the TCM. A first short primary peak is followed by a superposition of peaks in a broad secondary peak. The time-base relative to laser arrival is approximately equal to 0 ns. The zero-level is controlled by comparison to a fit from before to after the current pulse (orange dashed line) – here in good agreement.

Fig. 4. The transported charge from an aluminium target as obtained by numerical double-integration of the derivative measured by the TCM. The integral attains a plateau only slowly due to a slightly negative tail of the return current. The time-base relative to laser arrival is approximately equal to 0 ns.

Fig. 5. Average current and its standard deviation as obtained in laser shots of W cm at 1 Hz onto copper tape. The time-base relative to laser arrival is approximately equal to 0 ns. Multiple reflections across the conductive target yield a succession of multiple peaks.

Fig. 6. Average current and its standard deviation as obtained in laser shots of W cm at 0.5 Hz onto Kapton tape. The dielectric target allows one to produce single pulses. The time-base relative to laser arrival is approximately equal to 0 ns.

Fig. 7. (a) Total charge measured under variation of laser pulse duration, energy and the target material. (b)–(d) Select data obtained at best laser compression: (b) comparison with a semi-empirical model to derive the total charge from and a material constant; (c) spectral cut-off energies for protons in the target normal direction compared to available modelling^[38]; (d) the relation between target charge and proton cut-off energy.

Fig. 8. Average current and its standard deviation as obtained in laser shots of W cm at 1 Hz onto Al-e-K tape. The time-base relative to laser arrival is approximately equal to 0 ns.

Fig. 9. Average power spectrum density and its standard deviation as obtained in laser shots of W cm at 1 Hz onto Al-e-K tape. The time-base relative to laser arrival is approximately equal to 0 ns.