# Influence of Pre-Ionized Plasma on the Dynamics of a Tin Laser-Triggered Discharge-Plasma
## 2 Experimental Apparatus  
An experimental schematic diagram of tin LTD is given in Figure 1, which is mentioned in the previous works [19]. In all experiments, a 3 mm thick and 60 mm diameter high purity, planar tin target was the source for EUV generation. An Nd:YAG laser beam, produced by Continuum Minillite II laser system (8 ns pulse width, 1-50 mJ @1064 nm, 1-10 Hz), having a pulse energy of 20 mJ, was focused by an \(f=10\) cm focal length planoconvex BkZ glass lens, which was placed in the center of the hollow cathode. The laser beam was injected perpendicularly onto the tin target. The power density was \(10^{10}\) W/cm\({}^{2}\) at a focal spot diameter of 45 \(\upmu\)m FWHM. The distance between the hollow tungsten-copper anode and the tin cathode was 5 mm. Moreover, the laser pulse, which was used to produce a pre-ionized plasma, was not high enough to produce a detectable EUV emission. The tin cathode was rotated after five laser shots to avoid the effect of the electrode ablation [20]. In this paper, the measured spectrum is the average of the five shots. A 1 m Rowland circle grazing incidence spectrometer (GIS, McPherson 248/310 G) and an X-ray CCD (Andor, DO920P-BN) were used to record the time-integrated spectra. The spectral resolution is better than 0.04 nm. The spectrometer axis was vertical to the axis between anode and cathode electrodes, and coincided with the focused laser. An EUV energy monitor E-Mon (Bruker Advanced Supercon GmbH, 00M-EM-001), consisting of a Zr filter, two Mo/Si multilayer mirrors, and an X-ray photo-diode was used to measure pulsed EUV-energies and calibrated the GIS spectra detection system at central wavelength of 13.5 nm with a bandwidth of 2%. Figure 1: \(Cont.\)  
In order to avoid self-discharge and absorption of EUV emission, the vacuum chamber pressure was typically in the order of \(10^{-3}\) Pa, maintained by a mechanical pump and a turbo molecular pump. A Rogowski coil (Tektronix P6015A) and a high voltage probe (Pearson 5046) were used to measure the current and the voltage respectively. A diode (Thorlabs, DET10A/M) was used to monitor the laser signal. An EUV photo-diode (IRD, AXUV20 HS1 BNC) was used to observe the X-ray signal from a discharge. A 500 nm thick zirconium metal filter was placed in front of the IRD diode to select wavelengths from 6.5 nm to 16.8 nm [21]. These outputs of the current, the voltage, the signal of the laser, and the X-ray signal in one shot were recorded on a digital oscilloscope (Tektronix, MDO3054). The power supply was based on three-level magnetic pulse compression, which differs from conventional LTD sources [17; 22]. The high voltage can be generated and maintained by the pulse power system. When the pre-ionized plasma bridges the anode and the tin cathode, the current is generated. Then, the pre-ionized plasma is pinched to the center by the Lorentz force. In this way, high-temperature plasma is made to radiate EUV emissions. By modifying the delay between laser and voltage, this power supply can command the discharge process and the condition of the plasma. Figure 1b shows the time series diagram of the experimental setup. A computer gives two signals simultaneously to trigger the power supply and the digital delay generator (SRS, DG535). The inherent delay \(T_{1}\) is defined by the pulse power system and the final Z-pinch device, and it is a constant. Meanwhile, the delay \(T_{2}\) controlled by the DG535 was used to adjust the delay between the current and the laser signal. Figure 2 shows two typical waveforms for the voltage, the current, the IRD diode X-ray signal, and the laser signal under the laser-current delay \(\mathrm{d}t=0.68\) us and 1.60 us respectively. The laser-current delay \(\mathrm{d}t\) (\(\sim\)10% of laser pulse to 10% of current) is defined as the time interval between the laser irradiation (pre-ionized plasma) and the onset of the current (breakdown).