Fig. 1. Gas jet nozzle used during an experiment at the Vulcan Target Area Petawatt facility (a) before and (b) after a full-power laser shot. Melting can lead to occlusion of the nozzle aperture or even total rupture of the material. Images reproduced from Ref. [23] with permission.

Fig. 2. Schematic of the nozzle, gas and the cylindrical plasma channel formed by the laser. Here, is the radius of the plasma cylinder, is the separation between the channel and the nozzle and is the electric potential. The laser is directed into the plane of the page, along the axis of the plasma. Red crosses indicate a region of positive charge inside the laser-generated channel. Three curved arrows sketch the geometry of the electric field, E.

Fig. 3. Ion spectra collected in gas jet experiments. (a) Proton spectrum from an experiment on the Vulcan-TAP laser, measured at to the laser axis using a Thomson parabola spectrometer with BAS-TR image plate^[23]. The shaded region is the detection limit. (b) -particle spectra from an experiment at VEGA-3^[26], measured at from the laser axis using diamond time-of-flight detectors. The two spectra were recorded on different shots. See also Figure 5(a). (c) Proton spectrum from a separate experiment at VEGA-3^[24], measured using a Thomson parabola spectrometer at to the laser axis. The spectrometer dynamic range limits reliable measurements to energies more than approximately 1.3 MeV. The blue dashed line represents the background noise level. See also Figure 5(b).

Fig. 4. Ion energy deposited per mass of nozzle material as a function of laser-nozzle separation for the number of ions (a) and (b) normalized to the plasma height of . The red dot represents the theoretical melt threshold for a Cu nozzle. Red and green vertical dashed lines represent observed distances where a steel nozzle was destroyed and survived, respectively. The ion energy is 0.5 MeV.

Fig. 5. Schematic diagrams of two experiments conducted on the VEGA-3 laser system. (a) Setup described in Section 6.1. A Thomson parabola spectrometer (TP), two electron spectrometers (E-Spec) and four diamond time-of-flight detectors (Diamond ToF) are represented by coloured boxes. (b) Setup described in Section 6.2. Three Thomson parabola spectrometers are placed at 0°, 60° and 90° to the laser axis. The B-dot probe was positioned at variable distances ( m) from the gas target, with its measurement axis horizontal and orthogonal to the line-of-sight axis. A camera was used to take images of the gas at twice the laser fundamental frequency. Further details of these experiments can be found in Refs. [24,26] .

Fig. 6. Images corresponding to optical probe arrival approximately 150 ps after the drive laser in a He gas with a long-focus shock nozzle. (a) Raw interferogram. (b) Density map showing a plasma channel straddling the peak density region at μm. The laser focus position in the vacuum was set at μm.

Fig. 7. Interferograms of the VEGA-3 laser interacting with N₂ gas ejected from a short-focus shock nozzle. Probe times relative to the arrival of the pump beam are 40 ps (a) and 90 ps (b). The laser intensity is W/cm² and the gas density is cm⁻³.

Fig. 8. Three-dimensional graphic of the gas jet nozzle and solenoid valve assembly at VEGA-3. Arrows indicate dimensions relevant to electromagnetic emission.

Fig. 9. Comparison of EMP waveforms for solid and gaseous targets on VEGA-3. The signals were measured using a Prodyn RB-230(R) probe positioned at 60° to the laser forward direction at a horizontal distance of m from the nozzle and vertically in-line with the laser focal spot. The maximum amplitude of the magnetic field was a factor two to three times lower for the gas targets compared to 6-μm-thick solid Al foils.

Fig. 10. Variation of EMP maximum magnetic field with distance from the gas jet. Data was collected with the B-dot probe positioned at to the laser axis, with the line of sight to the target occluded. The fitted curve is for a 3-cm-tall antenna with the angle between the antenna axis and the probe assumed constant at for the different distances.

Fig. 11. Variation of the EMP electric field located 1 m from a 3-cm-tall nozzle for different values of the total laser energy and pulse duration, calculated using the model from Section 2. Laser and gas parameters have been chosen so that they are representative of experiments with the following: (a) under-dense gases on low-energy systems like the Gemini Target Area 2 laser^[71]; (b) near-critical gases and PW-scale lasers, such as VEGA-3; (c) under-dense gases and high energy, longer pulse duration lasers such as LMJ PETAL^[72] and NIF ARC^[73].

Fig. 12. Nozzle damage factor (ratio of the plasma ion energy deposited in the nozzle per mass of heated material divided by the nozzle melt threshold) for different values of the laser-nozzle distance and gas pressure, calculated using the model from Section 2. An exponentially decaying gas density profile with μm scale length is assumed, where the peak He gas density is taken at the nozzle surface (). A bold white line marks where the nozzle damage factor is equal to 1, corresponding to a deposited energy-per-mass equal to the melt threshold of Cu ( kJ/g).