As laser facilities have grown in size and power, understanding how electromagnetic pulses (EMPs) are generated has become an issue of great practical importance. High intensity lasers can induce strong fields (MV•m-1) and massive currents (MA) in solid targets, producing EMP radiation that disrupts electrical measurements and damages electrical equipment. A number of different mechanisms have been proposed to explain the broad spectral profile of laser-driven EMP, ranging from direct current processes up to transition radiation at terahertz frequency. When high-power lasers interact with materials, they accelerate hot electrons that escape from and electrically polarize the target. If the target is grounded, a neutralization current is pulled out of the chamber through the target support. It is thought that this current is responsible for the emission of intense electromagnetic pulses at gigahertz frequency that are disruptive to electronics. Today, there is growing interest in the applications of directed EMPs and fast current generation, though with the advent of intense, high repetition-rate lasers like the Extreme Light Infrastructure, strategies to limit EMP emission remain of considerable importance.

The research group had two objectives in the study: first to characterize the energy of the EMP emission (to understand how it varied with laser and target parameters) and second to see if it could be reduced. The research group of professor N. C. Woolsey used the Vulcan West laser system at the Rutherford Appleton Laboratory for our experiment, reaching a maximum intensity of ∼2×1019 W•cm-2 at best focus. The laser beam was directed onto copper targets mounted on a variety of support stalks. To measure the energy of the EMP, the researchers installed three passive probes behind glass windows on opposite sides of the interaction chamber. A Bdot and Ddot probe were positioned facing the front of the laser target and a further Bdot probe was directed towards the target rear. Probe signals at megahertz and gigahertz frequency were then integrated by first author P. Bradford to produce a measure of the total EMP energy. The results have been published in High Power Laser Science and Engineering, Vol 6, 2018 (P. Bradford et al., EMP control and characterization on high-power laser systems).

The first phase of the experiment looked at how EMP energy scaled with different lasers and target parameters, in order to assess qualitative agreement with theoretical models. Varying the laser energy from 7-70 J, the researchers observed a linear relationship with EMP energy. They also looked at the variation of EMP energy with laser pulse duration, pre-pulse delay and defocus. These scans suggested that the higher the laser intensity, or the more energy coupled to the plasma, the greater the EMP emission. When the researchers examined the effect of target size on EMP, they found that smaller foils and wire targets produced drastically reduced EMP. Indeed, EMP energy was over an order of magnitude less for wire targets (Ø=25-100μm) than for 3 mm×8 mm rectangular foils.

 

Since the EMP is generated by a current discharge mechanism (which can be pictured as a radio-frequency radio frequency control (RLC) circuit), a key experimental objective was to see if the EMP energy could be modified by changing the resistance, R, inductance, L, and capacitance, C, of the target mount. The research group fielded three different geometrical designs: a cylindrical stalk, a mount with sinusoidal surface undulations and a spiral stalk (see Figure 1). First, the research group replaced Al cylindrical stalks with plastic and found that there was a very significant drop in EMP energy (over one third reduced). The researchers attribute this to increased stalk impedance that limits the size of the neutralization current. Then they replaced the cylindrical plastic stalk with a plastic spiral and plastic sinusoidal design. For the spiral stalk the effect was clear: the researchers found that the plastic spiral stalk reduced the EMP energy by over an order of magnitude compared with Al cylinders.  The researchers also saw a significant reduction for the stalk with sinusoidal undulations, though the effect was less pronounced.

 

To verify whether the change in EMP was independent of the laser-target interaction, author Y. Zhang used an electron spectrometer to record the energy of emitted electrons emitted from the target rear surface. Her results showed that there was no significant reduction in electron emission for shots with the modified stalks.

 

To see if reduced EMP energy from the modified stalks was due to classical RLC effects, author F. Consoli ran a series of 3D particle-in-cell and electromagnetic simulations in which a cone of energetic electrons was emitted from a central target and the EMP energy measured at different points inside a virtual chamber. The simulations suggest that there will be a greater reduction in EMP than observed when using insulating versus conducting stalks and that geometry is a less important factor than stalk conductivity. It is therefore possible that other physical mechanisms may be required to explain our observations. For instance, charged particles and ionizing radiation from the laser-plasma interaction could be deposited along the length of plastic stalks, reducing the effectiveness of the insulator. This could also explain why the modified stalks were so successful, because their unusual geometry serves to partially shield the stalk surface against incoming particles/radiation and thereby guard against electrical breakdown. A second set of simulations were run with a stalk of half-length which showed much higher EMP energy and therefore provides us with tentative support for this theory. However, since the simulations did not take stalk ionization into account, more experiments are required before any definitive pronouncements can be made.

 

The experiment has demonstrated that a very significant reduction in EMP can be achieved by a simple modification of the target mount. In particular, a plastic spiral stalk has been shown to reduce the EMP energy by over an order of magnitude versus a metallic rod. The researchers are working on a complete explanation of why the stalks are effective using spectral analysis and by experimenting with other stalk designs. The researchers would also like to compare our laser and target parameter scans with leading theoretical models of EMP. Progress in this field depends on our ability to differentiate between the different mechanisms responsible for laser-driven EMP and, in understanding them, to tailor the emission according to our needs.

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