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Assessment and mitigation of electromagnetic pulse (EMP) impacts at short-pulse laser facilities

The National Ignition Facility (NIF) will be impacted by electromagnetic pulse (EMP) during normal long-pulse operation, but the largest impacts are expected during short-pulse operation utilizing the Advanced Radiographic Capability (ARC). Without mitigation these impacts could range from data corruption to hardware damage. We describe our EMP measurement systems on Titan and NIF and present some preliminary results and thoughts on mitigation.


Introduction
Electromagnetic pulse (EMP) is a known issue for short-pulse laser facilities such as Vulcan, Titan and Omega-EP [1]. We expect the National Ignition Facility (NIF) will be impacted by EMP in both long-pulse, which is the normal NIF operation mode, and short-pulse operation. However, we expect the largest impacts during short-pulse operation, such as with the short-pulse based Advanced Radiographic Capability (ARC). Therefore, understanding and mitigating EMP is an important part of the effective use of this and other laser facilities. Short-pulse (∼ few ps) lasers can produce very energetic (MeV) electrons that can be used to produce short-pulse, high-energy, x-ray backlighting as in ARC experiments. In general, the high energy of these electrons causes more to escape the target than the lower-energy "hot electrons" (<100 keV) often produced by laser-plasma interactions in long-pulse (∼ few ns) laser-plasma interactions.
Even in the case of short-pulse operation, the number of electrons that escape (∼ 10 12 ) is a small fraction of the total number produced in the target, and the associated charge is a small fraction of a Coulomb. However, the short duration causes the escaping electrons to produce very large transient currents and EMP. In addition, the short duration translates to a very broad spectrum with the potential of high-frequency EMP. In both cases (short-and longpulse) escaping electrons can be a significant source of EMP.
For effective shielding it is critical that the spectrum of the EMP, as well as the time-domain behavior, is known. Both measurements and simulations are necessary to understand EMP in short-and long-pulse lasers. We have fielded a large suite of diagnostics, including B-dot (loop) and D-dot (monopole) sensors, inside and outside the Titan short-pulse laser target chamber, which is a part of the Jupiter Laser Facility at Lawrence Livermore National Laboratory (LLNL) [2], in order to characterize both the EMP and the escaping electrons. We have used our Titan experience to field an EMP measurement system with B-and D-dot sensors [3] inside and outside of NIF's target chamber. In order to establish predictive capabilities and improve mitigation techniques, we have extended the 3D electromagnetic simulation code EMSolve [4] to model EMP generation. A specified number and spatial distribution of escaping electrons are used as input to EMSolve simulations. In this paper, we describe our measurement systems and signal processing techniques employed on Titan and NIF. We also give some preliminary results and thoughts on mitigation. Refer to [5] for information on how we model EMP in EMSolve.

Measurement systems
We fielded a wide variety of diagnostics at the Titan short-pulse laser facility in order to understand EMP and measure escaping electrons in such facilities. Our diagnostics include multiple B-and D-dot sensors, electron spectrometers, TLD's (Thermo-Luminescent Detectors), UTLD's (Ultra-Thin TLD's), image plates, and a Faraday cup. Our B-and D-dot measurement system is well shielded, consisting of differential sensors for common-mode noise rejection; shielded conduit to attenuate fields impinging on baluns and measurement cables; and a shielded box for the oscilloscopes. We have B-dot sensors both inside and outside the laser chamber and a D-dot sensor inside to measure the levels of electromagnetic fields. It is important to measure both the time-varying magnetic and electric fields because the ratio of those fields inside resonant chambers generally is not the intrinsic impedance of free space (377 Ω). Figure 1 is a photograph of the interior of the Titan chamber. The labeled sensors are a Prodyn RB-270 B-dot sensor (mid frequency); two RB-230 B-dot sensors (high frequency); and a Prodyn RB-50 sensor (low frequency). The remaining interior sensor in the picture, a Prodyn AD-55 D-dot, is located in the upper, right portion of the photograph. Our NIF EMP measurement system is similar. The present configuration in NIF is a temporary, early deployment for initial NIF operation that does not provide the full Faradaycage shielding that was used on Titan. However, measurements with sensors covered with electromagnetic shielding indicate little, if any, corruption due to extraneous EMP noise in the present configuration. A well-shielded measurement system, similar to the Titan system, is being installed for longer-term operation.  CAD image of EMP diagnostic in target chamber.

Signal processing techniques
In order to remove the effects of the measurement system, we perform channel compensation, or deconvolution. We construct calibration files for each component of the measurement system, consisting of sampled transfer functions in the frequency domain. The measurement system transfer function is then simply the product of all of the frequency-domain transfer functions of the individual components. A straight-forward method of performing the channel compensation would be to transform the data from the oscilloscopes into the frequency domain; apply the inverse of the system transfer function; and then transform back into the time domain. However, this method tends to magnify noise in the frequency bands where the transfer function is small, producing unrealistic signals. In order to reduce this noise magnification, we use Tikhonov regularization with L-curve analysis to determine the optimal regularization parameter [6]. Additionally, we low-pass filter the channel-compensated signal at the frequency of the analog low-pass filter in the measurement system.

Preliminary results and thoughts on mitigation
We applied the signal processing techniques above to the measured data from Titan and NIF. First, our preliminary results from short-pulse Titan (∼ ps) indicate large EMP, on the order of tens to hundreds of kV/m, that has a duration (before decaying into the noise) of up to about a few hundred ns. For example see Fig. 3, which is processed data from one of our RB-230 (high-frequency) sensors located about 32 cm from target chamber center (TCC). The resulting magnetic flux density values (B field) have been converted to electric field strength (E field) by multiplying by c, the speed of light in vacuum: E ≈ cB. Note that this conversion is only approximate, since the relationship does not hold in general for resonant cavities. The peak magnitude of the resulting E-field signal is 167 kV/m. The transient duration before decaying into the noise is ∼ 400 ns. A smoothed estimate of the one-sided energy spectral density (ESD) using just the transient portion is displayed in Fig. 4. The ESD indicates significant EMP out to greater than ∼ 5 GHz. We cannot observe much higher than ∼ 5 GHz because of the oscilloscope analog bandwidth (∼ 6 GHz) and a 5 GHz low-pass filter in the measurement system. Further, we have observed a clear trend with target size in the Titan data (Fig. 5). Note that the peak magnitude of the E field tends to increase with the target size, a possible explanation being that a larger target reduces the electrostatic field allowing more electrons to escape. The data in Fig. 5 is for square silver targets 12 µm thick. The laser pulse width is 2 ps with laser energies ranging from 123 to 155 J (∼ 75 TW average power). Our preliminary results from long-pulse (∼ ns) NIF indicate EMP with peak magnitudes up to ∼ 10 kV/m. For example see Figs. 6, 7, and 8, which are processed data from our RB-230 (high-frequency), RB-270 (mid-frequency), and RB-50 (low-frequency) sensors, respectively, located about 4 m from TCC. The shot is N090808 (233 kJ, 2 ns, 116 TW, vacuum hohlraum). Note that if we assume 1/r 2 scaling, the fields could be ∼ 16× higher 1 m from TCC. Again, the measured B-field values have been converted to E field using E ≈ cB. A smoothed estimate of the one-sided ESD using just the transient portion of the signals is displayed in Fig. 9. The ESD is plotted for the different sensors only over the frequency range where they are "valid", which is judged by comparison with the noise floor. The ESD indicates EMP with a maximum at ∼ 1 GHz with significant EMP out to greater than ∼ 5 GHz. As before we cannot observe much higher than ∼ 5 GHz because of the oscilloscope analog bandwidth and a 5 GHz low-pass filter in the measurement system. Also, we measured volt-level noise, likely due to EMP, on terminated signal cables on a NIF diagnostic instrument manipulator (DIM) inside the target chamber. We have not yet measured any EMP above the noise outside of the target chamber.
Based on these preliminary findings, we are reassessing diagnostic shielding inside the target chamber, starting the process of preparation for ARC operation, which we estimate could produce roughly an order of magnitude higher fields. The present focus of mitigation efforts is on placing high-frequency, Faraday-cage shielding around cables and diagnostics inside the target chamber. The amount of shielding required will vary depending on the incident EMP, coupling parameters, noise level requirements, and the susceptibility of the diagnostic's electronics. Accordingly, it is difficult to make a general statement about the level of EMP fields that would affect the diagnostics. However, in view of our current measured data we estimate that for normal NIF operation at least 60 dB of shielding is required for the DIM signal cabling, and flexible shielded conduits and other shielding methods are being evaluated for this purpose. Similarly, we are providing Faraday-cage shielding for airboxes and diagnostics mounted on the DIMs. Additionally, fingerstock contacts on the DIMs are being installed. Ferrites for noise current suppression and electromagnetic interference windows for shielding apertures are also being evaluated.

Summary and conclusions
We have deployed measurement systems on both Titan and NIF. Our preliminary data from Titan indicate high-level (on the order of tens to hundreds of kV/m), broad-spectrum EMP. We have also observed a clear trend in EMP level with target size. Our preliminary data from long-pulse NIF indicates EMP up to ∼ 10 kV/m, which we estimate to be higher (∼ 100 kV/m) 1 m from TCC. We postulate that these fields are generating the volt-level signals that we are observing on the diagnostic cables. Based on these preliminary findings, shielding for target chamber diagnostics is being reassessed, starting the process of preparation for ARC operation.