How to protect against EMP (ElectroMagnetic Pulse) is a key issue when dealing with HPEM, either generated by high power microwave weapons for defense applications or by intense laser-plasma interaction in the framework of scientific research. The protection can be considered only if EMP is well characterized, in terms of peak electric (E) field strength, spectral content, rise time...
For that purpose, two solutions exist: the “old” D-dot and optical electric field probe from Kapteos.
D-dot:
requires data post-treatment as it exhibits a time-derivative response
presents low E-field strength withstand
has limited high-frequency cut-off
induces interference on the E-field to assess due to its size and metallic composition
On the other hand, the optical electric field probe:
presents E-field strength withstand > 10 MV/m
has ultra small size
exhibits interference-free behavior
The electric field probe environment may be very harsh with UV, X rays and gamma rays, depending on the EMP generation, like in the case of:
generation by intense laser-plasma interaction,
generation by Z-pinch.
Non-Nuclear ElectroMagnetic Pulse (NNEMP)
The comparative measurement between D-dot and optical electric field probe has been carried out in a GTEM cell using a high voltage pulse generator. The two sensors were positioned at locations in the GTEM cell separated from each other by 0m75, Kapteos electric field probe being closer to the GTEM apex. Indeed, position of D-dot is much more restricted due to limitations:
E-field strength must not be too high,
a minimum distance to metallic parts should be maintained to avoid erroneous assessment of the E-field strength.
Kapteos probe curve corresponds to the raw signal recorded by the digital sampling oscilloscope (DSO), while D-dot curve corresponds to the time integration of the signal generated by the D-dot. Kapteos electric field probe gives a sharper signal with faster rise & fall times as its frequency bandwidth is not limited to only a few GHz. Moreover, it is almost transparent to the incident pulse whereas the D-dot generates a strong interference in the pulse wavefront.In case of outdoor giant EMP, distance from sensor to data acquisition shelter could reach one hundred of meters. No pulse attenuation (nor distortion) occurs with optical electric field probes as their insertion loss is < 0.2 dB/km.
An echo, induced by GTEM load impedance mismatch with its wave impedance (50 Ω), is clearly observed on both curves, the D-dot echo arriving ~ 5 ns before the electric field probe echo. This corresponds precisely to the EMP echo time-of-flight difference (2 x 0m75 / speed of light). Other differences can be seen just before and after the echo. Indeed, with Kapteos electric field probe, the noise is time independent as no post-treatment is required. With D-dot, noise increases linearly with time at first order as a time integration is carried out to get the “true” signal. As a consequence, the greater is the delay considered after the pulse rise front, the less accurate is the measurement carried out with a D-dot. This key consequence is generally underestimated, or in the worst case, not taken into account.
EMP generated by intense laser-plasma interaction
EMP generated by interaction of a > 1 PW laser pulse and the laser-induced plasma on a target has been carried out at PALS with ASTERIX laser at Prague (CZ). On the picture the target and the electric field probe with its black shielding sheath against high photon flux (IR, visible, UV, X and γ rays) are seen. The EMP generated by shot 54862 with a 623J energy has been obtained using a PTFE screen between target and probe in order to avoid photo-ionization at the surface of probe sheath. With that screen, the probe senses only the EMP without photo-ionization artifact. The instantaneous bandwidth of measurement setup was ~ 1.8 GHz and the FWHM of first peak is ~ 200 ps. A deeper analysis shows that the spectral content (@ -3dB) of the beginning of EMP burst is between 500 MHz and 2.5 GHz at the minimum, occurring at much higher frequencies than encountered in the framework of NNEMP, thus presenting high EM hazard for electronics.
Key features of the Kapteos optical Electric Field Probe for HPEM characterization
Withstand E field: > 10 MV/m
ultra harsh environment compliant: vacuum, UV, X rays and γ rays
non-perturbative optical technology with low permittivity E-field probe: εr ~ 3.6
ultra high orthogonal E field components rejection ratio: > 50 dB
ultra high spatial resolution: < 1 mm
high measurement reproducibility: 0.15 dB
single probe to cover an ultra wide band: 1 kHz → 65 GHz
ultra compact electric field probe: ∅ = 5.5 mm
Courtesy of CEA DAM, PALS & ENEA (Fabrizio Consoli)
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