Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1Development of EMP Shield for Vent Using Additive
Manufacturing
AKIO IKEDA*1

TAKASHI IKEDA*2

YOSHIAKI ARAKAWA*3

HISASHI SHIMIZU*1

SHUJI TANIGAWA*4

MASASHI KITAMURA*4

Intense electro-magnetic waves generated by nuclear explosions at high altitudes orgenerated as intentional electro-magnetic interference using electro-magnetic pulse generators or
the like are called EMPs (electro-magnetic pulses). EMPs have a great impact, causing damage toor destruction of electronic equipment and extensively paralyzing society. In today's highly
electronics-oriented society, countermeasures against EMPs are an urgent issue. Generally, thecountermeasures include shielding the electro-magnetic waves that propagate through space (e.g.,blocking with metal, etc.) and absorbing the overvoltage and overcurrent induced in cables byEMPs with protection circuits. However, vents and other openings cannot be blocked because of
their function. Mitsubishi Heavy Industries, Ltd. has developed, created, and fabricated a structurethat can provide vents with countermeasures against EMPs while minimizing the reduction in their
cooling (pressure loss) performance that should be inherent, utilizing detailed simulation, additive
manufacturing, and other technologies. Then, we have confirmed that this structure can actuallyprovide both electro-magnetic shielding and pressure loss performance.

|1. Introduction
If irradiated by EMPs, electronic equipment may malfunction or be damaged or destroyed.
EMPs can have long-term effects on critical infrastructure, etc., and can cause large-scaledisruption to the economy and daily life. In recent years, the U.S. and other countries have become
increasingly active in taking countermeasures against EMPs. Countermeasures against EMPs are anurgent issue for the information society, and we cannot be allowed to fall behind. This reportpresents a case study of the examination and evaluation of a countermeasure structure assuming
application to vents, which needs to be established while ensuring their performance as an original
cooling function, among the countermeasures against EMPs.

|2. Target EMPHigh altitude electro-magnetic pulses (HEMPs) generated by nuclear explosions at highaltitudes are classified into three pulses, E1, E2, and E3, depending on their occurrence conditions.
The E1 pulse has a electric field strength of 50 kV/m and a frequency of several hundred kHz to300 MHz, which is higher intensity and higher frequency than that of lightning (1). Intentional
electro-magnetic interference (IEMI) caused by high-intensity pulse generators, microwaves, etc.,
has a higher frequency than HEMP, and the upper frequency limit is often set at 10 GHz (2). Since
general countermeasures against electro-magnetic waves are insufficient for protection against
HEMP E1 pulses and IEMI, we used them as targets of the countermeasures against EMPs.
Figure 1 shows the relationship between electric field strength and frequency of HEMP and IEMI
as defined by IEC, etc. The required countermeasure performance (electro-magnetic shieldingperformance) is 80 dB (1/10,000 attenuation) at 1 GHz (1). There is a difference between HEMP andIEMI. HEMP is a single pulse, whereas IEMI is a burst wave in which the pulse is applied
repeatedly.
*1Electron･Physics Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*2

Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

*3

Decarbonization Business Department, Mitsubishi Heavy Industries Engineering, Ltd.

*4

Manufacturing Technology Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
2Figure 1 Frequency and electric field spectral intensity of EMP (2)|3. Concept of EMP shield for vent
In general, as a countermeasure against electro-magnetic waves for vents and the like,honeycomb shields are used. A honeycomb shield consists of thin metal plates laid in a honeycomb
shape. The smaller the honeycomb diameter and the longer the honeycomb shield in the depth
direction, the higher the electro-magnetic shielding performance. To provide electro-magnetic
shielding performance, the honeycomb diameter needs to be less than 1/2 of the wavelength.
Countermeasures against EMPs for vents require high shielding performance against frequencies
above GHz. The insertion of honeycomb shields for satisfying the requirements significantly
increases the pressure loss and makes cooling performance unsatisfactory, which is an issue.
Honeycomb cells have a shape that maximizes the opening area in plain view. In order tosolve the above issue, however, it is necessary to create a structure that can reduce pressure loss
compared to honeycomb shields. As shown in Figure 2, by making the honeycomb cell tapered, itis possible to lay more cells in terms of the projected area. For this reason, we decided to use
tapered honeycomb cells as a concept for the EMP shield structure.

Figure 2

Tapered honeycomb cell

|4. Shape examination of EMP shield for ventsThe EMP shield for vents was constructed as an assembly of tapered honeycomb cells as
described in chapter 3. To optimize the pressure loss and electro-magnetic shielding performance,
we repeated simulations of pressure loss and electro-magnetic shielding performance, andexamined a shape that satisfies both performance requirements.
The electro-magnetic shielding performance is affected by the honeycomb diameter, length,
and taper ratio (i.e., ratio of inlet and outlet cell diameters) of the tapered honeycomb cell. In orderto determine a honeycomb cell shape that satisfies the shielding performance required as aMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
3

countermeasure against EMPs, we performed a 3D electro-magnetic field simulation of the taperedhoneycomb cells with different inlet and outlet diameters shown in Figure 3 using thefinite-difference time-domain (FDTD) method to evaluate the effect of the taper on theelectro-magnetic shielding performance. Table 1 shows the evaluation result. The electro-magnetic
shielding performance of the honeycomb cells with taper ratios of 1.5 and 2.0 was lower than thatof the reference honeycomb cell without taper, and the ratio of the decrease in electro-magnetic
shielding performance was in general agreement with (d) intermediate cell size ratio with reference
to the intermediate cell size of the reference honeycomb cell without taper. To optimize the shape,
it is necessary to change the inlet and outlet dimensions and taper ratios of the honeycomb cell inmany ways, but this result shows that by simulation only the reference honeycomb cell without
taper, it is possible to obtain the ranges of electro-magnetic shielding performance for different
taper ratios and to efficiently examine the shape for its optimization.
The pressure loss performance is affected by factors including the shape of individual taperedhoneycomb cells, the degree of aggregation (number of cells), dead space where air cannot flow,
and distortion of air flow inside the structure.

Figure 3

Models to evaluate effect of tapered honeycomb cell on electro-magnetic shielding

Table 1

Evaluation result for effect of tapered honeycomb cell onelectro-magnetic shielding

Simulation condition(1) Reference
honeycomb cell(without taper)
1.00
1.00

(2) Taper
ratio = 1.50(3) Taperratio = 2.00(a) Inlet cell size
1.00(b) Outlet cell size
1.50(c) Intermediate cell size
1.00
1.25
(Average of inlet and outlet cell sizes)(d) Intermediate cell size ratio(Ratio of intermediate cell size with
1.00
0.80
reference to (1))
Electro-magnetic shielding performance [-]
1.00
0.86
@10 GHz* All values in this table are relative to (1) reference honeycomb cell (without taper).

1.00
2.00
1.50
0.67
0.79

Figure 4 shows shape plans. When aggregated simply, tapered honeycomb cells form ahemispherical dome shape as shown in Figure 4(a). In this case, a large dead space is formed at theedge of the dome and the air flow distortion increases more toward the edge. In order to reduce thedead space and air flow distortion and to equalize the flow velocity distribution, we created anelliptical dome shape by aggregating honeycomb cells with different honeycomb diameters,
lengths, and taper ratios at different locations, as shown in Figure 4(b), and added a rectifying
structure for the dead space to further reduce the pressure loss.
We studied the parameters of pressure loss for different shapes of tapered honeycomb cells
and determined the final shape that minimized the pressure loss and provided satisfactory
electro-magnetic shielding performance. Figure 5 shows the final shape. The final shape has astructure in which elliptical honeycomb cell domes are aggregated, but there remain areas thatcannot be dome-shaped between the domes. Therefore, honeycomb-shaped holes were made inthese areas in order to allow air to flow through.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
4Figure 4

Shape plans of EMP shield for vents

Figure 5

Final shape of EMP shield for vents|5. Fabrication and performance evaluation of EMP shield for vents
5.1

Fabrication of test piece

Because the shape determined in chapter 4 is very complex and cannot be fabricated bygeneral machining, we employed the additive manufacturing method. This structure has thin-walledhoneycomb cells, the overall manufactured size is large, and the dome shape has a large overhang,so it is necessary to use support materials during the manufacturing process. However, thehoneycomb structure may be damaged in removal of the support materials due to the honeycomb
cells being thin-walled, so the difficulty of fabrication is high. Therefore, we evaluated several
methods to remove the support materials without damaging the honeycomb shape. Table 2 showsthe evaluation result. It was confirmed that not only support materials but also burrs could beremoved without damaging the honeycomb structure by using Skill Touch (soft grinding wheel) toroughly remove the support materials and then performing shot blasting.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
5Table 2

Evaluation result of methods to remove support materials from dome honeycomb
structure

Using the above method, test pieces for performance evaluation were fabricated. Figure 6shows the fabricated test piece.

Figure 6 Metal test piece fabricated by additive
manufacturing

5.2

Evaluation of pressure loss

We evaluated the flow velocity distribution in the final-shape honeycomb dome by CFD(Computational Fluid Dynamics) simulation. Figure 7 shows the evaluation result. It is indicatedthat the elliptical dome shape and rectifying structure resulted in equalization of the flow velocity
distribution and reduction of the pressure loss.

Figure 7

CFD simulation result of final shapeMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
6

We evaluated the pressure loss in the fabricated test piece while varying the air volume, test
piece installation angle, and temperature. Figure 8 shows the test setup, which allowed thisevaluation to be made by measuring the wall pressure in the static pressure recovery space at theoutlet and eliminated the need to adjust the instrumentation according to the installation angle ofthe test piece during the test. Figure 9 shows the evaluation result of the pressure loss performanceobtained through the simulation and test. It was confirmed that the pressure loss of the ellipticaldome honeycomb was significantly reduced compared to the normal honeycomb shield which hasdimensions with the same electro-magnetic shielding performance as the test piece, and that thesimulation and test result were in good agreement.

Figure 8

Pressure loss evaluation test setup

Figure 9 Evaluation result of pressure loss performance of
final shape

5.3

Evaluation of electro-magnetic shield performance

The evaluation of electro-magnetic shield performance was conducted using a setup inaccordance with IEEE-STD-299, a representative method to evaluate electro-magnetic shieldMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
7

performance, as shown in Figure 10. Radio waves were emitted from the transmitting antenna
located in the transmitting room with and without the test piece installed, and power received withthe antenna located in the receiving room was measured. The electro-magnetic shielding
performance was calculated as the ratio of the received power values with and without the testpiece. In addition, as in the evaluation of tapered honeycomb cells, the same setup as the test wasmodeled as shown in Figure 11, and a three-dimensional electro-magnetic field simulation using
the FDTD method was performed to evaluate the electro-magnetic shielding performance. As in thecase of the evaluation by the test, the electro-magnetic shielding performance was calculated as theratio of the electric fields that passed through the dome honeycomb with and without the test piecein the evaluation in the simulation. Figure 12 shows the results of the test and simulation. The twowere in good agreement, and it was confirmed that the target performance of 80 dB@10 GHz wasachieved.

Figure 10

Test setup for evaluation of electro-magnetic shield performance

Figure 11

Setup for evaluation by electro-magnetic field simulation

Figure 12 Evaluation result of electro-magnetic
shield performanceMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
8|6. Conclusion
In recent years, as equipment has become more electronic and information-oriented,
countermeasures against EMPs have become an issue. We sought a new shape for the shieldstructure for countermeasures against EMPs to be applied to vents, which need to be open due totheir function and have a small design margin for cooling performance due to the restrictions, that
surpasses the existing honeycomb shield shape. We have created an optimum shape that satisfies
both pressure loss and electro-magnetic shielding performances by repeating trade-offs between thetwo performances using CFD and FDTD methods based on tapered honeycomb cells. Since thecreated dome honeycomb shape is complicated and the honeycomb cells are thin-walled, it isdifficult to manufacture it by ordinary machining as well as casting, so we adopted additive
manufacturing. Since the shape of the dome is large and has a large overhang, its additive
manufacturing required support materials, which were difficult to remove. We devised theirremoval method. By conducting an actual evaluation using the fabricated test pieces, we confirmedthat the simulation and test results were in agreement and that the target electro-magnetic shielding
performance and pressure loss performance were satisfied.

References(1)(2)

IEC 61000-2-11 Electromagnetic compatibility (EMC) -Part2-11: Environment-Classification of HEMP
environments
IEC 61000-2-13 : Environment-High-power electromagnetic (HPEM) environments-Radiated andconducted