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While defense has historically been the primary driver of pulsed power system development (see our previous blogs: 5 Things to Know About Pulse Power and More Pulse Power History), a broad range of civilian applications now exist for fast high-voltage pulses. In this article, we present several types of high-voltage switching technology used in pulsed power, along with some examples of how these devices could find use in both defense and in civilian industry, bringing a larger market to help accelerate the pace of innovation.

Interestingly, the traditional model of defense-spending and government-funded research driving innovation is changing with private sector investment in cutting-edge technologies, such as fusion energy, and beginning to outpace those being made by the public-sector. These developments could not only benefit the companies involved but may also prove advantageous for defense.

A Pulsed Power-Adjacent Example

An example of this trend can be found in high-temperature superconductors (HTS), a technology adjacent to pulsed power. Along with private investment driving the market demand for components related to fusion energy, ARPA-E has funded the University of Houston to achieve a 30-fold reduction in the cost of high-temperature superconductor tape; a critical technology used in the magnets that confine plasma in fusion reactors [1]. Although small fusion energy sources could enable deployable defense applications, this cost reduction will also drive advancements in civilian technologies such as medical imaging, transportation systems, and, most notably, efficient power transmission for electrical utilities.

Additionally, HTS tape manufacturers, like Faraday Factory and Fujikara in Japan, are ramping up production to meet the increasing demand for fusion energy even without the support of government-funded R&D [2]. As the supply increases, we would expect prices to fall, and if the demand dramatically outpaces that supply, the research funded by ARPA-E would ideally help to keep the cost in a range that makes fusion a viable technology and industry.

The Problem: Availability of fast, solid-state high-voltage switches

Although HTS tape is not typically used in pulsed power generators, this example illustrates the potential impact of similar investments in technologies like high-voltage/high-current silicon carbide (SiC) switches. APELC has worked with a wide range of customers over the past 27 years, several of whom have expressed the need to incentivize semiconductor fabs to produce SiC MOSFETs and silicon opening switches used in counter-UAS/UAV applications by finding a larger market for these unique devices in the civilian realm. An additional challenge for the Department of Defense (DoD) in securing these materials from trusted sources for critical defense systems lies in the fact that manufacturing of many semiconductor devices and materials has shifted abroad- notably to Taiwan [3]. In fact, Silicon Carbide is listed as a “Material of Interest” by the Defense Logistics Agency [4], and the Defense Microelectronics Activity (DMEA) has its own facilities for producing SiC components critical to national defense when industry cannot supply them [6].

Despite early successes by Cree Semiconductor, which began developing SiC devices at North Carolina State University in 1987, there remains a need for higher voltages, currents, and faster closure times. The U.S. Air Force, Navy, DARPA, and other agencies have continued to fund SiC development, leading to significant advancements in SiC MOSFETs. These devices have enabled pulsed power systems that were not feasible 10 to 20 years ago [5], but as we will discuss below, significant challenges remain.

Meeting the Needs for Counter-UAS Systems

Solid-state technology is not yet capable of fully replacing conventional spark-gap pulsed power systems that generate the hundreds of kilovolt, nanosecond-level pulses required for high-power microwave (HPM) systems used in counter-UAS applications. For instance, Wolfspeed’s SiC MOSFETs can handle up to 1,200V and several hundred amps with rise times under 100ns. To achieve tens of kilovolts, multiple devices must be combined in series, which increases equivalent series inductance and further slows the rise time. That said, recent developments in low-inductance topologies and faster MOSFET devices, such as the impedance-matched Marx generator, have demonstrated pulse voltages on the order of 20kV with risetimes of <8ns [7]. The researchers in the article cited here suggest that new switching technologies such as gate-boosted SiC MOSFETs or gallium-nitride high-electron-mobility transistors (GaN HEMTs) may aid in decreasing the rise-time even further.

 

Figure 1 AFRL’s THOR HPM system for counter-UAS

One potential solution for creating a pulsed-power driver capable of hundreds of kV and nanosecond rise-times would be a direct replacement of the spark gap switches used in conventional Marx generators with a solid-state device. To maintain a compact footprint and low-impedance, the limit on the number of stages would dictate a charge voltage/stage voltage in the tens of kilovolts (see our blog entry 5 Things to Know About APELC Marx Generators to learn more about why this is the case). As mentioned above, the maximum voltage of individual solid-state MOSFET switches is <2kV and would yield a pulser roughly as large as APELC’s MG15-3C-940PF 300kV Marx generator to achieve 20kV. This is not mentioned to speak ill of the incredible research being undertaken by those researchers, but to demonstrate the limitations in the present state-of-the-art.

Ongoing research is attempting to produce switches with the desired capabilities. In this article, we focus on two such technologies: photoconductive semiconductor switches (PCSS) and drift-step recovery diodes (DSRD). Both can switch up to 20kV in a compact footprint but still have challenges to overcome.

The Technology

Photoconductive semiconductor switches (PCSS)

PCSS switches use the photoconductive properties of wide-bandgap semiconductors to eliminate the control junction and provide faster closure times [8]. While well-known wide bandgap materials such as SiC, Gallium-Arsenide (GaAs) and Gallium Nitride (GaN) are being used in these devices, researchers at Lawrence Livermore National Laboratory (LLNL) have taken advantage of the ideal optical, electronic, and thermal properties of diamond to create a PCSS capable of both high-voltages and currents by conducting through the bulk of the material as opposed to the individual filaments common in many PCSS devices. These discrete filaments limit both the current density and lifetime of PCSS switches [9]. Other researchers, such as those at Texas Tech University have investigated the additional issue of packaging PCSS devices for use up to 100kV [10].

 

Figure 2 LLNL’s diamond PCSS

PCSS devices are still limited by a number of issues, although many of these are being actively addressed and, in some cases, overcome. One factor in the utilization of PCSS switches for deployable applications, such as counter-UAS systems, is the footprint of the laser required for switching. In many cases, a laser with pulse energies in the micro-joules is required [11], and while this may sound relatively low, the Q-switched Nd:Yag  lasers that are commonly used are often the driving factor in the total prime-power and physical footprint of the system. That said, researchers are investigating means of utilizing compact laser-diodes to achieve fast, high-voltage/current switching of semiconductor devices [12].

Drift-step-recovery diodes (DSRDs)

Drift-step recovery diodes (DSRDs) are silicon-based devices that operate like conventional diodes, conducting in the forward direction and blocking current in reverse. However, DSRDs have the unique ability to block high voltages and generate fast-rising pulses when switched rapidly from forward to reverse bias in hundreds of nanoseconds. Unlike MOSFETs or PCSS, which act as closing switches, DSRDs are opening switches that rapidly release the energy stored in an inductor’s magnetic field when the switch opens [13].

When combined with pulse-sharpening circuits, such as silicon avalanche diodes (SAS) or non-linear transmission lines, DSRD-based circuits can achieve peak voltages in the tens of kilovolts with sub-nanosecond rise times [14]. Additionally, DSRDs have the advantage of not requiring lasers for activation, which makes them suitable for compact and deployable systems.

Despite their potential, DSRDs still face obstacles before they can be widely adopted for both defense and civilian use. These include the limited availability of domestic fabs and foundries to produce DSRDs and the losses associated with resistance when stacking the diodes in series for higher voltages. Currently, most DSRDs available in the U.S. were originally sourced from Russia or produced in small batches domestically.

The technology discussed here demonstrates that solid-state pulsed power devices for long-term, low-maintenance use are on the verge of becoming widely available. Similar to the investments made in high-temperature superconductors for fusion energy, financial support for these devices could accelerate their development, reduce costs, and increase availability. Although there is certainly a concern about making a technology widely available that has a defense-related end-use, variations in these devices could potentially maintain their usefulness in the civilian realm, while handicapping them for use in defense systems. This is important to safeguard national security, limit potential restrictions placed on their sale and export, and to ensure a strong presence in the commercial market. One potential example of this would be the elimination of pulse-sharpening for a DSRD so that it is no longer capable of producing frequency content pertinent to counter-UAS applications, while still producing the 10-30kV/<10ns rise-time pulses useful in low-temperature plasma applications. The discussion below provides a summary of non-defense applications for pulsed power that have the potential to create a larger market for high-voltage solid-state devices.

 

 

1. Inactivation of biological contaminants

Along with coronal discharges and ultra-violet radiation, non-thermal atmospheric plasmas (NTPs) are demonstrating strong potential and promising results for sterilization and decontamination. By creating chemically reactive species, such as OH radicals, these low-temperature plasmas provide a means of deactivating contaminants on surfaces, as well as in gases and liquids. Moreover, studies are showing that they are often a more energy efficient alternative compared to other methods. This makes them useful tools in medicine, agriculture and the environment [15]. The figure below shows the three main methods of NTP treatment

 

Figure 3. Three methods of NTP treatment. Reproduced from [16]

With the recent concern over per-and polyfluoroalkyl (PFAS) substances’ impacts upon human health, the potential of utilizing pulsed systems for drinking and wastewater treatment is a very timely solution to a critical problem. Pulsed power-based solutions are already being implemented by laboratories and companies, such as the work being done at Clarkson University [22]; however, much of this technology remains dependent on spark-gap based pulsed power and is in urgent need of solid-state devices to ensure longer-term solutions.

2. Cancer treatment

A phenomenon known as electroporation was first described in scientific literature during the 1970s [17] and is a means of opening pores within the walls of a cell by means of applying a pulsed electric field (PEF). Both the destruction of cancer cells via electroporation and the efficient delivery of chemotherapeutic drugs into the cell member have been studied and used in practice for the treatment of certain cancers [18].

3. Wound healing

Utilizing the same principles mentioned in regard to biological decontamination, non-thermal atmospheric plasmas can be used to directly treat wounds by killing bacteria in the wound and accelerating the healing process. While these systems are presently used by the medical community, research has shown that advances in pulsed power switching can increase the efficiency of these systems by creating ideal pulse characteristics for the electrochemical reactions involved [19].

 

Figure 4 Wound healing with NTP (https://www.tierklinik-stuttgart.de/th-en/besondere-leistungen/cold-atmospheric-pressure-plasma-therapy.php)

4. Chemical Production

The generation of fuels and chemicals for renewable energy resources and industrial applications has brought significant attention to the process of electrolysis. Whereas electrolysis is typically thought of as the application of direct current to drive a chemical reaction, research has shown significant increases in efficiency and selectivity by utilizing pulsed voltages [20].

5. Biofuel and Phytochemical Production

Significant research and development over the past two decades has shown the benefit of using pulsed electric fields in the processing of biological feedstocks for fuels and chemicals. Using the same phenomenon of electroporation mentioned in regard to cancer treatment, PEFs can increase the permeability of cell membranes in biomass for the extraction of both starches/sugars (for fuel) or useful phytochemicals [21]. Unlike the corn used in ethanol production, which is problematic due to its large agricultural footprint, water usage, and availability as a food crop; lignocellulosic biomass does not readily give up its starches for the production of energy. Bioreactors utilizing PEFs make this possible and can take biomass that was once an agricultural waste product and convert it into a high-value fuel source. With the need for sustainable, clean sources of energy, this technology could provide a very lucrative and large market for pulsed power semiconductors.

6. Increasing the efficiency of combustion engines

Transient Plasma Systems Inc. (TPS) is perhaps the best example of a company who has already seen a commercial opportunity for nanosecond solid-state pulsed power and worked to bring it to market. Stemming from technology developed at the University of Southern California, TPS commercialized a method of using transient plasmas to dramatically increase the efficiency of the ignition process in combustion engines [23].

7. Metal forming and manufacturing

The process of electromagnetic forming uses a pulsed magnetic field to apply forces (the Lorentz force ) to metal tubing or sheet to force it into a desired shape [24]. The French company Bmax provides a fantastic and unique example of this technology being used in industry. In 2017, Bmax received a contract from Dior perfumes to create an intricate metal cap with the look of handmade craftsmanship for one of their perfume bottles.

 

Figure 5 Dior J’adore L’Or perfume bottle with metal collar manufactured by Bmax’s electroforming process

8. Methane treatment

Waste sludge produced by both humans and agriculture generates incredibly large amounts of methane gas. Not only is methane a powerful greenhouse gas, but if captured and treated, can be utilized as a clean-burning fuel source. Focus Pulsed (FP) pre-treatment of waste sludge demonstrated an increase of 40% in biogas production while decreasing odor and energy usage compared to other methods [26].

9. Ground-penetrating and Ultra-wideband radar

Ground-penetrating radar (GPR) uses scattered electromagnetic waves in the range of 10 MHz to 3 GHz to image subsurface features. This technology has been used for over 30 years in applications ranging from archaeology to road maintenance [27]. While the ultra-wideband (UWB) spectrum generated by GPR is heavily regulated by the FCC, thereby limiting the output power of these systems, advances in solid-state pulsed power have the potential to increase the depth and resolution presently available.

 

Figure 6 Ground penetrating radar example (https://www.nysm.nysed.gov/research-collections/geology/research/ground-penetrating-radar)

10. Drilling and Mining

Companies such as Tetra Corporation of Albuquerque, NM have already demonstrated the successful use of pulsed power to replace conventional drilling methods for the petroleum and geothermal industries. Their research has demonstrated up to a 10X increase in the drilling speed compared to conventional drills by using pulsed power to break hard rock [28].

11. Fusion Energy

Last but not least, fusion energy stands to be one of the largest markets for pulsed power systems. As mentioned in our blog How APELC Supports the Fusion Energy Industry, fusion is the process of atomic nuclei combining under extreme temperatures and pressures and releasing immense amounts of energy. While Department of Energy labs have been working at this on a large scale for decades, the recent announcement by LLNL that the National Ignition Facility (NIF) achieved a gain in energy (Q greater than 1) has spurred massive private and public investment into commercial fusion enterprises [29]. Because these systems must generate a high-energy plasma to feed the fusion process, pulsed power is often used as a driver. Although many methods exist for this, advances in solid state switching are certainly needed to realize a system capable of continuous, low-maintenance operation.

 

Figure 7 National Ignition Facility at LLNL

 

 


 

REFERENCES

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