In the realm of electrical engineering, pulsed power systems have gained prominence for their ability to generate fast-transients at high-voltages and currents. These systems find applications in a wide range of fields, from flash X-rays and lasers to particle accelerators and fusion energy. To ensure the efficient transmission of high-voltage, fast-rising pulses, coaxial cables have become the go-to choice. However, maintaining reliable connectivity and managing high-electric fields in these systems posed a significant challenge. In this blog post, we delve into the development of a groundbreaking solution—the quick disconnect connector for high-voltage coaxial cables. This proprietary innovation emerged from the expertise of Applied Physical Electronics L.C. (APELC) and promises to revolutionize connectivity in pulsed power systems.
THE MARX GENERATOR
The Marx Generator is a circuit created by Erwin Marx in 1924. The idea behind the Marx is simple: Charge up a bank of capacitors in parallel and then rapidly connect them in series, discharging the energy into a load. Each capacitor (a.k.a. “stage”) in the circuit is isolated on both the high and low sides by a charging element (either resistor or inductor) that serves the dual role of defining the charge rate of the capacitor, and isolating each stage so that when switched into a series combination, the RC time constant of the parallel circuit is much greater (>10X) than the time constant of the series discharge circuit. In this way, the voltage on each stage does not have time to fall off before the series circuit is fully formed and the pulse is delivered at the output. An example of a Marx Generator circuit is shown in Figure 1.
Figure 1 Simple 3-Stage Marx Circuit
While Marx generators have existed for a century, they have most commonly existed as extremely large, oil-insulated systems relegated to laboratory environments. Given the sizable dimensions of the Marxes, the associated stray inductance made for slow rise-times and high impedances that limited their usage for lower impedance loads (vacuum diodes, high-power RF, flash x-ray etc…). In order to drive a lower impedance load with a faster rise-time, secondary pulse conditioning such as pulse forming lines (PFLs), pulse forming networks (PFNs), and peaking switches are often required. Whereas PFLs and PFNs help to shape the width of the pulse and provide a known impedance by acting as a transmission line, the peaking switch is used in conjunction with the secondary charge storage (PFL, PFN, or a peaking capacitor) to hold-off the entire erected voltage of the system and discharge the portion of energy associated with the lower inductance secondary circuit before the remainder of the Marx energy adds to the pulse. While this combination provides a fast-rising pulse, with a controlled pulse-width and impedance, the associated hardware adds to the overall footprint of the system, again making for a system that is far from portable.
APELC was founded in 1998, based upon the work Dr. Jon Mayes performed for his dissertation in which compact Marx generators were used to inject fast-rising pulses onto a structure for the generation of microwave energy using photo-conductive switches [1]. Dr. Mayes employed an existing Marx generator design created by Dr. David Platts of Los Alamos National Lab (LANL), in which the stray inductance and capacitance of the Marx is managed through the use of a tightly-coupled coaxial ground-plane. Known as the “Wave-erection Marx Generator” [2], this style of Marx could be made in a cylindrical, pressurized metal housing, and provides a fast rise-time, at a relatively low impedance, without the use of additional pulse-forming. The resulting systems are not only compact but have a coaxial output ideal for driving high-power RF loads. APELC has since matured these generators into turn-key systems for use by the Department of Energy, Department of Defense, and civilian customers for a variety of applications. An example of such a system is the MG15-3C-940PF Marx generator shown in Figure X. The MG15 has a ~50 ohm source impedance, a 3-5 ns rise-time and a 25-30ns pulse-width at 300kV into a matched load, as shown in Figure 2.
Figure 2 APELC MG15-3C-940PF Marx Generator
DEVELOPMENT OF A QUICK-DISCONNECT CONNECTOR FOR HIGH-VOLTAGE COAXIAL CABLE
Given the coaxial nature of APELC’s Marx generators, a connector design was sought to provide a quick-disconnect termination of the output cable from the Marx or load in a manner that maintained the coaxial impedance and managed the high-electric fields which can lead to break-down of the cable insulation. The resultant design was implemented as a system component on APELC Marx systems. Prior to the development of the APELC quick-disconnect connector for high-voltage coaxial cable, many labs (including APELC) often used a simple, but crude plumbing part to terminate the ground braid of the cable onto the pulse generator and load: the hose clamp. The length of the insulated center-conductor is set to a length that can hold-off the voltage either at atmosphere, or in insulating oil, depending on the application. A section of the ground braid is exposed and clamped onto the system ground using the hose clamp. A real-world example of this is shown in Figure 3. While this does provide a simple means of maintaining the coaxial structure and terminating the cable, it can be time-consuming in removal/installation, and can create a significant field-enhancement at the cable-to-ground interface that often leads to break-down of the cable dielectric. As these types of coaxial cable are often used on multi-million dollar systems for defense and energy research, the aforementioned cable failures could cost laboratories significantly in down-time.
Figure 3 A hose-clamp used to terminate the ground-braid on high-voltage coaxial cable
During a meeting at APELC’s Spicewood, TX facility with DOE personnel involving pulsed power for a large accelerator system, APELC’s coaxial QD connector was noticed for its functionality and suggested for use on the system being designed. Eventually, APELC won a contract for supplying connectors for the Scorpius system as part of the Advanced Sources and Detector (ASD) program at the National Nuclear Security Agency (NNSA). Once completed, Scorpius will be a 125-foot-long linear induction accelerator designed to deliver 20 MeV X-rays as an imaging tool for the United States nuclear stockpile [3]. The accelerator is comprised of a large number of induction cells each driven in parallel by multiple pulse generators. The resultant design required thousands of connectors to make the connection between the pulse generators and the accelerator structure. Because of the number of connections and the difficult-to-access nature of the connections, a design was sought that was not only reliable in its design, but provided a means of visually identifying the connection. APELC honed the existing quick-disconnect connector design to create the QDF-RG217 connector shown in Figure 4.
Figure 4 APELC’s QD-RG217 Quick-disconnect connector for high-voltage coaxial cable
The QDF-RG217 utilizes a high-visibility ring to indicate full engagement of the connector. If the connector is not fully inserted and engaged, the red ring is clearly visible from a distance and provides a fail-safe means of ensuring connectivity for the thousands of connections on the DOE’s accelerator system. Moreover, the QDF-RG217 employs canted-coil springs for the contacts on both center conductor and ground. This style of contact ensures for a contact that will remain intact over many connect/disconnect cycles without the risk of metallic particles falling into the connector, which could occur with connectors using finger-stock or threads to engage the outer ground conductor, and potentially lead to electrical break-down at high-voltage. APELC utilizes this same design on a full line of connectors for pulsed voltages as high as 500kV.
APELC’s connectors are not limited to LANL’s application alone. They find relevance in a range of industries, including Particle Accelerator Labs, the Communications Industry, Aerospace, and Defense. Aircraft and aerospace systems can benefit from the high-reliability, easily connected/disconnected contacts offered by APELC QD connectors. These connectors provide convenient service and maintenance options for installations requiring high-voltage and/or high-current coaxial cable connections, further enhancing ease of use and operational efficiency.
To learn more about APELC and its advanced connector solutions, click here.
REFERENCES
- Mayes, Jonathan Robert. The injection wave generator. University of Missouri-Columbia, 1998.
- Platts, M. P. Hockaday, D. Beck, W. Coulter and R. C. Smith, “Compact flash X-ray units,” Digest of Technical Papers. Tenth IEEE International Pulsed Power Conference, Albuquerque, NM, USA, 1995, pp. 892-896 vol.2, doi: 10.1109/PPC.1995.599725.
- Shining a bright light on plutonium, https://str.llnl.gov/2021-04/zika (accessed May 24, 2023).
