Development of compact seed source with high-precision discharge switch for magnetic flux compression generator

1 Introduction

High-voltage fast rising pulse is necessary in the area of food sterilization, X-rays for medical use, gas laser for plasma technology, and plasma treatment surface techniques and so on [1]. Explosive magnetic flux compression generator (FCG) is a kind of single-use pulse power source. It is characterized in compact structure and small bulk which is mobile and low-cost. Meanwhile it has high specific energy storage. It was shown that FCGs can produce pulsed currents with amplitudes of in a very short period of time (microsecond level) [2]. So, the FCGs are often used as energy conversion device of high power electromagnetic pulse weapons. Driving FCGs requires to feed-in energy by applying a high seed current usually in the kiloampere range (10³ ∼ 10⁴ A) for midsized helical FCGs through the direct or indirect way. Thus the initial magnetic flux is established between the stator coil and armature [3]. This initial current is supplied by a high-current seed source that is capable of driving an inductive load. Under the condition of special applications such as missile, geometry size of electrical source, energy storage density, output efficiency and other stringent requirements are put forward. The stringent requirements for the high-current seed source such as geometry size, energy storage density and output efficiency are put forward when the high-current seed source is applied in special occasions especially the application of the ammunition. M. Elsayed from Texas Tech University has designed a self-contained (battery powered with full charge time less than 40 s), single-use compact seed source (CSS, No. 1 in Table 1) using solid-state components. The CSS developed is a system 𝛷152 × 305 mm³ volume and weighing 3.9 kg capable of delivering over 360 J (≮12 kA) into a load (the load inductor measured and had a resistance of at 10 kHz) with a trigger energy of microjoules at the TTL triggering level. The deliverable-energy-per-weight ratio is 92 J/kg and the deliverable energy density of the unit as a whole is 60 mJ/cm³. The upgraded system integrates the blasting power of main charge of FCG, and it shortens the time necessary to charge to 250 ms or below. Morihiko Sato from Gunma University has developed a high-frequency pulsed power generation system using twenty MOSFETs in series. The repetitive operation was achieved at an output voltage of 10 kV and a frequency of 1 kHz by using twenty MOSFETs [4]. Sun Qizhi from Chinese Academy of Engineering Physics has designed a compact seed source (No. 2 in Table 1) using hollow cylindrical capacitor with a rated working voltage of 5 kV, a rated capacity of 200 μF. The discharge switching scheme was not described in detail. The total energy storage is 2.5 kJ, which can provide electromagnetic energy greater than 1 kJ for the 04 series explosive magnetic compression generator [5]. Yu. Tkach et al. from Ukrainian Electromagnetic Research Center has also designed a primary compactseed source (No. 3 in Table 1) for explosion magnetic compression generator. The rated working voltage was 30 kV and the rated capacity was 0.06 μF. The charging time was not reported. It contained a coaxial-type explosive switch discharge, and could apply a seed current of 250 A to an inductive load of 383 mH, and the energy was 14 J [6]. But we have found no experimental data for dynamic matching between the CSS with FCG in the published literatures [7–10]. This paper will discuss a kind of high-precision CSS (No. 4 in Table 1) used explosive-driven flyer discharge switch, and analyze the dynamic coupling performance between the CSS and FCG. Table 1 shows the characteristics between different compact seed sources.

2 The component and working principle of primary compact seed source

The functional block diagram of the CSS design in this paper is shown in Fig. 2. The solid line box in the Fig. 1 represents the main component of the system, including a single battery prime power source, the step-up dc-dc converters, the explosive-driven flyer discharge switch, a pulsed energy-storage capacitor, the exploder and Circuit Control Module(CCM). The dotted box shows post-FCG, the arrowed heavy line represents charging and discharging within the system or detonation energy transfer, the arrowed thin lines represent trigger or control signals. The concept of compact includes not only compressing the device to a certain volume of space according to the reasonable layout, but also includes the reasonable connection between components to reduce the parasitic inductance and resistance with the purpose of improving the efficiency of the output of the electric impulse source.

The working process of the CSS: external signal incentives the CCM for the first time, the CCM powers up and works, to activate the battery and control the low power semiconductor switch integrated in the internal circuit to conduct according to the scheduled program. The pulse energy-storage capacitor is charged by the high voltage step-up dc-dc converters from the battery. The charging process lasts for dozens of seconds time. Then, external signal incentives the CMM for the second time, CCM triggers the explosive-driven flyer discharge switch according to the scheduled program, the switch is closed, the pulse energy-storage capacitor feeds current to FCG through the switch, the initial magnetic flux is set up. After microsecond precision of time delay, the CMM triggers the detonator, the detonation energy transfers to the latter FCG. So far, the work process of the CSS is finished.

The experimental device is processed out after analyzing the composition and the principle of CSS as it’s shown in Fig. 2. All the selection and layout of the components follow the principle of miniature and compact. After the integration of CSS, the volume of the CSS (V _S ) is 𝛷132 × 200 mm³, the quality is 12.5 kg, the working voltage is 5 KV, the theory charging time is t = UC∕I _C = 20 s, and the theory of energy-storage density is D = CU ²∕2 V _S ≈ 274 mJ/cm³.

Lithium batteries or thermal batteries can be used as power source. Lithium batteries have the advantages of high energy-storage density, strong power strength and high voltage platform. Commercial lithium batteries have cost advantages over thermal batteries in a normal application. Thermal battery is a kind of single-reserve power supply which has many advantages: high specific energy and specific power, small volume, quick activation, long storage time, and strong adaption to environment. Besides, the appearance of reasonable size of the thermal battery can be designed according to practical usage [11]. The thermal battery was activated electrically in this paper. The output voltage and current were respectively 22.6 ∼ 27.7 V and 3.1 A. The activation time was less than 1.5 s while the working time was more than 30 s. Choose the metallized-film pulse capacitor as energy-storage capacitor. Winding-type metalized film capacitor with high energy density and good reliability is a kind of energy storage device used widely in pulsed power systems. It can work stably in high-voltage circuits because of its breakdown self-healing nature. Winding structure makes its energy storage density higher than that of cascading structure. Extract electrode from metal-coating terminals is very efficient to minimize the internal resistance and inductance of the capacitor and reduce the discharge loss [12,13]. The rated voltage of the capacitor chose in this paper is 5 KV, the nominal capacitance is 60 μF, the dielectric loss angle is less than 5% and the expected lifespan is 8 times at most.

The major method to charge the energy-storage capacitor at present: constant voltage DC charging, LC resonant charging and high frequency switching transformer charging. The method of constant voltage DC charging is unsuitable because of its low efficiency and long charging time. Neither is the method of LC resonant charging, because it’s bulky both in volume and in weight and it doesn’t conform to the requirements of the compact. Compared with the two charging methods, high frequency switching transformer charging has smaller size and higher efficiency, and the characteristics of constant current output generally [14]. This high voltage direct current changer uses LCL-type resonant push-pull main circuit with input voltage 24 ± 1 V (DC), output voltage 5000 ± 10 V (DC, positive pole, load regulation ≤0.5%) and output current 15 mA (IC). The isolation between high voltage output and low-voltage components by structure designing and insulation materials.

The circuit control module (CCM) is integrated with the signal reception and processing circuit, low-power semiconductor charging switches and double-way delay trigger circuit. The CSS requires higher accuracy to the double-way delay trigger circuit. And external factors (such as temperature and voltage) will cause interference on the oscillation frequency of the oscillator circuit. It’s imperative to design a kind of compensation algorithm to compensate for the frequency deviation caused by temperature and voltage during the design process. The precision of delay time of the circuit can reach 0.1% for any temperature within the scope of −40° ∼ 60° with the constantly changing of power supply voltage. The explosive-driven flyer discharge switch has been described in detail vide supra.

The initiators of FCG and is similar to ammunition fuze equipped safety and arming device. Choose electric detonator with high accuracy of microsecond as ignition detonator. Explosive train includes detonating explosive, when calculating the time t _M should be taken into consideration.

3 The model of the best output time sequence

The CSS outputs the dual channel energy, electrical energy and detonation energy were transferred respectively to the post-FCG during the work process. The sequence of the precision of dual channel energy output is the primary factor to guarantee the working performance of the FCG. The best principle for time matching is: while the seed current fed in FCG from the CSS reaches the maximum value (that is, the initial magnetic flux reaches the maximum value), the armature in the FCG expands to the position of crowbar ring (means the crowbar ring closing), the best output sequence model is shown in Fig. 3.

In the figure, a is the second external excitation signal for the CCM, b is the pulse signal of the CCM to trigger the discharge switch, c is the current signal fed in the FCG from the CSS, d is the pulse signal of the CCM to trigger the initiators in the FCG, e is the close signal of pry break switch in the FCG; T ₀ is the moment of external signal incentiving; T ₁ is the initiation moment of the explosive-driven flyer discharge switch detonation, T ₂ is the starting moment of the CSS outputting the current, T ₃ is the triggering moment of the initiators in the FCG, T ₄ is the moment of the seed current reaching to peak value and crowbar ring closing in the FCG. the response time of the discharge switch closure: t _S = T ₂ − T ₁, the time from the triggering moment for initiators in the FCG to the moment for the crowbar ring closure: t _M = T ₄ − T ₃; t _S is determined by the closure performance of the discharge switch, t _M is determined by the detonating sequence and the expansion performance of the armature in the FCG, then introducing the characteristic time for the discharge circuit of the pulse energy-storage capacitor τ _C = 4(T ₄ − T ₂),The best trigger delay for dual channel of the CSS is: (1)

5 Experimental results and discussions

A schematic diagram (ignoring the circuit of capacitor charging) of the experimental setup that was used to investigate the closure and discharging performance of the switch when it was placed in high-voltage circuit is shown in Fig. 7(a). The energy-storage capacitor withstand voltage is about 6 kV/60 s (DC), and the rated capacity value is 47 μF. The inductance load coil was made as coaxial turns (about 28 μH, 60 mΩ).

The loop current was measured with a Rogowski Coil (210 A/V). The signals from the Rogowski Coil and the CSS were recorded with Agilent Oscilloscope (bandwidth of 500 MHz/2 GSa/s). When the CSS sent out the initiation signal to the switch, the signal was sent to the oscilloscope as synchronous trigger signal at the same time. Figure 7(b) was the current flow waveform in the switch performance test which was placed in high-voltage circuit. The step-function signal was the initiation signal of the switch in the figure, the periodic damped oscillator signal was the through current waveform of the switch.

Table 2 shows the discharge performance data of six switches driven by different voltages. The electric loss of the switch mainly concentrates in the closure moments. The charge-transfer rate k is lead up in this paper to represent the through-current capability of the switches (the other parts of the circuit remain the same when replacing the switch): (2) Q ^′ is the quantity of electric charge the switch transfers in τ _C ∕4 time, Q ₀ is the initial quantity of electric charge the capacitor storages, U is the charging voltage of the capacitor, C is the capacitance value of the rated capacity. In the table, R is the resistance of the electric detonator in the switch. I _max is the peak value of the current, t _L is the time of duration for the switch conduction.

There were convex surfaces on the front and reverse side of the switch after the closure, but the shape of the switch keeps intact, so it will not bring about any effect or change to other devices in the CSS during the time of the switching working. The switch had passed the high voltage breakdown and withstand test, and the withstand voltage of the switch exceeded 6 KV.

The distribution of closure response time averaged over six experiments of this series of tests was τ _aver = 49 μs, and the standard deviation was 1.67 μs. Completions of these experiments confirm the switch meet requirements and achieve the aim of controlling the time-sequence, precision and charge circuit. The current flowed stability without any jitter the instant the switch closed by the recording waveform. The full current through-flow waveforms in Case 1, 3, and 6 indicated the switches had closed completely. The switch-on times in Case 2, 4, and 6 were greater than 250 μs, and exceeded the value of τ _C ∕4.The switch-on time was enough to ensure the current rise to the peak. The charge-transfer rates reached a stable level (about 85%) over six experiments, no any obvious departures. That confirm the switch-on processes were consistent, the values of resistance, and inductance kept constant, and current could flow stability. All of the analysis above show that the design of explosion-driven-flyer discharge switch met requirements for the CSS system.

The experiment was repeated thrice to test the dynamic coupling performance of the CSS and the FCG. The FCG is a kind of experimental device which inductance of the output load distributes in nH level. The waveform of the current supplied from CSS to FCG at static condition is shown in Fig. 8(a), and the dynamic current waveform of the load after the system initiating is shown in Fig. 8(b).

The parameters and the results over three experiments are listed in Table 3. U _a was the measured working voltage of the CSS, C _a was the capacitor’s capacity and t _c was the charging time. L _FCG and R _FCG were the measured resistance and inductance (10 KHz) of the FCG. I _a was the practical coupling current when the crowbar ring closed at dynamic condition, t _a was the interval time through the current feeding to the FCG. I max ′ was the maximum current after amplification. The effective energy-storage density and energy output efficiency were: (3)

The results show that the measured working voltage and rated value less than the difference, the same as the capacitor’s capacity. So the CSS has a constant initial energy storage (≈750 J). But the measured charging time exceeded 30 s, more than the theoretical value of 4 ∼ 9 s. The main reason was that the output power of the thermal battery became lower along with the development of the chemical reactions of its internal medium, which in turn caused the decrease of charging power of the pulse energy-storage capacitor. The waveform of the current fed from the CSS to the FCG (about 42 μH, 78 mΩ) at static condition was stable. The current rise time was approximately 44.8 μs, the peak current exceeded 5.4 kA and the output energy was 600 J at least., The coupling times was “lag of 1.8 μs, ahead of 3.8 μs, lag of 4.2 μs” compared with the time of peak current when the CSS fed to FCG at dynamic condition over three experiments. The error of sequential control was not more than 5 μs. The coupling current differed with peak current 70 A, 220 A, 250 A accordingly. The coupling current was not less than 5.2 kA and the coupling energy was not lower than 550 J. The sequential control error was mainly caused by dispersing of function time of the discharge switch and the initiator. The peak current I _max was used in calculating the value of D _e and 𝜂 in literature [4]. While the formula (3) used current I instead after considering timing error correction. This gave the CSS a deliverable energy density of the unit as a whole of D _e ≮ 200 mJ/cm³ and the energy transfer efficiency was 𝜂 ≮ 75%. The current increased rapidly after the amplification of the FCG, the peak current reached about 20 KA. At the same time the CSS performed its work. The magnification was associated with the performance of FCG. The experimental results at dynamic condition show excellent coupling performance of the CSS and FCG. The CSS is capable of supplying the FCG effectively.

Primary electrical pulse source is the first part of magnetic flux compression generator systematic chains. The design of CSS in this paper is suitable for power supply applications in the limited volume and out of the ground. On basis of the analysis of the system composition, working principle and the best output sequence model, the test of coupling performance with FCG at dynamic condition has been carried out. The system has smaller overall dimensions, higher output energy and effective energy-storage density compared with the design by M. Elsayed from Texas Tech university but the charging time is significantly longer so it applies to the application which is undemanding for time such as gun-launched non-nuclear electron-magnetic pulse bomb. It has reduced the sequential control error and increased the coupling current to satisfy the requirements of the system with the use of explosive-driven flyer discharge switch. It is of important significance for FCG engineering use.