I. Introduction
There are several ways to form a Z-pinch in an experiment. In the past years, several Z-pinch configurations have been studied. For example, there are the original compression Z-pinch filled with gas, capillary discharge, the gas-embedded pinch together with micro-discharge, the wire array pinch, the gas-puff pinch, the fiber pinch, multiple shell pinch, the plasma on wire pinch etcetera [1]. In our researches, most of the attention has been paid to studying the wire array pinches; mainly involving single wire array [2-4], nested wire array [5], conical wire array [6], cingulum array [7], quasi-spherical wire array [8-10], for the purpose of producing an ideal x-ray source to heat inertial confinement fusion target. Professor Peng presented a novel energy technology with a local-volume-ignition fusion target, to form a Z-pinch driven fusion-fission hybrid reactor in China [11]. The problems on relatively low-temperature (2–3 keV) volume ignition of DT fuel have been studied, indicating that the burning temperature will decrease as the areal density increases at the point of ignition [12]. Further, simulation results revealed that forming an azimuthally uniform, and radially compact plasma shell at the moment of plasmas colliding on the foam convertor, was a key factor for the local-volume-ignition. A thin cylindrical liner with a perfect shell in the beginning seems a good choice. Liner Z-pinches have an extensive research history. Specifically, a relatively massive hollow cylinder of Al foils of 2 cm in height, 5 cm in radius and 0.25–0.75 μm thick was driven towards the axis on the Pegasus II facility, and emitted soft x-rays of 125 kJ in a 200 ns pulse [13]. Similar liner implosions were also studied by Degnan et al on the Shiva Star facility [14-15]. When the surface material of a conductor melts, and non-uniform heating appears owing to temperature dependence in the conductivity of a material, electrothermal instabilities form immediately, and act as a significant seed to subsequent MRT instability growth [16-17]. Later, research presented a way of mitigating density perturbations arising from an electrothermal instability, by using thick dielectric coatings [18]. In this paper, towards the goal of gaining a load design suitable for the local-volume-ignition fusion target, a novel configuration Z-pinch which seems like a thin cylindrical liner surrounded by a thick dielectric substrate on the outer surface, has been studied on Qiangguang-I facility. In addition, some initial results have been analysed. This type of novel configuration allows to conduct liner-like experiments on relatively small-scale Z-pinch facility, and get a flexible shape of the metallic load, by changing the shape of the substrate tube on the future Z-pinch facilities.
II. Experimental setup
The experiments have been carried out on Qiangguang-I facility, which is able to deliver a current of 1.4–2.1 MA, with 80–100 ns rising time (10%-90%) into a vacuum short-circuit load at Northwest Institute of Nuclear Technology in China [19~20]. The experiments comprise two stages: the preliminary experiment (called PRE-experiment), and the formal experiment. According to the structure of electrodes and experimental experiences, the load is designed as in Fig. 1. In detail, the hollow cylindrical substrate is SiO2. The grade of SiO2 is JGS1, which has a transmittance of over 85% for the electromagnetic wave, with wavelengths from 200 nm to 800 nm [21]. The corresponding metallic layers are grown on the inner surface of SiO2 substrate via magnetron sputtering technique [22-23]. Both 1# coating and 3# coating are sputtered with Cu. They are 200 nm±10% in thickness. 1# coating is utilized to make contact with the anode, and 3# coating is utilized to make contact with the cathode. 2# coating is sputtered with Al, and it acts as the imploding load. To gain a suitable thickness of 2# coating which creates a suitable imploding mass, different thicknesses of 2# coating which are listed in Table 1 are explored in these experiments. The ideal design is that the current passes through 1#, 2# and 3# coatings.
| Type No. | Thickness(nm) | Line mass(μg/cm) | Shot No. |
|---|---|---|---|
| 1 | 235 | 200 | 18187,18198,18199,18200,18204 |
| 2 | 470 | 400 | 18188,18192,18193,18194,18195 |
| 3 | 705 | 600 | 18189,18190,18191, |
To improve the thickness uniformity, and reduce surface roughness of 2# coating, the cylindrical substrate is divided into four quarters, or two halves before the magnetron sputtering process. In addition, all surfaces of the segments are polished. The surface which is sputtered as 2# coating is called 2 # surface. It is the same rule for the 1# and 3# surfaces. Figure 2 illustrates an example of polished surface characterization data from one of 2# surfaces. After being polished, 1#, 2# and 3# surfaces have an RMS roughness of 1–1.3 nm. Finally, the corresponding metallic layers are sputtered on 1#, 2# and 3# surfaces. 1#, 2# and 3# coatings are continuous for the electrical conduction at the end of the magnetron sputtering process.
Table 1 Different thicknesses of 2# coating of different loads.
An experimental assembly of load is illustrated in Fig. 3. In the PRE-experiment, to diagnose the current distribution, and gain a specific experiment condition of the formal experiment, the main diagnostics include radial ultraviolet (UV) frame cameras and magnetic mini-probes. In the formal experiment, for the purpose of analysing imploding dynamics, the main diagnostics include radial radiation monitor, axial power measurement system, axial UV frame cameras and load current monitor. The radial radiation monitor constructed with a photoelectric cell has a time response of less than 1.6 ns, and a spectral response of wavelengths from 200 nm to 700 nm. The axial power measurement system has a time response of less than 1.6 ns, and a flat energy response from 50 eV to 1500 eV [24-25]. Its measurement uncertainty is approximately20%. The axial UV frame cameras have four frames, and a spectral response of wavelength from 262 nm to 272 nm. Its static spatial resolution is better than 70 μm, and its exposure time is 0.5 ns. The schematic diagram is similar to that in [26] and [27]. However, in this experiment, without using a laser, these cameras record the UV radiations which come from the load itself.
III. Results and analysis
In the PRE-experiment, the line mass was 200 μg/cm for all of the metallic loads. In addition, the current had a prepulse. The interior current of the load was measured by using two magnetic mini-probes, placed 2.5 mm away from the center of the load in a symmetrical position that is illustrated in Fig. 3(a). The schematic diagram is similar to that in Ref. [5]. The total current was also measured by the Rogowski coil, which was located at 22.5 cm away from the axis of the load [28]. The time zero, which is similar in this article, is defined by the linearly extrapolated leading edges of the main current pulse (between 10% and 90% of peak), and the results were illustrated in Fig. 4. During the prepulse of current, all of the currents flowed through the interior metallic load. The measured current in the interior of the liner is different between 1# mini-probe and 2# mini-probe for the same shot. The results in the same position of shot17273 and shot17274 are also different. The main reason is that the distribution of current during the prepulse is not azimuthally symmetrical. Later, the result of the magnetic probe was significantly less than the total current during the main pulse, indicating that some of the current was still flowing through the SiO2 substrate. The radial UV radiations were also measured by the radial UV frame cameras, illustrated in Figure 5. Since there was a step of 2 mm height at the cathode, and the H is 20 mm in the PRE-experiment in Fig. 3(a), the height of the load in the radial view was exactly 18 mm. Before the main current, the metallic load was heated by the prepulse. The UV radiations from metallic load could be distinguished, indicating that all of the current flowed through the metallic load during the prepulse. However, the radiations were not uniform, as illustrated in Fig. 5(a). During the main current pulse, as the SiO2 was also heated, it was hard to distinguish where the radiations originated from. Since the length of the dielectric was not large enough to prevent the currents from flowing through the outer surface of the SiO2 substrate, the SiO2 in the outer surface was also ionized, as illustrated in Fig. 5(c). To reduce these influences, the prepulse of current has been weakened as low as possible, and the H in Fig. 3(a) has been increased from 20 mm to 40 mm in the formal experiment.
A. Current and radial radiation
In the formal experiment, the total currents were measured by the Rogowski coil. The radial radiations with wavelengths from 200 nm to 700 nm were recorded by the radial radiation monitor. Their results are illustrated in Fig. 6. The radial radiations increased along with the current, and decreased before the current peak. The main reason is the breakdown of the SiO2 substrate [29], which decreases the transparency of electromagnetic wave of wavelengths from 200 nm to 700 nm. At the start of current, the metallic load is heated by the current, and then is ionized and moves inward, and the radiations (200 nm–700 nm) can transmit through the SiO2 substrate. Later, the SiO2 substrate is also heated by the metallic load plasmas, until the metallic load is peeled from the substrate [18], in addition, the SiO2 substrate is ionized, and 2# surface is destroyed. Furthermore, the transparency of the electromagnetic wave of wavelengths from 200 nm to 700 nm within the surface of ionized substrate decreases. After the metallic load is peeled from the substrate, the emission of electromagnetic wavelengths from 200 nm to 700 nm coming from these plasmas cannot be transported through the destroyed 2# surface. At that time, the signal recorded by the detector is mainly from the region of ionized substrate. Therefore, the peak time of the radiation monitor could be adopted for the indication of Al separated from SiO2. As illustrated in Fig. 7, although the rise time of the current does not increase significantly, the peak time of the radiation lightly increases when the line mass of the metallic load increases.
The implosion process is simulated by the one-dimension MULTI program [30-31]. This program considers all the materials as cold plasmas with 0.0258 eV in the beginning of simulation. Therefore, the liner ablation is not considered carefully. In addition, the SiO2 substrate has a large resistivity, enough to prevent the current from flowing through it at the start. The substrate is mainly heated by the heat exchange and radiations of plasmas. During the simulation, the experimental current and parameters of shot18187, shot18188 and shot18189 are served as the input. In addition, the simulation is carried out in one dimension, beginning from -20 ns and considering the radiation magneto-hydrodynamic process. The implosion trajectories along with the current, are illustrated in Fig. 8. At the start of current, because the electrical resistivity of SiO2 is significantly larger than that of Aluminum, all the current flows through the Aluminum coating. The metallic coating is converted into hot plasmas, and imploded by the magnetic field. As the line mass of metallic load increases, more time is needed for the metallic load to be turned into hot plasmas and separated from the SiO2 substrate. The time increases from 67 ns to 70 ns, to 72.5 ns, for the experiment at an approximate 3 ns rate in Fig. 7(a). It increases from 28.3 ns to 34.5 ns, to 40 ns, for the simulation at an approximate 6 ns rate in Fig. 8. However, the given time of Fig. 8 is earlier than that of Fig. 7(a). The main reasons are that the load is considered as cold plasma, the given time is defined as that the trajectory of Al just lightly moves 70 μm away from the trajectory of SiO2 and the width of Al plasmas has not been taken into account carefully at this moment in the simulation. These conditions are different from the experiment.
B. Axial radiation property
The axial radiation power is measured by the axial power measurement system, and the result is illustrated in Fig. 9. Since there was a prepulse of current for shot17270, its axial power rose earlier, and increased slower than shot18187 although they had the same line mass of the load, as it is illustrated in Fig. 9(a). As the line mass of metallic load increases, more time is spent in separating Al from the SiO2, thus more SiO2 is heated with more energy depositing in the substrate. Accordingly, the axial power rises consequently later and slower, and the peak decreases, as illustrated in Fig. 9(b).
The frame times of the axial UV frame cameras are presented in Fig. 10, and their results are illustrated in Fig. 11. The eight dark sectors are the shadows of the opaque structure of REH illustrated in Fig. 3(b). There are a hot-region in the center, and some sharp jets near the assembled gaps of segments before 30 ns (indicated as 18198-1 and 18193-1). As illustrated in Fig. 9, when the load is heated by a prepulse of current, a negligible amount of metal is ionized, and very few precursory plasmas are formed and moved inward. These negligible axial radiations have been recorded by the axial power measurement system. The hot-region is a result of these precursory plasma radiations. The metallic layer is heated, expanded and ionized near 30–40 ns (indicated as 18198-2 and 19192-1). In addition, plasmas are peeled from the substrate, and start to implode near 60 ns (indicated as 18198-3 and 18192-2). These plasmas implode inward, and reach the center near 100–110 ns (indicated as 18199-2 and 18193-4). Its average implosion velocity is 10 cm/μs. At that time, there are still lots of plasmas being formed from the SiO2 substrate. Later, these plasmas also take part in the implosion (indicated as 18199-3 and 18194-2). There is no clear outer boundary of these implosion plasmas (indicated as 18199-4 and 18194-3).
Before 60 ns, the radiations were almost azimuthally uniform (indicated as 18199-1 and 18193-2). After radial radiation peak of the radiation monitor, the plasmas are peeled from the substrate, and the radiations are not uniform anymore (indicated as 18199-2 and 18193-4). This asymmetry is more serious for the load assembled with two half-cylindrical substrates (indicated as 18200-2 and 18194-1). The uniformity of the load mass distribution in the half-cylinder is worse than that of the quarter-cylinder. One of the reasons is the asymmetry of resistivity distribution. When the temperature of the load increases, its resistivity increases rapidly. The metallic load sputtered on the substrate via the magnetron sputtering process with loose microscopic structure, will magnify the asymmetry of resistivity. However, the current flowing through the load is not azimuthally uniform. Accordingly, it leads to a worse electrothermal instability [16]. In addition, other factors, such as the early plasma ablation and implosion, can also produce these non-uniformities at a later stage.
In details, one of the eight light sectors in Fig.11(a) is chosen as the sample region to analyse the average intensity distribution with the radius at different frame times, illustrated in Fig. 12. Firstly, the metallic layer is heated, and ionized from 12.6 ns to 33 ns near the position of radial 5mm. Secondly, it expanded from 0.2 mm to 0.5 mm, at the time from 33 ns to 54 ns. These plasmas are peeled from the substrate with a width of about 1.2 mm at 62.1 ns. There are lots of plasmas taking part in the implosion until 153.6 ns. During these time, the edge of substrate moves outwards. At last, implosion collapses near the center at nearly 184.1 ns.
IV. Conclusion
Z-pinch loads of a novel structure have been studied at Qiangguang-I facility for the first time. Before radial radiation peak of the radiation monitor, the metallic load can be converted into plasmas, and peeled from the SiO2 substrate, because most of the currents flow through the metallic load. During the implosion, the SiO2 substrate in contact with metal is also heated, and takes parts in the implosion. A novel material with a higher adiabatic factor should be chosen as the substrate, instead of SiO2 in the future experiment, to reduce this influence. In the pictures of axial UV frames, the radiations are uniform before 60 ns, but the uniformity becomes worse, after the metallic plasmas are peeled from the substrate. As the mass distribution of load is not uniform enough, it leads to asymmetry in resistivity which seeds the electrothermal instability for this novel structure. In the next stage, we will try to improve the uniformity of mass distribution, and reduce the loose microscopic structure of the metallic load, for example adding the anneal process. The most important goal is to improve the quantity of current passing through the metal during the implosion. In addition, the simulation needs to be improved to two dimensions.
A review of the dense Z-pinch
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