The assessment of the atmospheric air breakdown voltage generated by the interaction between microwaves and metallic wires

The aim of this research is to understand the plasma initiation process generated by metallic wires when interacting with high energy density microwaves. Lead (Pb) and molybdenum (Mo) wires of 0.5 mm diameter were investigated in this experiment. The tip of the metallic wire was placed into the nodal point of a waveguide cavity attached to a microwave generator, where it was exposed to the high energy density of the microwave field. Following the interaction between microwaves and the metallic wire, a plasma was initiated having as effect the wire vaporization. The experiments were conducted in atmospheric air at ∼1 bar pressure. From optical emission spectroscopy investigations it was observed that electronic excitation of the plasma has high values and it is in a local thermal equilibrium. The theoretical calculation of the voltages induced in the metallic wires when exposed to high energy density microwave field are similar to those measured in air breakdown experiments. The scanning electron microscopy analysis of the tips of the metallic wires showed that the field emission process is responsible for the ignition of the metallic wires and plasma generation.


Introduction
The interaction of microwaves with metals is an important topic in microwave, plasma and materials physics.
Understanding this process can bring new contributions needed for the development of industrial, medical and space science technology. Presently, the plasma initiation process using the interaction between small metallic objects and microwave field is not completely understood. Most of studies were focused on the microwave plasma diagnostic, metallic powders characterization and less on the investigation of microwave plasma initiation process.
Therefore, the goal of this research is to elucidate the plasma initiation process by studying the direct interaction between high-power microwaves and metallic wires (Mo and Pb) with very different melting points (2623°C versus 327.5°C). Many studies about the interaction between microwaves and metals in wires or powder form have been conducted so far.
In 2001, Cheng et al exposed to microwaves various small samples (powdered metal compact samples and metal samples) placed under nitrogen atmosphere. During the experiment it was observed that the metallic samples were heated and in some cases arcing electric discharges occurred. The heating of metalic samples by the microwave field was attributed to the Eddy curent process [1].
In 2008, Mondal et al iraditated with microwaves Cu metallic powders placed under a hidrogen atmosphere. Folowing micrwave absorbtion process by metallic powders these were heated up to 1000°C. The heating of metalic powders by the microwave field was also attributed to the Eddy curent process [2].
Other studies about interaction of the microwaves with metal powders mixed in a liquid solvent showed that the metal powders produce sparks and the liquid was heated.
The experiment indicated that numerous factors which depend on nature of the metals and solvent were responsible for the sparks generation [3].
Given the behavior of the metallic powders under the action of the microwaves, numerous studies on interaction between microwaves and small metallic objects (metallic wires or electrodes) placed in vacuum conditions have been lately performed. Sudies about interaction of the microwaves and metallic electrodes placed in vacuum conditions showed that the metallic electrode generate a high magnitude electric field [4]. Other studies highlighted that when a photomultiplier ( used as a detector of cosmic rays) was irradiated with microwaves, this generated an electrical signal in the output [5].
If the metallic objects are placed in gas or liquid medium and irradiated with microwaves these can generate a plasma.
In 2015 Popescu et al conducted a study about interaction of the microwaves with a titanim electrode in atmospheric air. Using an experimetal device they generated a plasma with a high electronic temperature value (∼0.4 eV) [6].
In 2018, Yukun Feng et al showed that a discharge initiated by microwaves on metallic paper clips under a gaseous acetone atmosphere can decomposes the acetone .
The experimental results showed that various factors influenced the microwave discharge and acetone dissociation [7]. Other studies conducted under high pressure of nitrogen showed that plasma generation during interaction of the microwaves with metallic wires is caused by heating of the metalic wires having as effect an emmision of electrons by the thermoionic effect [8].
We recently conducted experimental studies about the interacton of the microwaves with a tungten wire placed under CO 2 atmosphere.
The results highlighted that when plasma was generated, the metallic wire was vaporized and ionized and CO 2 molecules were dissociated [9].
This research is focused on generation of the microwave discharge in air at atmospheric pressure, investigation of the plasma, morphological analysis of the tips of the metallic wires and theoretical determination of the voltage induced by high-power of the microwaves in the wires.
To generate plasma from an electrically insulated metallic wire a new high-power microwave generator was developed. The theoretical results highlighted that when a metallic wire interacts with high-power microwaves, a high voltage is generated.
The analysis of the surface morphology of the tips of the metallic wires showed that the plasma is initiated in the tip region and that the metallic wires are not melted in volume. From optical emission spectroscopy investigations it was observed that the electronic excitation temperature of the plasma has high values and it is in a local thermal equilibrium.

Materials and methods
In general, the plasma ignition depends on the gas nature and pressure, shape of the electrodes [10] and the electric voltage between the two metallic electrodes [11][12][13].
In this experiment the plasma is generated in air at atmospheric pressure (∼1bar) through interaction between microwaves and an electrically isolated metallic wire.
The microwaves are emmited by a microwave generator and the metallic wire behaves as an absorber of the microwaves. In this case the metallic wire is a negative electrode, and the microwave generator is a positive electrode. The microwave generator is composed of a power supply, a microwave source (a commercial magnetron with frequency 2.45 GHz and up to 800 W RF power-Whicepart Electronic LTD, model 2M2013-01TAG, Zhejiang, China) coupled using an antenna with a single-mode waveguide cavity. The waveguide was constructed in accordance with the TM 011 propagation mode (transverse magnetic mode).
The waveguide is used to focus the microwaves generated by the magnetron in the focal point, which is located on the central axis of the waveguide and has a small volume of approximately 1 cm 3 . Figure 1 presents the design of the experimental setup. If the magnetron emits microwaves with 800 W power, then in the focal point of the waveguide one will have 8 MW cm −2 power of the microwaves. If an electric isolated metallic wire is placed with one tip in the focal point of the waveguide and then it is irradiated with microwaves, it will be quickly evaporated and then a plasma will be generated.
The plasma generated inside the waveguide is based on the breakdown voltage of the air induced by high power density of the microwaves in metallic wires.
To experimentally analyze the microwaves propagation of transverse magnetic mode in waveguide, after generating the plasma we turned off the magnetron power supply. Figure 2 displays an image of the metallic wire, which was taken just after the microwaves were switched off. It clearly shows that the metallic wire was heated only in the focal point of the waveguide To study the behavior of the metallic wires when these interact with high power density microwaves, experimental studies were performed on Pb and Mo wires. These metals were chosen because Pb has a low value of the melting point, while Mo has a high value of the melting point. The metallic wires were 0.5 mm in diameter and 5 cm in length. To determine the composition purity of the metallic wires, they were investigated by scanning electron microscopy (SEM) using an Apreo S microscope from Thermo Fisher Scientific with energydispersive x-ray spectroscopy (EDS) system, fixed silicon detector and integrated Peltier element as a cooling system. For EDS, the used beam spot was 6.5 μm-7 μm in diameter, the working distance was 10 cm, and the dead time during signal collection was 30 s. It was also operated at 10 kV acceleration voltage and 6.3 pA electrical current.
The results of EDS investigations for metallic wires, mass percentage, and atomic percentage are presented in tables 1 and 2.
Regarding the chemical composition of the metallic wires, special importance was given to their homogeneity and purity. EDS analysis of the wires showed mostly Pb (97.32% mass percentage, 94.22% atomic percentage) and, respectively Mo (98.46% mass percentage, 94.87% atomic percentage), with very few other alloying elements.
After identifying the focal point of the waveguide cavity, the experiment started by placing the metallic wire along the cavity symmetry axis with one tip located in the focal point of the waveguide. The microwave source was turned on and then the power of the microwaves was increased until the metallic wire was ignited. The power of the microwaves required to ignite the plasma was recorded for each used metallic wire.  In figure 3 the image of the plasma generated by a Pb metallic wire in interaction with microwaves is presented.
Under these conditions, each metallic wire was exposed to microwaves for 10 s. The plasma composition was analyzed using the optical emission spectroscopy (OES) method [14,15] with an Ocean Optics USB 2000 ++ spectrometer (Ocean Optics Inc., Orlando, FL, USA).
The optical emission spectrum of the plasmas generated by each metallic wire in interaction with microwaves was recorded with a 1 ms integration time. The optical resolution according to the datasheet of the Ocean Optics USB 2000 + spectrometer is: FWHM ∼ 0.1 nm [16]. Before starting the plasma characterization, the spectrometer was calibrated using a broadband light source (Ocean Optics DH-mini UV-vis-NIR Deuterium-Halogen Light Source).
To identify each chemical element from plasma and correct the emission spectrum of the plasma, the experimental results were compared to three of the most intense spectral lines for each element [16] from the National Institute of Standards and Technology (NIST) database [17].

Results
During the interaction between the microwave field with Mo or Pb wires a plasma was generated, whose emission spectrum in the UV-vis-IR region was recorded.  Using Span V.1.7 Spectrum Analyzer software [18], the recorded emission spectra of the plasma generated by each metal was analyzed and the results are displayed in figures 4 and 5, respectively.
One can observe that during the interaction between microwaves and Mo or Pb wires the plasma emitted lines corresponding to metallic excited atoms and ions : Mo I, Mo II, Pb I, Pb II and gas excited atoms and ions: OI, OII, NI, NII [19]. These spectral lines correspond well with the spectral lines from the NIST database.
Using the Boltzmann Plot method, which assumes that local thermodynamic equilibrium (LTE) is met within the plasma, the electronic temperature of the plasmas generated in microwave induced discharges was estimated. From the Boltzmann Plot results we noticed that the obtained plasma is thermal [20] for the excited neutral atoms species from microwave discharge.
The electronic excitation temperatures of Mo I and Pb I species are shown in figures 6 and 7. The energetic domain presented on X-axis from the Boltzmann Plots corresponded with the frequency of the spectral lines selected for determining the electronic excitation temperature of the excited neutral atoms species.where: lnI gA l is: I-intensity of the spectral lines, λ-wavelength of the spectral lines, A -transition probability, g-the statistical weight of, upper energy, Excitation energy (eV) -energy level of upper state.
To evaluate the voltage induced by microwaves in metallic wires, a theoretical calculation was performed. Using equation (1) fom [21] and physical parameters from table 3, the resistances in alternating current for metallic wires with electrical conductivity between σ = 4 × 106 S m −1 and σ = 6.5 × 107 S m −1 was determined as: where: R AC -electric resistance in alternating current, μ r -relative magnetic permeability, f -frequency of the E-M radiation, σ -electric conductivity, l -length of the metallic wire, a -radius of the metallic wire.
The relative magnetic permeability (μ r ) from volume magnetic susceptibility (χ m ) is in accord with the equation (2) from 22 was used:      In figure 8 it is displayed the dependence of electrical resistance in alternating current on electric al conductivity.
The results from figure 8 highlights that R AC of a metallic wire when this is exposed to microwaves depends on their electrical conductivity. Acording to forumula (1) the R AC values for Mo and Pb are 624 Ohm, and 1275 Ohm respectively, at 20°C.
To determine the microwave power when the plasma is generated, the power of the microwaves was increased until the metallic wire was ignited and then the value of the power of the microwaves was recorded. This stage was performed for the two metallic wires. The experimental results showed that the Mo and Pb wires were ignited at 8 MW cm −2 microwaves power.
Knowing that a metallic wire can be associated with a resistor, the voltage generated by the wire is in accord with Ohm law is (3).
where: P -the power of the microwaves, V-electric voltage, I-electric intensity.
Using the value of the microwaves power recorded when the Mo and Pb wires were ignited and R AC values from figure 8, the dependence of the electric voltage on the R AC was determined (see figure 9).
The figure 9 displayes the dependence of the electric voltage induced by microwaves on the R AC of the metallic wires.
The electric voltage induced by microwaves in Mo and Pb wires when the plasma was generated was estimated to be ∼71.000 V for Mo and ∼100.000 V for Pb.
These values of the voltage were obtained considering only physical parameters of the Mo and Pb wires without the effect of impurities contained in wires.
The electric voltage was estimated using a theoretical method because any instrument introduced in the waveguide during the microwave discharge can influence the measurement results.

Discussion
So far many experimental studies about microwaves interaction with small metallic objects were conducted.
To generate the plasma from metallic powders or wires the researchers used comercial devices such microwave ovens or other dedicated devices. The distribution of the microwaves intensity from the waveguides of these devices is localized in the entire volume of the waveguide with multiple focal points of the electric and magnetic field.
Therefore, the magnitude of the electric field induced by microwaves in metallic wires is much smaller.
To observe the interaction mechanism of the microwaves with metallic objects it is necessary that they interact with high power E-M fields. Using a simple microwave device, the behavior of metal wires when they interact with the high power of microwaves was studied.
From experimental results, one can note that Mo and Pb wires are ignited at 8 MW cm −2 microwaves power. From theoretical calculation one can note that the plasma is initiated when the metallic wire generates a threshold voltage coresponding to the air breakdown voltage value [25]. Given that the experiment is conducted in air under atmospheric conditions, the theoretically estimated value of the breakdown voltage is close to the experimental data from other papers [25,26]. In figure 10 the flowchart of the interaction mechanism of the high-power of the microwaes with an metallic wire is displayed.
To understand the plasmas initiation process generated by the metallic wires in interaction with microwaves, the Pb and Mo wires were exposed for a short time to the action of the microwaves. After plasma initiation the power supply of the magnetron was turned off as fast as possible.
Using a SEM equipment working at an acceleration voltage of 25 kV, in the magnification range of 65×-15,000×, at 10 cm working distance the morphology of the tips of the metallic wires was analyzed. The images of the tips of the metallic wires and the affected zone after plasma initiation are shown in figures 11 and 12.
Figures 11(a) and 12(a) show the images of samples at a magnification of 65×. When the surface is analyzed in detail, at a magnification of 15,000× (figures 11(b) and 12(b)), we observe with clarity that plasma initiation process begins at the tips of the wires. From the analysis of the affected zone it is observed that the metallic wires were only melted at their tips, therefore the melting point of the metallic wires is not an important parameter in the plasma initiation process. Another observation is that the Mo and Pb wires were not heated in volume, these behaving similarly in interaction with microwaves.
If the metallic wire is exposed at lower microwaves powers (below 800 W RF emitted by magnetron) or the dimension of the wire is shorter than 5 cm or the diameter is greater that 0.5 mm then the plasma is not initiated.
If the metallic wire is irradiated a long period with microwaves the plasma will be generated until the wire will be completely vaporized from focal point of the waveguide.
In figure 13 it is displayed a Pb wire exposed for 1 s time at microwaves. The experiment showed that the Pb wire was melted only at the tip placed in the focal point of the waveguide. Following interaction between the Pb wire and microwaves the Pb wire was heated and got oxidized.
Using the parameters from table 3, the theoretical calculation showed that the metallic wires generate high voltages when they are irradiated with microwaves. Therefore, initially the electrons are generated by the metallic wire when it is exposed to a strong electric field through the field emmision process (the yellow marked zones in figures 11(a) and 12(a)) [27].
After this, the ions are generated by the interaction of the gas atoms with free electrons. Once the plasma was initiated, the end of the wire is strongly heated and locally melted due to electron-ion collision process.
From Boltzmann plots results it was observed that electronic excitations of the plasma have high values (∼4 eV) and that the microwaves discharge generates a thermal plasma. The heat generated in microwave discharge leads to the generation of electrons through the thermionic effect [28], having as effect the growth of the plasma volume.
The interaction between high microwave power and the wire produced a clean vaporization of the wire without wire melting residues. This clean vaporization was also observed for other tested metals (In, W, Mo, Fe, Al, Cu, Zn, Pb, Sn and graphite). If the experiment is conducted in air, it generates pure metal oxide powders [29]. Development of the high-power microwave generator and the results presented in this paper bring new informations about interaction of the microwaves with metals, that can be use by the different branches of industry but also in space science area. Because most of the metals have paramagnetic and diamagnetic properties, the theoretical calculations can apply on the electrical and electronic engineering. In industry this technique allows for thin films and metal coatings to be deposited rapidly in atmospheric air and in different gases. Given as the microwave generator is very simple and inexpensive this is suitable for space science area applications. The microwave generator can be easily integrated in actual space electric propulsion systems having a plasma generator role. In this way can be making posible developing a new generation of the space electric propulsion systems which use a gas-metal plasma as propelant.

Conclusions
The theoretical results highlighted that when a metallic wire interacts with high-power density microwaves this generates a high voltage. The electrical conductivity of the metal play a crucial role in the generation of the high voltage.
If the metallic wire is expose at high power of the microwaves in gas atmosphere then a plasma is generated. During the interaction between microwaves and metallic wires, the plasma initiation process begins at the tip of the wires. This results showed that the plasma is initiated through a field emission process.
In microwave induced discharge process the metallic wire is locally melted in the tip area and not heated in volume.
The melting point of the metallic wires is not an important parameter in the plasma initiation process. From investigations of the plasma composition we observed that during microwave discharge the metal and air are   ionized. From Boltzmann plots results, we observed that both metallic wires generated a thermal plasma and electronic temperature of the plasma parameters have high values.
This results showed that the wires were strongly and locally heated leading to growing of the plasma through thermionic effect.