Ultra-high temporal resolution images of a homogeneous electric field short air gap negative streamer at overvoltage and atmospheric pressure

Air gap discharge is one of the most basic scientific problems in the field of high-voltage engineering. The homogeneous electric field 1.5 mm air gap negative streamer at overvoltage and atmospheric pressure is observed by a high-speed 4-channel framing camera. The ultra-high temporal resolution images of a single negative stream are captured (the exposure time is 5 ns, and the inter-frame delay is no more than 0.1 ns). It is observed that the negative streamer formed in the middle of the air gap and grew bidirectionally towards both electrodes. At the same time, the electrical measurement is also carried out.


M
odern high-voltage supplies easily create negative streamers, 1) an in-depth study of air gap discharge characteristics is helpful to understand the mechanism of streamers and improve the refinement level of the insulation design of power transmission and transformation equipment. 2)Based on Townsend's electron avalanche theory and considering the distortion effect of space charge on the electric field, streamer theory was proposed by scholars Raether, Loeb, and Meek around 1940. [3][4][5][6] The influence of photoelectrons formed by a photoionization effect on the development of discharge is emphasized in streamer theory. Apositive streamer grows from the positive electrode towards the negative electrode and is usually created at the air gap breakdown voltage.7) The primary avalanche needs to pass through the air gap and then transform into an initial streamer at the positive electrode.However, a negative streamer is created when the air gap voltage exceeds the breakdown voltage.If the ionization degree of its inside is strong enough, the primary avalanche can transform into an initial streamer without passing through the air gap and photons will be emitted in the vicinity of the initial streamer.Electrons produced by photons initiate secondary avalanches on their motion path.The primary avalanche converges to the tail of the secondary avalanches which are in front of the primary avalanche.The primary avalanche electrons intermix with secondary avalanche ions and form a quasineutral plasma (anode-directed streamer) growth towards the positive electrode.7) Due to the direction of the resulting field, the secondary avalanches behind the initial streamer are pulled into the trail of a primary avalanche.The secondary avalanche electrons intermix with primary avalanche ions (cathode-directed streamer) and grow towards the negative electrode.1,[8][9][10][11] This is called a double-headed streamer in Ref. 10.
Experimental research is the most intuitive and accurate method to study the evolution mechanism of streamers, especially high-speed imaging technology.][14][15][16][17][18] However, limited by the inter-frame delay, it is impossible for the ICCD to capture the complete evolution process of a short air gap single streamer which is usually in the order of tens of nanoseconds.Although multiple observation experiments can be summarized by photographing the images of different evolution periods of multiple streamers, 19) the dispersion and randomness of the streamer will have a certain impact on the accuracy of the results. 2)][22][23][24][25][26] However, the streak camera has only onedimensional spatial resolution. 27,28)The morphology of the streamer cannot be recorded by a streak camera.More details of the differences and advantages of imaging equipment can be found in Table 2 The high-speed multi-channel framing camera has outstanding advantages in the imaging of the nanosecond discharge process.It integrates multiple ICCDs into one, distributes the optical signal by a fixed beam splitting system, 29) transmits it to each ICCD unit, and uses electronic control technology to control and integrate multiple ICCDs to achieve ultra-high-speed exposure.In single-frame mode, the inter-frame delay can be shortened from ICCD inter-frame delay to fast photoelectron control delay, which greatly improves the frame rate. 30)n this study, an experimental platform with an ultra-high temporal resolution is established to observe the evolution process of a negative streamer channel in a short air gap.The experimental results show the evolution of the homogeneous electric field with a 1.5 mm air gap negative streamer which is at overvoltage (in comparison with the breakdown voltage) and atmospheric pressure underlying the ultra-high temporal resolution of 5 ns exposure time and no more than 0.1 ns inter-frame delay.Finally, the physical mechanism and electrical characteristics of a negative streamer are analyzed in detail.
Figure 1(a) is a schematic of the experimental observation platform with ultra-high temporal resolution.The homogeneous electric field short air gap device is set in a discharge chamber to ensure that the observation system is not disturbed by the electromagnetic radiation generated by the discharge experiment.An arbitrary generator (Tektronix; AFG3152C) and ultra-high-speed high-voltage amplifier (Matsusada; AMP-20B20, maximum output voltage 20 kV, slew rate 700 V μs −1 ) compose the high-voltage power which is used to drive the discharge.The arbitrary generator also has the function of triggering the air gap discharging and high-speed framing camera shooting synchronously.
The electrode structure with its geometric size is shown in    current in a circuit containing the air gap.The sensor signal is connected to the mixed signal oscilloscope (Tektronix; MSO64, 6 GHz, 25 GS s −1 ), and the digital oscilloscope also collects the internal shutter TTL signal output of the camera IntGtP port to obtain the shutter action time.
We use two output signals of the arbitrary generator to trigger the ultra-high-speed high-voltage power amplifier and XXRF framing camera respectively.Considering that the delay may be involved in the observation system, the digital oscilloscope cable transmission delay, XXRF internal delay, and AMP-20B20 voltage rise time are measured.An AFG3152C two-channel signal output delay is set according to these delays to control the camera shutter to operate within the duration of the high-voltage pulse and ensure the synchronization of electrical and optical measurements.
The single-pulse streamer observation experiment is carried out in a dark room at atmospheric pressure with a fixed air gap of 1.5 mm.The single-channel ICCD gating time of XXRF is set to 5 ns, and the relative trigger signal is delayed by 0, 5, 10, and 15 ns, respectively.The inter-frame delay is no more than 0.1 ns, and the total shooting time is 20 ns.The time when Tektronix MSO64 is triggered by XXRF IntGtP signal is set as the time zero.
The inherent error of the framing camera splitting system which leads to a certain degree of deviation in the position of the images taken by each ICCD is necessary to be corrected.
The method is to keep experimental conditions exactly the same (including the working mode and parameters of XXRF), except for setting up an external illumination and no discharge in the air gap, to take another set of images.Superimpose those images of each channel with the streamer images obtained by the corresponding channel experiment, as shown in Fig. 2(a).Adjust the transparency of the

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd superimposed images of each channel to make the reflection positions of the electrodes in the superimposed images of each channel coincide, that is, the correction of the deviation of the XXRF spectral system is completed, as shown in Fig. 2(b).
Figures 3(a) to 3(d) display the typical experimental image results of the negative streamer in the homogeneous electric field.It shows that the streamer channel is a single filament without obvious bifurcation, and the streamer is formed in the air gap and grows towards the electrodes until the breakdown.Figures 3(f) to 3(h) show a schematic of the mechanism of the formation and evolution of the negative streamer, corresponding to stages (b) to (d).
Figure 4 is a typical evolution of the current and voltage of the discharge circuit.According to the shutter action signal fed back to the XXRF, the rectangular cyan block is used to mark the shutter time and exposure time on the current curve.In particular, Fig. 3(a) is blank, and Fig. 4(a) shows that there is no current at this stage, so there is no discharge in the period of 0-5 ns.This ensures that the subsequent experimental images capture the evolution process of the streamer's initial formation.The sudden drop of the air gap voltage (at 21 ns) is a significant feature of the streamer channel through the air gap, therefore, the streamer is in the formation and development stage during the 0-20 ns period.
As shown in Fig. 3(f), the primary avalanche moves to the anode under the action of the external electric field.Photographs of avalanches are obtained by using light emission due to the excitation of molecules and atoms.In this sensitivity level of the recording equipment, we can see the outlines of the primary avalanche.The main body of the avalanche has a well-pronounced wedge shape, rounded at the head [Fig.3(b)].
Electrons are almost at the head of the avalanche, while most of the positive ions remain behind.Space charges produce their own field that adds up vectorially with the external field, E 0 , and distorts it in the vicinity of the avalanche. 31)As shown in Fig. 3(e) the fields in front of the avalanche head add up to give a field stronger than E 0 .The fields in the zone between the centers of the space charges of opposite signs point in opposite directions and the resultant field is weaker than E 0 .
This effect becomes strong as charge multiplication continues and influences the subsequent process of ionization.The distorted electric field induces the recombination and antiexcitation of space charges and produces photoionization in the vicinity of the primary avalanche.In this case, the primary avalanche goes far from the negative electrode but does not reach the positive electrode and transforms into an initial streamer.As a result of radiation-causing photoionization, secondary avalanches are produced in front and behind the initial streamer [Fig.3(g)].As shown in Fig. 3(c), the streamer channel shows a sharp cone and the light intensity in the middle is obviously enhanced and weakened on both ends.The front electrons of the head of the primary avalanche, moving rapidly in the distorted field, join the ionic trails of secondary avalanches that are in front of it and together form the anode-directed streamer.The secondary avalanches behind it join the ionic trails of the primary avalanche and together form the cathode-directed streamer.This is the mechanism of growth towards both electrodes of the negative streamer.
As shown in Fig. 4(d), the anode-directed streamer arrives at the positive electrode and connects the positive electrode with a weak ionization channel which suppresses the ionization at other locations in space.The streamer channel is significantly contracted, and the brightness is weakened.On the other hand, the cathode-directed streamer continues to grow toward the negative electrode.According to the optical resolution of ICCD 60 lp mm −1 , the vertical length of the negative streamer (86 pixels) is calculated to be about 1.43 mm at 20 ns, and the cathode-directed streamer is almost reaching the negative electrode.
The circuit current reflects the formation and dissipation of charged particles in the process of the streamer. 32)When the anode-directed streamer grows close to reaching the positive electrode, the electrons at the head of the avalanches sink into the metal quickly, resulting in a positive current pulse (at 15 ns) in the circuit.It is produced by the discharge current.In the following, we try to use Maxwell's equations to construct the relationship between conduction current and Evolution of the current in a circuit containing the discharge gap, double current pulses produced at 15 ns (positive) and 21 ns (negative).

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd distorted electric field, the results will show the mechanism of a negative current pulse caused by a displacement current.
When the streamer grows, a redistribution of the electric field strength occurs, 33) which means there are displacement current flows in the gap and a corresponding conduction current flow in the wires.The cathode-directed streamer is a quasineutral plasma in that the electric internal field is at a small value, but the electric field in front of it is strengthened by the positive ions that are in the trail of the primary and secondary avalanches [Fig.3(h)]. 9)The electric field at the place where the cathode-directed streamer arrives is suddenly weakened [Fig.5(a)].
In the observation region, the air gap can be regarded as a circular plate capacitor with a radius of R, as shown in Fig. 5(b).We choose a closed loop, l, which is perpendicular to the wire, and B is the magnetic field of tangential of l.According to Ampere's Law, the closed loop integral B dot dl is where μ is the magnetic permeability of air, and I con is the conduction current in the wire.We make an open surface, A, attached to l and right through the negative electrode [Fig.5(b)].E is the penetration of A of the electric field.Apply Maxwell's 4th equation In the strict sense, formed by space electron motion, I pen is not zero, so we preserve it in Eq. (3).However, I pen is a small cause of the charged species large decay through electron-ion recombination before the arrival of the streamer. 34)Before the cathode-directed streamer propagates to the negative electrode, there is no rapid change in the electric field in the vicinity of the negative electrode.According to Eq. ( 3), I con is dominated by I pen , and the experimental result proves that its value is much smaller during the 17-20 ns period.We hold that the main effect on the I con during the 20 In summary, we established an experimental platform with an ultra-high temporal resolution to observe the evolution process of a short air gap negative streamer channel at overvoltage and atmospheric pressure.The experimental image results show the evolution of the negative streamer in the Townsend stage in the early, and then the streamer stage.The primary avalanche transforms into a streamer in the short air gap and grows towards both electrodes.The existence of the double-headed streamer supports the evolution physical mechanism of the negative streamer.We consider that the current pulse is an important electrical feature of the anode-directed or the cathode-directed streamer that reaches the electrode surface in the evolution of a negative streamer.

Fig. 1 (
b), both of the electrodes are made of copper.The edge of the cylindrical electrode (upper) is rounded to prevent the discharge of the electrode edge caused by the excessive local electric field.
Figure 1(c) shows that the electric field of the air gap in the white dotted line region is evenly distributed, and we limit the observation region according to that result.The optical measurement part uses a high-speed 4-channel framing camera (Stanford Computer Optics, XXRapidFrame, XXRF) which is based on 4-QuikE ICCD camera technology, with an optical resolution of 60 lp mm −1 , a dynamic range of 12 bit, and a jitter time of less than 20 ps.Single-channel ICCD has a minimum gating time of 1.2 ns, independently controlled by electronic devices and a minimum inter-frame delay of 0.1 ns.It can shoot 4 consecutive images at a frame rate of 10 10 fps in singleframe mode.The USB 2.0 port of XXRF is used to connect with the computer, and the working mode, parameters, and shutter action delay of each channel ICCD camera are set by computer software.The Trig port is an external trigger signal input (TTL signal falling edge trigger); the IntGtp port is the internal shutter TTL signal output.The electrical detection part adopts a high-voltage probe (Tektronix; P6015A) to measure the voltage at both electrodes of the air gap, and the current probe (Tektronix; TCP0020) is used to measure the

Fig. 1 .
Fig. 1.(a) Schematics of the experimental observation platform with ultra-high temporal resolution, (b) schematics of the electrode configuration, and (c) the short air gap electric field simulation by Maxwell 16.0.

Fig. 2 .
Fig. 2. The correction principle of XXRF splitting system, (a) image superimposition of a channel and (b) correction between the each channels.

Fig. 3 .
Fig. 3. Typical experimental image results of a negative streamer in homogeneous electric field and schematic diagram for the evolution of negative streamer channel.(a) Blank, (b) and (f) the primary avalanche moves to the negative electrode, (c) and (g) the primary avalanche transforms into an initial streamer and emits the photons, (d) and (h) streamer is almost through the air gap, and (e) diagram of the distorted electric field due to space charge.
where ε is the dielectric constant of air, and I pen is the penetration of A of the current.It is obvious that Eqs.(1) and (2) are equivalent.Exchanging the operation order of the electric field derivative to time and integral to the open surface of Eq. (2), we can get the relationship between current and time-varying electric field: -21 ns period is the displacement current, dA , not I pen .When the cathode-directed streamer propagates to the negative electrode, the value of dE/dt (negative) caused by the sudden drop of the electric field in the vicinity of the negative electrode increases rapidly, it causes the negative current pulse.