Measurement of Rising part due to Starting of Micro Gap Discharge in Air Using Distributed Constant Line System

Ken KAWAMATA*, Shigeki MINEGISHI**, Akita HAGA** and Risaburo SATO**

* Department of Elechical Engineering, **Faculty of Engineering, Hachinohe Institute of Technology, Tohoku Gakuin University, 88-1 Ohbii Myo Hachinohe-shi, 1-13-1 Chuo Tagajo-shi, 031-8501, JAPAN 985-8537, JAPAN

Abstmct : Very fsFt voltage rising time in positive polarity negative polarity) due to the starling of the discharge in very and falling time in negative polarity tie to starting of gap wide band time &main [8]-[ll]. Vay little is known about the &s&a~e were investigated ia time &main. ‘~%e gap space WBS duration of voltage rise time and voltage fall time tie to gap set very small for voltages below 1500 V as a simulation of the discharge at voltages below 15OOV. The main purpose of the CD&~ ED md the gap &sdtarge of switch &im. ‘Ik present papa is to cIadtj the characteristics of tn+xISitiOn meawnaneat system consists of a &rib&d constant. line duration as EM1 source. due to gap discharge at voltages below system with a tapered maxial electrode which has a matched 1500 V. impe&moe for the chawteristic imp&we of the distributed It is &sir&k to obswe the transition duration me to gap constant line system. lk insertion loss of the tapered coaxial discharge in distributed constant system, because the transients eIectm& WBF within -3dB in the fnqcncy range below 4.5GHz are very rapid. In the first place, a measurement system using the llte atmosphae around the electrode is or&wy air. This dstributed constant system was established to observe the very axpezimmtal system enables to measure the high speed fast transition &rations. It WBS confirmed that the experimental trsasieats of about 100 ps &e to gap discharge in time Qmsin. system enables to measure the high speed voltage transients of As a cansquence of the experiment, the relationship between the about loops [12]. In this paper, relationship between sou~oe discharge voltage and transition &nation were mnfimwi The voltage and the transition ckation were invstigated using this voltage rise time was slowed down gra&aIIy in positive experimental system. polarity, while the voltage fall time was slowed down Aa a coaseq~eace of the experiment, the voltage rise time remarkably in negative polarity for the 0.1 mm needle. slowed down from about 100 ps to about 300 ps according as the

source voltage inaeaw from 400 V to 1300 V in positive INTRODUCTION polarity. While in negative polarity, the voltage fall time slowed

Qwa remarkably from about 130 ps to about 450 ps in the case It is well known that the very fast transients of ~$0.1 mm radii of curvature of needle &&ode.

eledmmagnetic field are arisen from gap &charges of ESD (eledmstatic dsdwge) and eledrical contacts. The transient tie DiSTRIBUTED CONSTANT EXPERIMENTAL

to gap disdmge is a very wide band (high frqency) SYSTEM aIectmmagaetic noise sounx. Over the past few years a considaable number of studies have bean ma& on The qedmentaI system using distributed mnstaat line electromsgaetic noises of the ESD and contacts from the point system shown in Fig.1 wls set up. The system consists of a of view of the eledmmagaetic compatibility. The power supply, a tapered maxial electrode, a diRdionaI mupIer ektmmagnetic noise chawteristics of gap discharge. m (HP778D) as coupled trammission lines and sani-rigid coaxial gmddIy becmning clearer [l]-[7]. cables (50 62 ) BS distributed constant lines. The directional

Howeva, there has beea only a little amount of information mupler WBS used to observe the transition durations. When the about voltage waveforms of the tmasition durations (voltage drectionaI muplex was &iven by 35 ps rise time pulse, it had a rising time in positive polarity and voltage falling time in response of 68 ps rise time. ‘Ihe tapered maxial electrode and a

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source side semi-rigid coaxial cable were constructed in a body. A wide band transient digitizer (TEKTRONlX SCD.5000, 4SGHz) connects an output of the coupled transmission lines at terminal @ via coaxial attenuator of 10 dB.

Fig. 2 shows a cross section view of the tapered coaxial electrode. The electrode consists of an inner conductor as plane electrode, an outer conductor, a needle electrode and a micro meter head Each conductor was made of copper. The design of a taper was carried out by equation (1). The characteristic impedance Zc of an air-insulated coaxial line is given by

Z,=60~ln(b,/a,) CQI - l - (1)

where a, is a diameter of the inner conductor at x, and b, is an inside diameter of the outer conductor at x. In Fig. 2, q=20.Omm, b,=46.2 mm at x=1, andH0.0 mm. The taper was designed by the equation (1). This is a linear physical taper. The characteristics impedance will be constant (Zc) at each point of the tapered coaxial electrode. So, impedance matching between the cable and the coaxial electrode was accomplished The needle electrode was made by sharpening the inner conductor of the source side semi-rigid coaxial cable. The two needle electrodes have curvature of radius r=O.l mm, and r=O.S mm, respectively.

Fig.3 shows an insertion loss of the electrode measured by a network analyzer (HP8753D, 3OkHz-6GHz). The connection between needle and plane electrodes was made by mechanical connection. Fig. 3 (a) is needle electrode of r=O. 1 mm and @) is ra.5 mm. These insertion losses are within -3dR in the frequency range below 4.5GI-L

In the experiment, the semi-rigid cable with the needle electrode is moved by micro-meter head The gap space is reduced gradually. Waveform of voltage transients are observed with the wide band transient digitizer. The duration time (10%-900/o) of voltage rise curves and fall curves at the instance

of discharge were observed, where the voltage of power source was increased from 400V to 1300V. Experimental parameters are the radius of curvature of needle electrode, the source voltage and the voltage polarity.

Coaxial electrode Power Supply Coupled t ransmksion lines Semi- Rigid Cable

Fig. 1 Experimental system using the distributed constant line system

N-type Connector Needle Electrode \

f- Outer Conductor / <Source Side>

L-d=2 <Coaxial Electrode>

Fig. 2 Cross section view of the tapered coaxial electrode

-9L I I III1 I II I

3OkHz 1GHz 2GHz 3GHz 4GHz 5GHz Frequency

(a) inmtion loss in 0.1 mm radius of curvature of needle

_I I I I I I I I Ird.5mmI -Y( I I I I I I I I I

3OkHz 1GHz 2GHz 3GHz 4GHz 5GHz Frequency

(b) insertion loss in 0.5 mm mdius ofcurvatureof needle Fig. 3 Insertion loss of the tapered coaxial electrode

duration in positive polarity and Fig.5 shows the falling part in EXPERIMENTAL RESULTS AND DISCUSSION negative polarity. In both figures, (a) is for the source voltage of

600 V, and @) is for 1000 V, respectively. Converted voltage The example voltage waveforms of transition duration are value of the vertical axis for an attenuation of 30 dR (2OdB of the

shown in Fig.4 and 5, when the radius of curvature of needle coupler and 10 dR of the attenuator) is about 33 Vldiv., and for electrode is 0.1 mm. Fig.4 shows the rising part of transition the horizontal axis is 500 ps/div., respectively. The voltage on a

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(a) source voltage is 600V

(b) source voltage is 1OOOV Fig. 4 Waveforms of rising part of transition duration

in positive polarity

distributed constant line at the load side should rise (fall) to a half amplitude of the source voltage. However, the peak value of the wavefonns was lower because of the influence of the coupling characteristics of the coupled transmission lines. Especially, in Fig.5 @), the peak value was low, because the high frequency components were decreased due to slowdown of transition duration. In this report, the study was limited only to the relative duration time of the voltage transients.

In Fig.4, the voltage rise time of (a), and (b) is about 130 ps, and 170 ps respectively. In Fig.5, the voltage fall time of (a), and (Jo) is about 150 ps, and 260 ps respectively. These waveforms show an acceptable reproducibility. Relationship between source voltage and the transition duration due to an inaeasing of the source voltage from 400 V to 1300 V in

positive and negative polarity are shown in Fig.6. The source voltage polarity and radius of curvature of needle electrode are as follows; (a-p) needle of rd.1 mm in positive, (b-p) needle of ~0.5 mm in positive, (a-n) needle of r-=0.1 mm in negative, and (b-n) needle of r=OS mm in negative polarity. In these figures

we show the minimum, maximum and average of sixty measurements. The voltage rise time of (a-p) and (b-p) is from 116 ps to 245 ps, and from 107 ps to 290 ps respectively. Furthermore, voltage fall time of (a-n) and (b-n) is Corn 142 ps

(b) source voltage is 1oOOV Fig. 5 Waveforms of falling part of transition duration

in negative polarity

to 450 ps, and from 110 ps to 285 ps respectively. The relationship between the discharge voltage and the

transition duration were confirmed from these results. The voltage rise time was slowed down gradually in positive polarity, while the voltage fall time was slowed down remarkably in negative polarity for the A.1 mm needle. It can be consi&red that the cause of the difference in transition duration influenced the distribution of the electrical field in the gap electrode. The distribution of the field was decided by a gap length and a radius of .curvature of needle electrode. The field becomes uniform when the gap length is very short in comparison with the radius of curvature of the needle electrode. In the reverse, it becomes a nonuniform field when the gap length is long. In the case of a sphere electrode and a grounded plan electrode, formative limits of the uniform field are given by (M.Topler’s experimental equation in 1907)

d/(Z*R) < 1.7 l l - * - (2) d/(Z*R) < 0.9 l l l l l (3)

where d is the gap length, R is the radius of sphere electrode, and equation (2) is in positive polarity and equation (3) is in negative polarity. When, these equations apply to our experimental system, the formative gap length of the uniform field is below 0.34 mm in positive polarity, and below 0.18 mm

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0 200 400 600 800 1000 1200 1400

Source voltage [q (a-p) r=O.l mm needle in positive polarity

-..-..-...+-..- ..-..-, i -..-..-,,-..” ..-,, -..-.& ..-..-,.I,, &-.- ..--.-

600

0

--..-..I.. +” -...-.. -f..“.“‘“..-“‘-‘.-..“...-..-..-.~-..-..-,~ I

I I I I I O.ln

200 400 600 800 1000 1200 1400 Source voltage M

(a-n) r=O.l mm needle in negative polarity

600

-.-.J.- ._.,... - .._. +..- _..__ p”-.--- I I I I 1 0.5p

iO0 400 600 800 1000 1200 1400 Source voltage [Vj

(b-p) r=OSmm needle in positive polarity

o : f ; j I I I I 0.5n

200 400 600 800 1000 1200 1400 Source voltage M

(b-n) r=OS mm needle in negative polarity Fig. 6 Relationship between source voltage and the transition durations according as increasing of source

voltage from 400 V to 1300 V. (the duration is rise time in positive polarity, and the duration is fall time in negative polarity)

in negative polarity, where radius of curvature of the needle electrode is, HI.1 mm. Furthermore, the formative gap length is below 1.7 mm in positive polarity, and below 0.9 mm in negative polarity, where the radius of curvature is r=O.S mm. Fig.7.shows a relation between discharging voltage and the gap length in this experimental system. When the radius of curvature of needle electrode was 0.1 mm in negative polarity, the field became nonuniform at more than 1200 V. This voltage agrees approximately with the results of Fig.6 (a-n) where the fall time of transition duration was slowed down remarkably.

It can be said that the discharge gives breakdown directly in the uniform field, while the breakdown in nonuniform field is reached via a complex process of corona discharge (partial discharge). This is one of the cause of the difference of transition duration.

0.25

:

0 200 400 600 800 1000 1200 1400

Discharge Voltage yV] Fig. 7 Relation between discharge voltage aud the gap length

in this experimental system.

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CONCLUSION

The very fast transition duration due to the short gap discharge was investigated in time domain using the distributed constant line system.

As a consequence of the experiment using this measurement system, the voltage rise time was slowed down from about 100 ps to about 300 ps for an increasing of the source voltage from 400 V to 1300 V in positive polarity. While, the voltage fall time was slowed down remarkably from about 130 ps to about 450 ps when experiment was performed in negative polarity for the 0.1 mm needle electrode. The relationship between the discharge voltage and the transition duration was confirmed from these results. That is, the rise time of the transition duration shows similar characteristics in positive polarity, while the fall time was slowed down remarkably in negative polarity when the radius of curvature of needle electrode was 0.1 mm. It seems that one of the cause is the differential distribution of electric field in the gap of electrode.

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