ELECTRON ENERGY RELAXATION EFFECT ON THE DYNAMIC CHARACTERISTIC OF ARGON PLASMA ARC

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ELECTRON ENERGY RELAXATION EFFECT ON THE DYNAMIC CHARACTERISTIC OF ARGON

PLASMA ARC

H. Shindo, S. Imizu, T. Inaba

To cite this version:

H. Shindo, S. Imizu, T. Inaba. ELECTRON ENERGY RELAXATION EFFECT ON THE DYNAMIC CHARACTERISTIC OF ARGON PLASMA ARC. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-259-C7-260. �10.1051/jphyscol:19797127�. �jpa-00219099�

JOURNAL DE PHYSIQUE CoZZoque C7, suppte'ment au n07, Tome 40, J u i l l e t 1979, Page C7- 259

ELECTRON ENERGY RELAXATION EFFECT ON THE DYNAMIC CHARACTERISTIC OF ARGON PLASMA ARC

H. Shindo, S. Imizu and T. lnabax.

xFacuZtg of Engineering, Hiroshima U ~ ~ e r s i t y , Hiroshima 730, Japan.

Central Research I n s t i t u t o f E l e c t r i c Power I n d u s t r y , Tokyo 180, Japan.

1.Introduction:There have been, so far, many inves- Table I. Typical plasma parameters in the central core of the arc at 0.1 atm pressure.

tigations on arc dynamics, since it is important for circuit breakers. However, little work has been reported dealing with the energy relaxation effect of electrons on dynamic characteristics. The relax- ation time of the electron energy is generally a strong function of the electron density and tempe- rature. Therefore, in low electron densities and temperatures the energy relaxation time can proba- bly dominate the arc dynamic characteristic.

The objective of the present study is to investi- gate experimentally the influence of the energy re- laxation effect on dynamic characteristics. Argon arcs at 0.1 atm pressure, which were in the region of the electron density 5 . 1 0 ~ L 1 0 ~ ~ ( c m - ~ ) and the electron temperature 0.72-1.08(eV), were investi- gated.

2.Experiments:The arc source employed in this ex- periment is a wall-stabilized type arc which has an arc radius of 5 mm and a length of 7 cm. The inter- electrode voltage was traced when a small current was injected into the arc column burned at various constant DC currents. A small current was injected through a thyristor switch from a current source parallel with the main current source. The time history of the plasma parameter was obtained by spectroscopic measurements. In the measurements, argon gas seeded with hydrogen(0.1 % volume ratio) was partly used to determine the electron density from'the H line width.

B

3.Results and Discussions: Typical plasma parame- ters in the central core of the arc are listed in Table I for our experimental conditious. The arc current I is the value before the current injec-

s

tion. The electron density N and temperature Te were experimentally determined; namely N from the H line width and T from the absolute continuum

B

radiation strengthll]. The gas temperature T was obtained through a kinetic formula using the meas- g ured density and temperature of electrons.

The relaxation time -rKE for the electron kinetic energy is obtained by the following formula,

where the value reported by Devoto[2] was used for the integral i2. The relaxation time T~~ for the ionization is also calculated by the following equation,

where the value found in ref. [3] was used for the ionization cross section. It is found in Table I that T~~ is more than one order higher than T

KE in the same current, and that both relaxation times are strongly dependent on the electron density and temperature.

Figure 1 shows the time history of the ArI 6965 line intensity, where the value of the injected current is 20 % of the I value. The line intensity at first decreases from the pre-steady state value just after the current injection, resulting from the increase in the electron temperature. The time scale for changing the electron temperature is con- sistent with the value of T~~ in Table I.

Furthermore, the line intensity commences to in- crease after 50-100 (ps) from the current injec- tion. This time corresponded to the increase in the electron density, which was confirmed by the time variation of the H line profile. In Fig.1, the ionization relaxation time becomes longer at lower B arc currents. In the case of 19=30 (A), the line intensity does not seem to saturate in the obser-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797127

Fig.1, Time history of the ArI 6965 line inten- sity. Arrows indilcate the time of the current injection. The time scale is 100 (ps/div).

vation time, but it required more than 1 (ms). The figure makes clear that the ionization relaxation time becomes longer at lower electron densities and temperatures. This tendency is consistent with that in Table I. However, the absolute values differ from the values of T~~ in Table I by a factor of 4 or more. The discrepancy may be due to the uncertainty in the measured values of electron temperatures, since T~~ is a strong function of the temperature Te. For example, the difference of 10 % in Te gives an error in T~~ by a factor of 3.

As mentioned above, the time for changing the electron temperature is short, but the 'electron density changes more gradually. These facts verify that an assumption of LTE is not valid for analy- ses of dynamic characteristics in low densities and temperatures. The validity of the LTE assump- tion was verified for the steady state at arc cur- rents more than 150(A)[l]. The verification was made not only by spectroscopic measurements, but also by comparisons between the measured and cal- culated "ER-I/R" characteristics. The measured

"ER-I/R1' characteristics were in agreement with those calculated using the transport properties

reported by Devoto[4]. It can be concluded, from the above discussion, that LTE criteria for dynamic characteristics are more severe than those for static characteristics.

Figure 2 shows the time dependences of the inter- electrode voltages in the cases of I? 30 and 150

(a) .Is=30 (A) The time scale is 100(ps/Div)

.

(b) .Is=l50 (A) rhe time scale is 50 (ps/Div)

.

Fig.2, Voltage response. Arrows indicate the time of the current injection.

The voltage response is certainly effected by a long time constant for the ionization.In the case of I =150(A), the voltage transfers to the next .steady state in 400(ps). The transfering time is

almost the same as that of the ArI line intensity in Fig.l(c). The voltage in the case of Is=30(A) still does not reach the next steady state in the observing time. This behavior also corresponds to the response of the ArI line intensity in Fig.l(a), The later stage of the voltage response is found to be characterized by the relaxation time for the ionization.

4.Conclusion:Arc dynamic characteristics were in- vestigated from the point of the electron energy relaxation time. The time for changing the elec- tron temperature is short,but the electron density changes more slowly in low electron temperatures.

As a result,the voltage response is effected by the long time constant for the ionization.

5. References

[l] .H .Shindo,T. Inaba,SImazu:Trans

.

e,

1(1979), in Japanese.

[2].R.S.Devoto:Phys.Fluids

16,

[3].A.von Enge1:Ionized Gases,P57,Clarendon Press Oxford (1965)

.

[4].R.S.Devoto:ARL Report No.71-0075, Appendix-C (1971)

.