T. Sakuta, K.C. Paul, M. Katsuki, and T. Ishigaki*

Department of Electrical and Computer Engineering, Kanazawa University
Kodatsuno 2-40-20, Kanazawa 920, Japan
sakuta@t.kanazawa-u.ac.jp

*National Institute for Research in Inorganic Materials
Namiki 1-1, Tsukuba-shi, Ibaraki 305, Japan.

An argon/hydrogen (argon: 89% molar) pulse-modulated radio-frequency (rf) inductively coupled plasma (ICP) was generated, for the first time of its kind, at adequate power level (active plasma power up to about 15 kW) which is sufficient enough for studying contamination effects on properties of gas circuit breaker (GCB) arc-quenching media as well as for materials processing. A solid state power source, which supplies the electric power at nominal frequency of 1 MHz, was used as the pulsing generator. Experimental measurements were carried out to store the transient responses of ArI line intensity (at 751 nm wavelength) and current intensity for the supplied pulsing signal. Temporal ICP responses were investigated for different operating conditions: pressure from 200 torr to 760 torr, pulsing on-time from 2 ms to 20 ms, and plate power (with 90% matching efficiency) from 11 kW to 17 kW.

During the pulsing on-time, an overshoot was noticed in both ArI line intensity and current signal. This experimental prediction is in agreement with the theoretical prediction of Mostaghimi et al [1]. Before on-pulsation, plasma was at steady-state condition with low power level. Just after turning-on of the pulsing signal, the plasma was less conductive than it would have been at the new steady-state condition to dissipate the higher plasma power. So, the electromagnetic fields had to rise sharply and, thus, the current. With time the plasma conductivity increased by temperature rise and the fields/current decreased toward steady-state value. Same kind of explanation is applicable in the other way around, i.e., for off-pulsation. At 200 torr, average exciting temperature of the Ar atom varied between 5000 to 14,000 K corresponding to the on-and-off (5 ms on-time with 50% duty factor) of the pulsing signal. ArI line intensity changed considerably with on-time and steady-state was not achieved with 2 ms on-time. ArI line intensity also increased with pressure for obvious reason of augmented particle density [Fig.1], and the response or relaxation time of the ICP was higher.

[1] J. Mostaghimi, K.C. Paul, and T. Sakuta, `Transient response of the radio frequency inductively coupled plasma to a sudden change in power', J. Appl. Phys., Feb. issue, 1998.