Temperature Controllable Atmospheric Plasma Source - IEEE Xplore

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 6, JUNE 2015

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Temperature Controllable Atmospheric Plasma Source Takaya Oshita, Hiroaki Kawano, Toshihiro Takamatsu, Hidekazu Miyahara, and Akitoshi Okino, Member, IEEE

Abstract— An atmospheric pressure plasma source in which the gas temperature can be accurately controlled from below freezing point up to a high temperature has been developed. In general plasma devices, an electrical discharge is passed through a gas at about room temperature to generate plasma, so the plasma is at a temperature higher than room temperature; moreover, the gas temperature is determined by the discharge condition such as discharge power and plasma gas flow rate, so accurate temperature control is difficult. In the plasma source that has been developed in this research, the gas that is to be supplied to the discharge unit is first cooled using a gas cooler and then heated by a heater. The gas temperature of the produced plasma is measured, and feedback is sent to the heater. Thus, plasma at a desired temperature can be generated. Gas temperature control of the plasma over a range from −54 °C to +160 °C with a standard deviation of 1 °C was realized. Spectroscopic characteristics of generated plasma were investigated. This plasma source/technique will realize that effective plasma is applicable for heat-sensitive materials such as paper, textile, polymer, and especially human tissue. Furthermore, it enables us to generate the plasma at optimal gas temperature for chemical reaction of each plasma treatment. Index Terms— Atmospheric plasma, biomedical electronics, plasma medicine, surface treatment.

I. I NTRODUCTION

A

TMOSPHERIC pressure plasmas have significant advantages in industrial applications compared with conventional low-pressure plasmas, because they do not require a vacuum chamber and an exhaust system [1]. Equipments for plasma generation are simple and low cost. High-speed processing can be performed because of continuous processing and higher density plasmas than conventional low-pressure plasmas. Besides, living bodies and large objects that cannot be put in a vacuum chamber can be processed, so a wider range of processing object is applicable. In particular, because the generation of atmospheric pressure plasma at low temperatures—from room temperature to around 100 °C—has been feasible for a number of years, heat-sensitive objects have been processed. Consequently, atmospheric low-temperature plasmas have been widely used for various applications:

Manuscript received November 14, 2014; revised February 1, 2015; accepted April 26, 2015. Date of publication May 19, 2015; date of current version June 8, 2015. T. Oshita, H. Kawano, H. Miyahara, and A. Okino are with the Department of Energy Sciences, Tokyo Institute of Technology, Yokohama 226-8502, Japan (e-mail: [email protected]; [email protected]; [email protected]; aokino@es. titech.ac.jp). T. Takamatsu is with the Department of Gastroenterology, Kobe University, Kobe 650-0017, Japan (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2015.2428696

cleaning and hydrophilization of the surfaces of processing objects by oxygen included plasmas discussed in [2] and [3]; oxide removal on metal surfaces with hydrogen included plasmas discussed in [4]; sterilization of various bacteria [5], [6]; and surface coating [7]. In the case of processing heat-sensitive objects such as living bodies and polymer materials, for high-speed processing without damage to the target, dense but low-temperature plasmas were required. In atmospheric pressure, low-temperature plasma means that the gas temperature of generated plasma is low. In this paper, we call the gas temperature of the plasma the plasma gas temperature. Whereas low-plasma gas temperatures are required in the processing of heatsensitive objects, when processing objects have no limitation of temperature, to promote chemical reactions with reactive species high-temperature plasma is suitable. Therefore, the plasma gas temperature should be controlled to an optimal value depending on target objects. However, the plasma gas temperature is often proportional to input power to the plasma and it is not controlled precisely. For example, to generate low gas temperature plasma, a method such as limiting the input power to the plasma or increasing the gas flow rate is used. However, the energy per unit volume provided to the plasma is reduced by these method, and treatment effectiveness of the produced plasma decreases. As a plasma source that can generate lower temperature plasmas without reducing the input power to the plasma, the cryoplasma jet was developed [8]. In this plasma source, gas supplied from a cylinder is cooled by liquid nitrogen, after which plasma is generated. Using this plasma source, helium cryoplasma jet (5–296 K) can be generated [9]. Using this method, the plasma gas temperature can be controlled without varying input power to the plasma. In this paper, we developed a new plasma source in which the plasma gas temperature can be controllable not only below room temperature but also above room temperature without varying the input power to the plasma (patent number in Japan: 4611409). Moreover, the plasma gas temperature can be controlled to a desired value accurately. In this paper, detail of developed plasma source and spectroscopic characteristics of generated plasma will be reported. II. M ATERIALS AND M ETHODS A. Temperature Controllable Plasma Jet Fig. 1 shows comparison between general plasma source and the developed plasma source. The general plasma source generates plasma from a room-temperature gas, so gas

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Fig. 3.

Experimental setup for spectroscopic measurement.

Fig. 1. Comparison between (a) general plasma source and (b) temperature controllable atmospheric plasma source.

Fig. 4. Relationship between the input power to heater and the plasma gas temperature.

B. Spectroscopic Measurement System

Fig. 2.

Detail of discharge unit.

temperature of the generated plasma is higher than room temperature. On the other hand, in the developed temperature controllable plasma source, gas supplied from a cylinder is cooled by a gas-cooling device that uses liquid nitrogen (−196 °C), after which the gas is heated to a desired temperature by a heater, and then plasma is generated. Feedback on the gas temperature of the generated plasma is sent to the heater, and the gas temperature of the plasma can be controlled to the desired value. Fig. 2 shows the detail of the discharge unit. Ring-shaped copper electrodes spaced 10-mm apart were provided at the periphery of a glass tube with a 3-mm i.d. and a 5-mm o.d., and a 9-kV electric power at 16 KHz was applied. In testing, helium plasma was generated with 10 slm of the gas flow rate. In the case of generating plasma from 22 °C helium gas by this discharge unit, the gas temperature raised up to 36 °C. slm, ring-shaped copper electrodes spaced 10-mm apart were provided at the periphery of a glass tube with a 3-mm i.d. and a 5-mm o.d., a 9-kV electric power at 16 KHz was applied, and thus a plasma jet was generated. Before generating plasma, the gas temperature was 22 °C. After generating plasma, the gas temperature was raised up to 36 °C. From heat capacity of helium and temperature change, calculated power to heat plasma was 2.1 W.

Spectroscopic characteristics of the plasma whose gas temperature is controlled by developed plasma source were investigated by spectroscopic method. Fig. 3 shows experimental setup for the spectroscopic measurements. A multichannel spectrometer (Maya 2000 PRO and HR4000, Ocean Optics Inc., FL, USA) and a 50-cm focal length Czerny–Turner monochromator (wavelength resolution: 0.027 nm) equipped with a photo multiplier (R928, Hamamatsu Photonics Company, Hamamatsu, Japan) were used. Emission from the plasma, axially viewed, was observed with a 5-mm distance to an entrance of an optical fiber on the other side of a quartz glass 1-mm thick. Signals from the monochromator were monitored and recorded with a digital oscilloscope (TDS-680B, Sony/Tektronix Corporation, Tokyo, Japan). III. R ESULT AND D ISCUSSION A. Temperature Control of Helium Plasma The gas temperature of the generated plasma was controlled by varying the input power to the heater and it was measured with a thermocouple with a 2-mm distance to the plasma outlet. Fig. 4 shows the relationship between the input power to the heater and the gas temperature of the generated plasma. The gas temperature right after cooling by the liquid nitrogen was about −150 °C but it rose to −69 °C by heat loss between the gas-cooling device and the plasma source until it reached the outlet. Therefore, when the heater was not used, the gas temperature after plasma generation was −54 °C. Under this condition, 10 μL of water droplet was frozen in 10 s because the plasma gas temperature was below freezing point; the ice produced by this plasma irradiation is shown in Fig. 5.

OSHITA et al.: TEMPERATURE CONTROLLABLE ATMOSPHERIC PLASMA SOURCE

Fig. 5.

Water droplet frozen by 10 s of plasma irradiation.

Fig. 6.

Example of temperature control.

As the input power of the heater was raised, the plasma gas temperature rose linearly, rising to 160 °C at 60 W. Fig. 6 shows example of temperature control. When the input power to the heater was changed, the plasma gas temperature started to change, and then it became constant in 10–15 min. With the device we have developed, control of the plasma gas temperature over a range from −54 °C to 160 °C with a standard deviation of 1 °C was achieved. B. Influence on Discharge Power of the Plasma Gas Temperature In the temperature controllable plasma source, the plasma gas temperature is controlled without intended control of the discharge power. However, density and flow rate of the gas change with change in the gas temperature, so the discharge power has a potential to change. Then, the discharge power was measured by varying the plasma gas temperature. The discharge power was calculated from the voltage between the discharge electrode and the current calculated from the voltage of shunt resistance. Fig. 7 shows the relationship between the plasma gas temperature and the discharge power. As the plasma gas temperature increased, the discharge power was decreased. In this experiment, a constant voltage was

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Fig. 7. power.

Relationship between the plasma gas temperature and the discharge

Fig. 8.

Visible light spectrum of helium plasma.

applied to the plasma source, so it seems that the impedance of the plasma was increased with depression of the gas density. In the case of the conventional plasma source, the plasma gas temperature increased with the input power to the plasma. Therefore, developed plasma source possess properties differing from conventional plasma source. C. Spectroscopic Characteristics Figs. 8 and 9 show visible and infrared light spectra of helium plasma. The plasma gas temperature was controlled to 0 °C, 60 °C, and 120 °C. As shown in Figs. 8 and 9, the helium atomic emissions and oxygen atomic emissions were observed. The oxygen atomic emissions were derived from oxygen in air because the plasma outlet was open to

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Fig. 11.

Fig. 9.

OH rotational spectrum observed from 40 °C plasma.

Infrared light spectrum of helium plasma.

Fig. 12. Relationship between the plasma gas temperature measured by thermocouple and the OH rotational temperature.

Fig. 10. Relationship between the plasma gas temperature and the electron number density.

the atmosphere. As the plasma gas temperature was raised, the emission intensities of these atomic emissions decreased. Fig. 10 shows the relationship between the plasma gas temperature and the electron number density. The electron number density was measured using Stark broadening of Hβ line (486.133 nm) spectrum [10]. The electron number density was also decreased with the plasma gas temperature rise, and it was between 0.24 × 1015 and 0.34 × 1015 cm−3 . The cause of these decreasing could be attributed to reduction in the plasma gas density associated with the plasma gas temperature rise. This agrees with the result that the discharge power decreased with the plasma gas temperature rise. The gas temperature measured by thermocouple has the potential to be influenced by the effect of the electric field in the plasma, so the gas temperature should be measured

from many side. Therefore, to measure plasma gas temperature excluding the effect of the electric field, OH rotational temperature was measured by spectroscopic method. Under high particle density condition like an atmospheric pressure, the rotational temperature is in equilibrium with translational temperature due to collision between neutral molecules and excited molecules. Therefore, the gas temperature can be estimated by the rotational temperature. As shown in Fig. 11, the OH rotational spectra derived from water in atmosphere are observed when the plasma outlet is open to the atmosphere. Using three spectra (308.33, 308.52, and 308.73 nm), OH rotational temperatures were obtained by Boltzmann plot. Fig. 12 shows the relationship between the plasma gas temperature measured by thermocouple and the OH rotational temperature. The rate of variation of the OH rotational temperature was almost same as that of the plasma gas temperature measured by thermocouple, but the OH rotational temperatures were 6 °C–28 °C higher than the plasma gas temperature measured by thermocouple. In spectroscopic measurement, emission from the plasma was observed from axial direction, so it would appear that emission from the inside of the plasma outlet was integrated and the higher temperature was obtained. Fig. 13 shows the relationship between the plasma gas temperature and the helium excitation temperature. The excitation temperatures were obtained using two lines method using two helium emission lines (447.15 and 501.57 nm). As the

OSHITA et al.: TEMPERATURE CONTROLLABLE ATMOSPHERIC PLASMA SOURCE

Fig. 13. Relationship between the plasma gas temperature and the helium excitation temperature.

plasma gas temperature was raised, the excitation temperature increased, and it was 900 °C–1500 °C higher than the plasma gas temperatures. Thus, it was confirmed that the generated plasma is nonthermal equilibrium plasma. And nonequilibrium has increased with the gas temperature.

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for chemical reaction of each plasma treatment. For example, high gas temperature plasma increases hydrophilization effect on metal surface by plasma treatment [13]. Thus, by plasma treatment at higher gas temperature not to give a thermal damage to the target, higher treatment effect can be obtained. In recent years, the use of atmospheric pressure plasma has attracted interests in medical field, and a broad range of research has been conducted into directly sterilizing wounds, promoting blood clotting, activating cells, and so on [14]. When a treatment that directly irradiates a living body with plasma is implemented, it is unacceptable for the temperature of an exposed region to exceed 43 °C, the denaturation temperature of proteins, for a long period. Therefore, reliable temperature control is required. This problem can be solved using the method. This plasma source/technique will realize that effective plasma is applicable for heat-sensitive materials such as paper, textile, polymer, and especially human skin. We think this will be a must-use technology for field of plasma medicine. ACKNOWLEDGMENT

IV. C ONCLUSION

The authors would like to thank Plasma Concept Tokyo, Inc., Tokyo, Japan, for providing the power source for this paper.

Atmospheric pressure plasma source in which the gas temperature can be accurately controlled from below freezing point up to a high temperature have been developed. With the device we have developed, control of the plasma gas temperature over a range from −54 °C to 160 °C with a standard deviation of 1 °C was achieved. By controlling the plasma gas temperature using developed device, as the plasma gas temperature increased, the discharge power was decreased. In spectroscopic measurement, as the plasma gas temperature increased, intensities of atomic emissions from the plasma and the electron number density also decreased. This agrees with the result that the discharge power decreased with the plasma gas temperature rise. The OH rotational temperatures were higher than the plasma gas temperature measured by thermocouple, but rate of variation was almost same. The helium excitation temperatures were 900 °C–1500 °C higher than the plasma gas temperatures, thus, it was confirmed that the generated plasma is nonthermal equilibrium plasma. In processing of a material using conventional atmospheric pressure plasma, to prevent material temperature’s rising, apart from reducing the output power of plasma, cooling the material itself is often performed [11], [12]. However, this kind of processing is difficult to put into practical use from the viewpoint of heat removal and cost. In our plasma source, the gas temperature of the plasma can be accurately controlled independently of the discharge power. Therefore, all kind of materials can be irradiated with the plasma that are weak in temperature rise, and the plasma irradiation can be performed under the optimum temperature for the process requires. As a result, plasma irradiation to materials with low melting points, which are difficult to treat with conventional devices, is possible. Furthermore, developed plasma source enables us to generate the plasma at optimal gas temperature

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R EFERENCES

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[13] T. Oshita, T. Takamatsu, R. Sasaki, N. Nakashima, H. Miyahara, and A. Okino, “Development of temperature controllable atmospheric plasma jet source,” presented at the Plasma Conf., 2011, Ishikawa, Japan, paper P22036-P. [14] G. Lloyd, G. Friedman, S. Jafri, G. Schultz, A. Fridman, and K. Harding, “Gas plasma: Medical uses and developments in wound care,” Plasma Process. Polym., vol. 7, nos. 3–4, pp. 194–211, Mar. 2010.

Takaya Oshita was born in Tokyo, Japan, in 1987. He received the B.E. degree from the Tokyo University of Science, Tokyo, in 2011, and the M.E. degree in energy sciences from the Tokyo Institute of Technology, Yokohama, Japan, in 2012, where he is currently pursuing the D.Eng. degree.

Hiroaki Kawano was born in Oita, Japan, in 1992. He received the B.E. degree in electrical and electronic engineering from the Oita National College of Technology, Oita, in 2014. He is currently pursuing the M.E. degree with the Tokyo Institute of Technology, Yokohama, Japan.

Toshihiro Takamatsu was born in Hokkaido, Japan, in 1988. He received the B.E. degree from the Tokyo University of Science, Tokyo, in 2010, and the M.E. and D.Eng. degrees in energy sciences from the Tokyo Institute of Technology, Yokohama, Japan, in 2011 and 2014, respectively. He is currently a Post-Doctoral Researcher with Kobe University, Kobe, Japan. His current research interests include atmospheric plasma engineering and plasma medicine.

Hidekazu Miyahara was born in Tokyo, Japan, in 1977. He received the B.E. degree in applied chemistry from Chuo University, Tokyo, in 2000, and the M.E. and D.Eng. degrees in energy science from the Tokyo Institute of Technology, Yokohama, Japan, in 2002 and 2005, respectively. He was a Post-Doctoral Researcher with the Tokyo Institute of Technology, from 2005 to 2009, where he is currently an Assistant Professor. He is also a Chief Executive Officer with Plasma Concept Tokyo, Inc., Tokyo. His current research interests include atmospheric plasma engineering and analytical chemistry. Akitoshi Okino (M’95) was born in Kyoto, Japan, in 1965. He received the B.E. and M.E. degrees in applied physics from Osaka University, Osaka, Japan, in 1989 and 1991, respectively, and the D.Eng. degree in nuclear engineering from the Tokyo Institute of Technology, Yokohama, Japan, in 1994. He was a Visiting Professor with the Department of Chemical and Biomolecular Engineering, University of California at Los Angeles, Los Angeles, CA, USA, from 2006 to 2007. He established Plasma Concept Tokyo, Inc., Tokyo, Japan, in 2008. He is currently an Associate Professor with the Department of Energy Sciences, Tokyo Institute of Technology. His current research interests include the applications of thermal and nonthermal atmospheric plasmas for analytical-chemistry and medical fields.