Generation of a Strong Pressure Wave 10th Asia-Pacific Conference on Plasma Science and Technology and 23st Symposium on Plasma Science for Materials July 5-8, 2010, Jeju Island, Korea PC 257 Generation of a Strong Pressure Wave in Sea Water by a Pulsed Arc Discharge Sooseok Choi1, Seok-Geun Lee2, Kyoung-Jae Jung2 and Y.S. Hwang1, 2 Center for Advance Research in Fusion Reactor Engineering (CARFRE) at Seoul National University, Seoul, Korea 2) Department of Energy System Engineering, Seoul National University, Seoul, Korea E-mail : mouse4@snu.ac.kr * This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST). (No. 2010-0001827)

Circuit Diagram for the Pulse System Charging Switch Discharging Switch I total Sea Water Chamber (Φ100 / L600, Tube) High Voltage Power Supply ( ~ 20 kV) High Voltage Power Supply ( ~ 0.5 ㎌) V I loss = I total – I load I load

Research Motivation and Objectives 1. Before Discharge Energy Transfer from the Capacitor to Discharge Channel Expansion of Plasma-Water Interface 2. At Discharge Propagation and Attenuation of the Pressure Wave Identify the relationship between electrically stored energy in the capacitor and strength of pressure wave generated by a pulsed arc discharge under sea water. Examine the effects of electrode design such as electrodes gap distance and cathode material on the pulsed arc discharge. Find out an efficient energy delivery condition to generate a strong pressure wave in the inherently conductive sea water. 3. After Discharge

Experimental Setup for Measurements Rogowski Coil 1 (Total Current) Positively Charged High Voltage Capacitor Discharge Switch V Voltmeter (Load Voltage) High Voltage Anode Sea Water Chamber Discharge Channel 100 mm Pressure Gauge Viewport Grounded Cathode Camera Rogowski Coil 2 (Load Current)

Images from the High Speed Camera Time (㎲) 50 75 High Speed Images Shutter off Frame #26424 Frame #26423 Frame #26425 50 ㎲ is too short to observe the evolution of the pulsed discharge. Therefore, fixed exposure time of a second is used to investigate the discharge structure (D.S.) in photograph measurements.

Experimental & Photographic Conditions Pulsed Discharge Camera Setting Capacitance: 0.1 ~ 0.5 ㎌ (increasing by 0.1 ㎌) Applied voltage: 10 ~ 20 kV (increasing by 2 kV) Electrodes gap distance: 1 ~ 10 mm (increasing by 1 mm) Cathode material: W, W-Ce, W-La, W-Th Used camera & lens: Canon 50D, Pentax 200 mm Exposure time: 1 second Adjustment of luminous intensity: F8, F8 with ND400 Filter, F22 with ND400 Filter 0.5 ㎌, 20kV, 5mm, W 1/400 1/8 F8 without filter F8 with ND400 filter F22 with ND400 filter

V, I and P Measurements: ex) 0.3㎌, 15 kV, 5 mm, W V-I Curve Pressure Wave Electrical noise caused by the pulsed discharge Max. loss current Propagation time It was measured that a considerable difference between total current and load current because of an current leakage to the grounded chamber wall through an inherently conductive sea water. Sine the pressure sensor was located 100 mm away from the discharge axis and the speed of sound wave under water is 1500 m/s, the signal was detected around 65 ㎲ after burst of the discharge.

Effects of Input Voltage: D.S. (0.5㎲, 5mm, W) 12 kV 14 kV 16 kV 18 kV 20 kV -

Effects of Input Energy: Pressure (5mm, W)

Effects of Gap Distance: D.S. (0.5㎲, 18kV, W) 2 mm 5 mm 8 mm

Effects of Gap Distance: Pressure (0.5㎲, W) In the photographs for discharge structure, an arc channel connects between two electrodes when their gap distance is close and a strong pressure wave is measured at the same time, though the same energy is supplied. It is supposed that almost current will pass through the arc channel and large portion of electrical energy will be dissipated to the plasma when its resistance decreased significantly compared with that of sea water surrounded.

Effects of Gap Distance: I-V (0.5㎌, 15kV, W) 5 mm for Electrodes Gap Distance 2 mm for Electrodes Gap Distance Almost half of total current (0.5 kA) leaks to the surrounding sea water at peak current point in the case of electrodes gap distance of 5 mm. On the other hand, almost same curves for the total current and the load current of 4.0 kA in maximum are measured after the start of under-damping in the case of 2 mm.

Effects of Gap Distance: Resistance (0.5㎌, 15kV, W) Under-damping start Arc build-up The RLC circuit goes to the under-damping after an arc channel build-up and the load resistance is suddenly dropped under 2 Ω.

Effects of Cath. Material: D.S. (0.5㎲, 16kV, 5mm) W (4.5 eV for Work Function) Th (3.4 eV) added W La (3.5 eV) added W Ce (2.9 eV) added W

Effects of Cath. Material: Pressure (0.5㎲) Voltage Gap Distance W W-Th W-La W-Ce Pressure [Mpa] 15 kV 2 mm 3.035 3.089 3.199 3.567 5 mm 1.049 1.177 1.141 1.217 20 kV 5.529 5.238 6.161 6.256 2.573 2.263 2.698 2.819 The measurements were repeated three times and their averaged values are presented in the table, because measured peak pressures showed a fluctuation though a fixed voltage and electrodes gap distance were maintained. Although an arc channel was easily developed, slightly increased pressure waves were measured when a low work function material was added to the cathode.

Conclusions An pulsed arc discharge was successfully generated under inherently conductive sea water. It is basically affected by electrically stored energy in the capacitor that the strength of the pressure wave caused by the pulsed discharge. The shorter electrodes gap distance shows the stronger pressure wave, although the same input energy is used. An arc channel is easily developed and a slightly increased pressure is measured when a low work function material is added to the cathode with a fixed input energy and an electrodes gap distance.