Description Plasma generating apparatus and plasma generating method Technical Field [0001] The present invention is related to a plasma-generating apparatus which generates plasma in liquid, in particular a liquid-treating apparatus which treats the liquid by generating plasma. Background Art [0002] A conventional liquid-treating apparatus using a high-voltage pulse discharge is described in, for example, Patent Document 1. Fig. 10 shows a configuration view of a conventional sterilizing apparatus described in Patent Document 1. [0003] The sterilizing apparatus 1 shown in Fig. 10 comprises a discharge electrode 6 including a pair of a columnar high-voltage electrode 2 and a plate-shaped ground electrode 3. The high-voltage electrode 2 is covered with insulator 4 except for an end face of a tip portion 2a, to form a high-voltage electrode portion 5. The tip portion 2a of the high-voltage electrode 2 and the ground electrode 3 are opposed to each other with a predetermined gap, being immersed in to-be-treated water 8 within a treatment vessel 7. The high-voltage electrode 2 and the ground electrode 3 are connected to a power supply 9 which generates high-voltage pulses. The discharge is made by applying negative high-voltage pulses of 2 kV/cm to 50 kV/cm and 100 Hz to 20 kHz between both electrodes. Bubbles 10 of steam and a jet flow 11 caused by bubbles 10 are generated by evaporation of water with energy of discharge and vaporization involved by a shock wave and a jet flow 11 is generated by the bubbles 10. Plasma generated around the high-voltage electrode 2 generates OH, H, 0, 02~, O", and H202 to destroy microorganism and bacteria. [0004] Similarly, Patent Document 6 proposes a method for purifying liquid wherein the liquid is boiled and vaporized to form bubbles and the vaporized substance is ionized (turned into plasma) within the bubbles to form ions and the ion species in the plasma are penetrated and diffused in the liquid. Patent Document 6 describes that, in order to generate plasma, high-voltage pulses having a maximum voltage of about 1 kV to 50 kV, repeated frequencies of 1 kHz to 100 kHz and a duration of 1 us to 20 us, are applied to an electrode pair of high-voltage electrodes. [0005] Another conventional liquid-treating apparatus is described in Patent Document 2. Patent Document 2 discloses that a liquid-treating apparatus described in this document can reduce an applied voltage by interposing bubbles, which are supplied from outside, between electrodes in liquid, whereby power consumption can be reduced. Similar techniques are disclosed in Patent Document 3 and Patent Document 4 and Patent Document 5. Prior Arts Documents Patent Documents [0006] Patent Document 1: JP 2009-255027 A Patent Document 2: JP 2000-93967 A Patent Document 3: JP 2003-62579 A Patent Document 4: JP 2010-523326 A Patent Document 5: JP 3983282 B Patent Document 6: JP 2007-207540 A Summary of Invention Problems to be Solved by the Invention [0007] However, there was a problem that generation efficiency of plasma was low in the above-described conventional apparatuses, requiring a long time of period for treating liquid. Further, when the plasma is generated in bubbles formed by vaporization of the liquid, it is necessary to input high electrical power to vaporize the liquid since the electrical power was lost in the liquid, which requires a large-scale power supplying apparatus. Specifically, the power supplying apparatus is required to have ability of supplying electricity power of 4000 W or more in order to vaporize water, considering the loss. One embodiment of the present invention is to solve the above problems and provides a plasma-generating apparatus and a plasma-generating method which generate plasma efficiently so as to, for example, enable liquid to be treated in a short period of time and/or with a low power. [0008] A plasma-generating apparatus which is one embodiment of the present invention includes: a first electrode of which at least a part is placed in a treatment vessel that is to contain liquid, a second electrode of which at least a part is placed in the treatment vessel, a bubble-generating part which generates a bubble in the liquid when the liquid is contained in the treatment vessel such that at least surface where conductor is exposed, of a surface of the first electrode which surface is positioned in the treatment vessel, is positioned within the bubble, a gas-supplying apparatus which supplies gas in an amount necessary for the bubble-generating part to generate the bubble from the outside of the treatment vessel to the bubble-generating part, a power supply which applies voltage between the first electrode and the second electrode, and a control apparatus which controls one or both of the gas-supplying apparatus and the power supply such that the voltage is applied between the first electrode and the second electrode when the at least surface where the conductor is exposed, of the first electrode is positioned within the bubble. Effect of the Invention [0009] The plasma-generating apparatus according to one embodiment of the present invention can generate plasma efficiently and thereby enables the liquid to be treated with low power and/or in a short period of time, reducing loss of power consumption. Brief Description of Drawings [0010] Fig. 1 is a configuration view of a liquid-treating apparatus in a first embodiment of the present invention. Fig. 1-2 is a sectional side view wherein vicinity of an opening portion of an electrode in the first embodiment of the present invention is enlarged. Fig. 1-3 is a photograph showing bubbles generated in the first embodiment of the present invention. Fig. 2 is a graph showing spectral characteristics of plasma generated in the first embodiment of the present invention. Fig. 3 is a graph showing change over time in transmittance of an aqueous indigocarmine solution in the first embodiment of the present invention. Fig. 4 is a configuration view of a liquid-treating apparatus in a second embodiment of the present invention. Fig. 4-2 is a sectional side view wherein vicinity of an opening portion of an electrode in the second embodiment of the present invention is enlarged. Fig. 4-3 is a photograph showing bubbles generated in the second embodiment of the present invention. Fig. 5 is a graph showing relationship between complete decolorization time of an aqueous indigocarmine solution and a distance between an end face of insulator and an end face of the second electrode in the second embodiment of the present invention. Fig. 6 is a configuration view of a liquid-treating apparatus in a third embodiment of the present invention. Fig. 7 is a graph showing change over time in transmittance of the aqueous indigocarmine solution in the third embodiment of the present invention. Fig. 7-2 is a graph showing relationship between complete decolorization time of the aqueous indigocarmine solution and the distance between the end face of the insulator and the end face of the second electrode in the third embodiment of the present invention. Fig. 7-3 is a photograph showing bubbles generated in the third embodiment of the present invention. Fig. 8 is a graph showing spectral characteristics of plasma generated in the third embodiment of the present invention. Fig. 9 is a graph showing change over time in transmittance of the aqueous indigocarmine solution in the first embodiment of the present invention when power is varied. Fig. 10 is a configuration view of a conventional waste water treatment apparatus using high-voltage pulse discharge. Fig. 11 is a graph showing relationship between complete decolorization time of the aqueous indigocarmine solution and a flow rate of gas in the second embodiment of view of the resistance detection device as the bubble detection device when the bubble is generated. Fig. 20 is a schematic view showing another example of a bubble-detecting method wherein a light-emitting element and a light-receiving element are used as the bubble detection device. Fig. 21(a) is a graph showing change in light-emitting voltage of the light-receiving element of a pair referred to as "No. 1" in Fig. 20, and Fig. 21(b) is a graph showing change in light-emitting voltage of the light-receiving element of a pair referred to as "No. 2" in Fig. 20, and Fig. 21(c) is a graph showing change in light-emitting voltage of the light-receiving element of a pair referred to as "No. 3" in Fig. 20. Fig. 22 is a configuration view of a liquid-treating apparatus in a sixth embodiment of the present invention. Fig. 23 is a graph showing an emission spectrum of plasma light. Fig. 24 is a graph showing change in ratio of a Na spectrum to a H spectrum in the emission spectrum of the plasma light when conductivity of the liquid is changed. Fig. 25 is a graph showing an emission spectrum of plasma light. Fig. 26 is a configuration view of a liquid-treating apparatus of a variation of the sixth embodiment of the present invention. Fig. 27 is a graph showing bubbles in a seventh embodiment of the present invention. Fig. 28(a) is a graph showing relationship between a direction of an opening portion of the first electrode and an electrode coverage (at a flow rate of 100 ml/min) and Fig. 28(b) is a graph showing relationship between the direction of the opening portion of the first electrode and the electrode coverage (at a flow rate of 500 ml/min) and Fig. 28(c) is a graph showing relationship between a direction of the opening portion of the first electrode and the electrode coverage (at a flow rate of 2000 ml/min). Fig. 29 is a graph showing relationship between the direction of the opening portion of the first electrode and a bubble size. Fig. 3 0 is a schematic view showing ra and rb for determining the bubble size. Fig. 31 is a graph showing relationship between an inner diameter of insulator (alumina ceramic) and the electrode coverage. Fig. 32 is an equivalent circuit diagram of the device structure of the plasma generation part. Fig. 33 is a graph showing voltage applied to the device structure when R1>R2 in the equivalent circuit diagram. Fig. 34 is a graph showing voltage applied to the device structure when R1=R2 in the equivalent circuit diagram. Fig. 35 is a graph showing voltage applied to the device structure when R1