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Huazhong university an all-solid-state pulsed power generator for non-thermal plasma discharge applications


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  • 1. Poster Session An All-Solid-State Pulsed Power Generator for Non-Thermal Plasma Discharge Applications1 Kefu Liu, Jiang Qiu, Houxiu Xiao, Qiong Hu, Shengguo Xia Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China, Phone: 86-27-87541346, Fax: 86-27-87540937, E-mail: – Non-thermal plasma discharge requires in a low temperature background gas. Plasma dis-a pulsed power generator with a high repetition charge reactor is designed as a coaxial stricture con-rate, short pulse width, fast rise voltage and a long sisting of an outer and a center electrode. Typicallylifetime. This requirement will give a challenge to the gas to be treated enters the tube at one end andpulsed power generator, especially to the switch exits the structure at the other end. The plasma dis-devices. Normal switch devices tend to have limited charge can be represented as a lumped time varyinglifetime and average power level, which hinder resistance, occurs when the breakdown voltage of thetheir practical aplications. gas in the tube is reached, in parallel to a stray ca- In our work, an all-solid-state pulsed power pacitance. The energy per pulse dissipated in the dis-generator is presented. It consists of a resonant charge was approximately 60 mJ. The system could becharging circuit, a fast charger with high fre- operated at a repetition frequency greater than 1 kHzquency controlled by a high speed semiconductor with peak output voltage of up to 30 kV.switch, a step-up transformer, a magnetic com- Non-thermal plasma discharge can be achieved bypression switch and the load. In this circuit three applying a positive high – voltage pulse to a field en-stages of pulse compress from millisecond to nano- hancing wire electrode in the center of a tube. Thesecond is achieved. The resonant charging circuit high electric field developed in the gas region betweenprovides primary power supply in tens of millisec- the electrodes causes partial ionization of the gasonds scale. The fast charger charges the capacitor which provides a source of free electrons. The desiredin the last stage in several microseconds. At the last free radicals can be produced. In order to prevent thestage, the magnetic compression switch transfers undue acceleration of ions and the development of anlow inductance from high inductance and com- arc between the electrodes, high voltage pulse shouldpresses the current pulse into short one. A semi- be a fast rise time (~ 1000 V/ns) and relatively shortconductor switch controls the frequency of the load pulse duration (> 100 ns).discharge. The whole pulse power generator oper-ates with high frequency and efficiency. It has high 2. Basic Principles of Pulse Compressionreliability and long lifetime, which is very useful in A pulsed power generator is shown in Fig. 1. It con-applications of non-thermal plasma discharge. This sists of a resonant charging circuit, a fast charger withpaper discusses the basic principle of pulsed power high frequency (1 ~ 10 kHz), pulse magnetic switchgenerator and put the focus on the system analysis and the load. The load is the plasma discharge reactorand design. that can be represented as a lumped time varying re- sistance (R3) in parallel to a capacitance of reactor1. Introduction tube (C4). The resonant charging circuit is comprisedNonthermal plasma has been extensively investigated of L1, L2 and C1, it provides a resonant current forrecently years because it can be effetely used to mod- transformer TX1. During the normal operation, theify a number of hazardous compounds rendering them energy per pulse dissipated into the load is much lessless harmful to environment [1–3]. A principal ad- than the stored energy in C2, the charging time for C2vantage of nonthermal plasma processing is that the is charged from a residual voltage is much less thanmajority of the energy invested in the process is util- one cycle time as shown in Fig. 2. There is a relation-ized for the acceleration of electrons without signifi- ship between the voltage and charging time, C2(U –cantly heating the bulk of the gas. Therefore chemical Ure) =reactions that are normally associated with very high = Itch–1. The intermediate storage capacitor C2 isbulk temperature can be realized near room tempera- charged by rectifying bridge in one cycle time intervalture. Nonthermal plasma treatment of gaseous efflu- in the initial time. Capacitor C3 is charged by C2ents can be significantly more efficient than other through a inductance L3, which compensates the en-treatment methods. A nonthermal plasma can be pro- ergy per pulse dissipated into the load. The fastduced at atmospheric pressure by a transient electrical charger transfers the energy required by the load fromdischarge in which high energy electrons are created 1 The work was supported by Chinese Natural Scientific Fund under contract 90305002. 157
  • 2. Pulsed power technology Fig. 1. A pulse power generator with pulse compressionC3 to high-voltage capacitor C5 with less capacitance 200in microsecond time interval as shown in Fig. 3. Thehigh power semiconductor switch is used to controlthe frequency of load discharge. Once the capacitor in 150parallel to the secondary of transformer is fullycharged, the magnetic switch saturates and the ca- Cur ent A) r (pacitor switches on the load with tens of nanosecond 100rise time as shown in Fig. 4. Whole pulse power gen-erator has only one semiconductor switch which de-termines the frequency of plasma discharge. Three 50stages have independent working mode, that is, laterstage is not affected by former stage. But the pulsewidth of later stage is compressed into a short one. 0 0 5 10 15 20 25 10 Ti e( ) m ns Fig. 4. The current waveform from the plasma discharge 8 reactor Cur ent A) r ( 6 3. Resonant Charging Circuit 4 The advantage of the resonant charging circuit is that it has a relatively fast charging time and high charging 2 efficiency. The parameters in the resonant charging circuit meet the condition as following: 0 4 6 8 10 12 14 ω2 LC1 = 1 , (1) Ti e( s m m )Fig. 2. A current waveform from the resonant charging circuit where L = L1 = L2, ω = 2πf0 , f0 is the supply frequency. 70 According to the reference [4], when character- ωL 60 ristic coefficient Q = ≥ 20, coupling coefficient R 50 k = 0.95, the efficiency of resonant charging circuit is larger than 88%. It is favorable to improve Q and k for Cur ent A) r ( 40 the pulse power generator. 30 4. Fast Charger 20 10 The fast charger of a flyback mode of operation is chosen based on following considerations, first it im- 0 mune to transients associated with the discharge of the 0 2 4 6 8 10 12 14 load. In this mode the primary semiconductor switch m µ) Ti e( s of the resonant charger is in the off state when the Fig. 3. A current waveform from the fast charger capacitor in the secondary is discharged. Secondly it 158
  • 3. Poster Sessionoperates in high efficiency mode if the parameters ofthe primary and secondary of transformer match well. which the leading edge of the pulse rises from 10 toIn order to lower the over-voltage applied into the 90% of its peak value. It is given bysemiconductor switch and improve the working effi- Lsatciency of transformer, the secondary capacitor should tr = 2.2 , (8)be fully charged time in a quarter of cycle. At the Rlsame time all energy stored in the capacitor dis- where Lsat – saturated inductance of the magneticcharged into the primary of the transformer com- switch; Rl – load resistance.pletely during the switch is closed. The quarter of acycle time is determined by the inductance and ca- 20pacitance in. Here the resistance of the primary andsecondary in the transformer is neglected (normally, 16the resistance is much less than the impedance): 12 π π Current(A) tch − 2 = Lp C p = Ls Cs . (2) 2 2 8The probable maximal voltage in the secondary is 4 U 2 m = U0 C p / Cs . (3) 0If -4 0 10 20 30 40 50 C p = N 2 Cs ; (4) Time(ns) Ls = N 2 Lp , (5) Fig. 5. A current waveform when C3 discharge into the primarythe output voltage in the secondary is 6. Operation U 2 m = NU0 . (6) Typically current pulse in the primary windings of the set-up transformer in the resonant charge circuit is The conduction time of the semiconductor switch shown in the Fig. 5. The voltage waveform from theis matched with the charging time tch−2 , which pre- plasma discharge reactor is shown in Fig. 6, from which we can see that the pulse rise time in the load isvents the semiconductor switch from overvoltage at about 50 ns. Pulse duration is about 80 ns.the opening moment and improve the operation effi-ciency of pulse power generator. 35,0k5. Pulsed Discharging into Load 30,0kThe magnetic switch has two functions for the dis- 25,0kcharge of the load. First, it can isolate the charge of t V) 20,0kstorage capacitor from the load. Secondly it can Vol age(minimize the rise and fall time, which does not suffer 15,0kfrom many of the limitations, associated with dis- 10,0kcharge switches such as relatively long recovery timesand serious electrode erosion problem. Once the ca- 5,0kpacitor C5 is fully charged, the magnetic switch satu- 0,0rates discharging the capacitor C5 into the load. Dur-ing C5 is charged, the saturated delay time ts of - 0k 5, 0 50 100 150 200 250magnetic switch is Ti e( ) m ns SN ∆Bs Fig. 6. Voltage waveform from the plasma discharge reactor ts = , (7) UN 7. Conclusionts corresponds to the charging time tch of capacitor C5. In this paper we have presented a pulsed power gen- If the stray capacitance of plasma discharge tube is erator which can produce a high repetitive rate pulseneglected, the risetime of load voltage pulse is deter- with nanosecond duration and risetime. The operationmined by the L/R time constant of discharge circuit, principle and design has been described. 159
  • 4. Pulsed power technologyReference dielectric barrier discharges for pollution control”,[1] Andras Kuthi, Tom Vernier, Xianyue Gu et. al., Pulsed Power Conference, 1997. Digest of Techni- “Compact nanosecond pulse generator for cell cal Papers. 1997 11th IEEE International, Vol. 1, electroperturbation experiments”, Power Modula- 29 June–2 July 1997, pp. 97–102. tor Symposium, 2002 and 2002 High-Voltage [3] R. Korzekwa, L. Rosocha, M. Grothaus, “Analysis Workshop. Conference Record of the Twenty-Fifth of a pulsed corona circuit”, Pulsed Power Confer- International, 30 June – 3 July 2002, pp. 225–228. ence, 1999. Digest of Technical Papers. 12th IEEE[2] R. Korzekwa, L. Rosocha, Z. Falkenstein, International, Vol. 1, 27–30 June 1999, pp. 519–522. “Experimental results comparing pulsed corona [4] “Power supply for laser”, scientific press, 1980 and (in Chinese). 160