A compact tungsten electrode argon plasma torch operating in the range of 5 kW-50 kW is developed for high temperature plasma processing and available for transfer of technology to industry. It takes commercially available argon and converts that into controlled well defined jet of plasma with a maximum core temperature around 10000K and electro-thermal efficiency around 50% or more. The device produces plasma in the form of an intensely luminous extremely hot radiating atmospheric pressure cylindrical plasma jet controlled in terms of its length, diameter, velocity and power content. The plasma jet facilitates a very high enthalpy inert environment to engineer variety of high temperature process chemistry depending on reactants.

Introduction :

A thermal plasma jet of argon is a beam of huge concentrated thermal energy at very high temperature consisting of electrons, argon ions and neutrals. Such plasmas are naturally formed in atmosphere during thunderbolt and observed as a bright flash in the sky. Similar plasma is produced by the offered technology in the form of an intensely luminous extremely hot radiating atmospheric pressure cylindrical plasma jet controlled in terms of its length, diameter, velocity and power content using a small device called argon plasma torch. The novel technology offers a compact device that takes argon gas from a cylinder bank and converts it into a controlled well defined jet of argon plasma at an electrical power level of many tens of kW with efficiency greater than 50%. Low device cost, low operational cost, simple design, use of cheapest gas (argon), good efficiency (>50%), high temperature (~10000 K at the anode exit) and ease of control are some of the key features of the technology. .

1. Low device and peripheral cost: reduced installation cost.
2. Operation at low current and high voltage: Reduced diameter of current cables, easier handling and installation.
3. Light weight portable compact device: easier handling.
4. Simple design: easier maintenance, low maintenance cost.
5. Extremely low cathode erosion: long electrode life and low operational cost
6. Very high plasma temperature (at the exit >10000K): efficient heat transfer, faster process chemistry.
7. Low gas flow requirement (~20 slpm): reduced cost of gas delivery and post treatment section.

1. Very high temperature high enthalpy inert environment for thermal cracking of chemical compounds
2. Conversion of waste into energy through plasma pyrolysis
3. Hazardous waste destruction
4. Nuclear waste immobilization through melting and volume reduction
5. Chemical processing industries
6. Thermal barrier coatings
7. Steel and iron making industries
8. Metallurgical alloy making industries
9. High temperature, high heat flux testing of heat shield materials
10. R& D Applications

Internal components of the device are shown in Figure 2. The torch has adjustable, modular, segmented design. The same device can be adopted for variety of applications requiring different power levels of operations and process needs. It has one cathode segment, one auxiliary anode segment adjacent to it, one main anode segment at the exit end and floating segments in between. The plasma cathode is the source section that holds a 2% thoriated tungsten cathode along the central axis and gas injection ports surrounding it. The cathode is base cooled and emits electrons under thermionic emission. A coolant header supplies chilled water to all the segments. With the help of a radio-frequency igniter, the plasma (arc) is initiated in this section between the cathode and the auxiliary anode. Once the source is on, the arc is transferred to the subsequent sections and finally to the anode. The role of the intermediate segments is to streamline the plasma jet as well as develop appropriate voltage in the plasma column for operation at a given power level. For a given current and gas flow rate, the power of the plasma jet can be adjusted just by changing the length of this constrictor section. An extremely bright well-formed huge jet comes out from the device as shown in Fig.2.

Electrical connection and principle of operation of the torch are explained in Fig.3. A DC constant current power supply operates the plasma torch. Details of the required power supply are provided in the next section. Numbers of segments to be used in the torch are completely customized. The torch in Fig.3 is having 12 segments, electrically isolated from each other through Teflon insulator rings. Each of insulator ring has 25 equi-spaced gas injection ports of id 1mm along its periphery, which allow gas to be injected in the space between two adjacent rings. Segment '3' (auxiliary anode), segment '7' and segment '11' are connected with the main anode (segment '13') via high current contactors, namely, CT1, CT2 and CT3. Initially all the contactors are set in on position. Argon at a rate of 40 slpm is injected between segment '2' and segment '3'. To ignite the arc, momentarily a high frequency high voltage pulse (3 MHz & 3 kV) is applied between segment '2' (cathode) and segment '3'. Applied radio frequency pulse causes breakdown of the flowing argon gas. Already existing bias establishes an arc between the cathode and the auxiliary anode. Current is increased and CT1 is opened. This forces the arc to get connected to segment '7'. Current is increased further and CT2 is opened. This forces the arc to get connected to segment '11'. Desired final current (~400A) is set and lastly the contactor CT3 is opened. This establishes the final arc connection between cathode and the main anode (segment '13'). Typical arc voltage drop per segment is around 25 V. A total voltage drop of ~300V is expected. Total operating power reaches ~120 kW (400 x300=1200W). Operating current may be adjusted appropriately for operation at a desired power level. For generating plasma of any other gas like argon, nitrogen, CO2, CH4, H2O etc., the desired gas may be injected downstream through the gas injection ports.