Christian Teske

Energy transfer efficiency of a spherical theta pinch

A pulsed inductive plasma source for vacuum ultraviolet generation and plasma stripper applications with an operating frequency of 12 kHz has been developed. The setup consists of a series resonance circuit with a load capacitance of 27 μF and a large diameter induction coil surrounding a spherical discharge vessel with a discharge volume of 4000 ml. Measurements have been made to evaluate the transfer efficiency of the pulsed inductive discharge in argon with 2.8% hydrogen at gas pressures from 0.6 up to 100 Pa at load voltages from 4 to 8 kV. Pulsed coil currents reached a maximum value of 18 kA with current rise times of 2 kA/μs while achieving a maximum energy transfer efficiency of 85% between the driving circuit and the plasma. Pulsed power peak values inside the plasma were higher than 2 MW fore pulse energies of 1 kJ. The experimental evidence suggests that the high transfer efficiency obtained with the spherical theta pinch is due to the improved transformer coupling between primary circuit and d...

Pulsed Low Frequency Inductively Coupled Plasma Generator and Applications

In this paper, an inductively coupled pulsed plasma source with an operating frequency of 29 kHz is described. Using a series resonance circuit with a load capacitance of 12 muF and large diameter (approximately 20 cm) induction coils surrounding a spherical discharge vessel, a discharge plasma was produced and investigated in a pressure range of 0.1 to 100 Pa. Pulsed coil currents reached a maximum value of 9.6 kA with current rise times of 1.8 kA/mus while achieving an energy coupling efficiency of 80% between the driving circuit and the plasma. Pulsed-power peak values reached more than 800 kW. Moreover, the spectroscopic diagnostic revealed a high fraction of ionized particles and an emission spectrum in the near UV domain. Electron densities reached peak values of 1021 m-3. By using a fast camera system, the initiation of the discharge was investigated. The ignition of the discharge along an azimuthal path was documented, showing evidence of an entirely inductive plasma initiation without capacitive coupling.

Electron density and plasma dynamics of a spherical theta pinch

A spherical theta pinch for plasma stripper applications has been developed and investigated regarding the electron density and the plasma confinement during the pinching sequence. The setup consists of a 6 μH induction coil surrounding a 4000 ml spherical discharge vessel and a capacitor bank with interchangeable capacitors leading to an overall capacitance of 34 μF and 50 μF, respectively. A thyristor switch is used for driving the resonant circuit. Pulsed coil currents reached values of up to 26 kA with maximum induction of 500 mT. Typical gas pressures were 0.7 Pa up to 120 Pa with ArH2 (2.8% H2)-gas as a discharge medium. Stark broadening measurements of the Hβ emission line were carried out in order to evaluate the electron density of the discharge. In accordance with the density measurements, the transfer efficiency was estimated and a scaling law between electron density and discharge energy was established for the current setup. The densities reached values of up to 8 × 1022 m−3 for an energy of ...

Excimer Emission from Pulsed Tandem Microhollow Cathode Discharges in Xenon

This paper describes an extension of a basic single microhollow cathode discharge (MHCD) to a tandem MHCD, i.e., two discharges in series from an anode–cathode–anode configuration. When a high-voltage pulse is superimposed with a direct current (DC) tandem MHCD, an intense excimer emission along the discharge axis in a high pressure xenon gas is generated which is two orders of magnitude higher than that of the DC tandem MHCD. In addition, the emission intensity increases to almost twice by increasing cathode thickness from 250 to 1000 µm. The emission is further enhanced by increasing the gas pressure from 400 to 800 mbar.

Thyristor stack for pulsed inductive plasma generation

A thyristor stack for pulsed inductive plasma generation has been developed and tested. The stack design includes a free wheeling diode assembly for current reversal. Triggering of the device is achieved by a high side biased, self supplied gate driver unit using gating energy derived from a local snubber network. The structure guarantees a hard firing gate pulse for the required high dI/dt application. A single fiber optic command is needed to achieve a simultaneous turn on of the thyristors. The stack assembly is used for switching a series resonant circuit with a ringing frequency of 30 kHz. In the prototype pulsed power system described here an inductive discharge has been generated with a pulse duration of 120 micros and a pulse energy of 50 J. A maximum power transfer efficiency of 84% and a peak power of 480 kW inside the discharge were achieved. System tests were performed with a purely inductive load and an inductively generated plasma acting as a load through transformer action at a voltage level of 4.1 kV, a peak current of 5 kA, and a current switching rate of 1 kA/micros.

Design and Tests of a 13-kA/6.5-kV Thyristor Switch for a Pulsed Inductive Plasma Source

In this paper, the design, construction, and test procedure of a closing-switch prototype based on thyristors are described. In particular, details are given about the design criteria and the triggering board architecture, which is a high-side-biased self-supplied unit using electrical energy derived from a local snubber network for the gate drive. The structure guarantees a hard firing gate pulse for the required high-dI/dt application. Furthermore, the results of the prototype tests are presented and discussed. The stack assembly has a holding voltage of 6.5 kV and is used for switching a series resonant circuit with a ringing frequency of 12 kHz for a pulsed inductive plasma source. Maximum current amplitudes of 13 kA and pulse energies of more than 600 J were switched during the test procedure.

Density enhancement of an RF plasma in a magnetic quadrupole

A new method for the effective confinement of a low pressure gas discharge has been proposed by Christiansen and Jacoby. The principal component is a magnetic quadrupole superimposed upon an RF-driven gas discharge plasma. It has been suggested to use the device as an ion source for accelerator applications and as a plasma target to investigate the interaction of heavy ion beams with a magnetically confined plasma. A complete experiment including a capacitively coupled radio frequency (CCRF) discharge and an electric quadrupole magnet was set up and investigated by applying spectroscopic diagnostic methods. The plasma parameters for the magnetically confined CCRF discharge were measured using a Ar : He gas mixture. The electron temperature and the electron density as a function of the gas pressure and the magnetic field could be determined. A maximum of the mean electron temperature was identified as due to collisionless heating. Further, an ion beam was extracted and the mean electron density derived. The confinement of the plasma at low pressures between 0.4 and 1 Pa has also been obtained with an electron density of 3 × 1017 m−3.

Note: Design and tests of a 13 kA-6.5 kV thyristor switch for a pulsed inductive vacuum ultraviolet source

In this paper, the design, construction, and test procedure of a closing switch prototype based on thyristors is described. In particular, details are given about the design criteria and about the triggering board architecture, which is a high side biased, self supplied unit using the electrical energy derived from a local snubber network for the gate drive. The structure guarantees a hard firing gate pulse for the required high dI/dt application. Further, the results of the prototype tests are presented and discussed. The stack assembly has a holding voltage of 6.5 kV and is used for switching a series resonant circuit with a ringing frequency of 12 kHz for a pulsed inductive vacuum ultraviolet source. Maximum current amplitudes of 13 kA and pulse energies of more than 600 J were switched during the test procedure.

Radio frequency quadrupole confined noble gas discharge laser

A typical multiple wavelength noble gas laser, like an argon ion laser, consists of a capacitively coupled high current density glow discharge in the presence of a magnetic field. The output power of a noble gas ion laser is extremely dependent on the current density of the discharge. Typical conditions of such a plasma are current densities (J) between 100 and 1000 A/cm2. The upper laser level population N2 varies as N2~J2. Recently, a new kind of plasma source for a noble ion gas laser was proposed by J. Christiansen and J. Jacoby. With this new configuration, high power radio frequency is inductively coupled via a coil into the plasma. To achieve a high current density the plasma was focused by a quadrupole magnetic field. The main advantages of this system are the electrodeless configuration to avoid impurities, and the high temperature and particle density in the centre of the discharge

The spherical theta pinch and its applications

The Spherical Theta Pinch, formerly also referred to as low-frequency inductively coupled plasma (LF ICP) was developed at the Institute for Applied Physics in Frankfurt around 2008. It consists of a massive spherical coil of up to 10 windings which is arranged around a glas sphere of several 1000ml of volume. A capacitor bank is discharged through the coil, igniting a plasma inside the gas in the discharge vessel. The peak currents reach several tens of kiloamperes, the capacitor voltages more than 10kV. Gas pressures lie in the region of 1-200Pa. Average electron densities of more than 5×1022 m-3 have been measured. This paper is meant to act as a reference and to give an overview about the former, ongoing and upcoming spherical theta pinch experiments in Frankfurt. The different investigated applications as an ion source, a VUV-flash lamp and as a plasma stripper are examined regarding their special requirements followed by the approaches which were taken to live up to those needs. A short introduction into the switch development at the institute is given, to show the subsequent replacement of lifetime limited and, in the case of ignitrons, potentially hazardous gas discharge switches by long living thyristor stacks which also contributed to the advance of the spherical theta pinch devices built. Finally an outlook on parallel and future developments is presented.