Paschen Tests Research Articles

Experimental Study on Paschen Tests of ITER Current Lead Insulation

An experimental Paschen test setup has been established to analyze the quality of ITER current lead (CL) insulation and extend the research on Paschen's law under various conditions. Insulation problems can destroy a machine if a Paschen discharge is triggered by an insulation defect that is caused by faulty manufacturing, electromagnetic force, and thermal stress load with a certain degree of vacuum helium or pipe leakage. The results show that the CL insulation mock-up worked well under normal temperature and pressure. Besides, the mock-up also worked well in helium conditions and at 80 K temperature at different pressures. One area of CL insulation was severely destroyed when the 80 K test was conducted after 5 thermal cycles, resulting in Paschen discharge phenomenon. The breakdown voltage is maintained at a relatively low level under different pressure conditions; the change of breakdown voltage was mainly due to the change of pressure, and such change was in line with the Paschen law.

Paschen testing of ITER prototype cryogenic axial breaks

Axial insulation breaks are needed in forced cooled cryogenic high voltage devices for the separation of the high voltage area from the grounded pipe system. The ITER cryogenic axial breaks will be surrounded by good vacuum in case of normal operation but also under vacuum breakdown conditions sufficient dielectric strength is required for a reliable fast discharge of the coil system. A Paschen tight design of the ITER prototype breaks enables high voltage operation independent on the outer vacuum or gas conditions. Consecutively two pretested ITER prototype breaks were integrated in the insulation system of a Paschen test unit and high voltage tested. Two different ways to perform the Paschen testing were used for both breaks. The preparation of the breaks and the test setup are described and the test results are given.

Paschen Problems in Large Coil Systems

When a breakdown of the thermally insulating vacuum occurs in an energized high power coil system (e.g. a leakage of the He-cooling system or to the surrounding air) the coils warm up and have to be discharged fast to avoid destruction. Due to the coil inductance and the necessary discharge time, a high voltage is produced in the coil and the feeder system. Therefore an unspecified pressure coincides with the occurrence of a high voltage at the coil and feeder system. It is well known that under these conditions arcing can occur at a magnet when it is not perfectly insulated even for voltages 1 kV (Paschen-effect). The gas dependent breakdown voltage minimum occurs typically in the range of some 100 V at a pressure value that depends from geometry and temperature. For lower and higher pressures the breakdown voltage is increasing. For a high power magnet system an insulation is necessary that can withstand high voltage in the 10 kV range although any possible pressure is applied to the system. Usually a pressure independent solid insulation is used to solve this requirement. In order to test the reliability of this electrical insulation under all pressure conditions, a so-called Paschen test has to be performed. This Paschen test is a high voltage test with pressures varying between good vacuum and ambient pressure. In this paper examples for insulation failures on large coils found with Paschen testing are given. In a series of experiments with samples that have been prepared with intended insulation faults the necessity of Paschen testing is demonstrated to identify insulation problems that are not visible under good vacuum or ambient pressure. Therefore, a Paschen-test is also a useful high voltage diagnostic tool to localize insulation faults in solid insulated magnets.

Paschen tests in superconducting coils: why and how

The Paschen breakdown is the outcome of an avalanche effect related with the ionization of a gas under electric stress. Paschen's law defines the voltage breakdown of rarefied (low pressure) gases as a function of the product of pressure and distance between electrodes showing a minimum value (typically of the order of hundreds V) below which the breakdown cannot take place. The actual field level depends on the gas, on the shape, material and surface conditions (roughness, cleanness and presence of oxide layers) of the electrodes; hence there is a big dispersion in the results available due to the referred number of parameters influencing the voltage breakdown. The design of the electrical insulation of superconducting magnets targets a safety factor of ×10 on dielectric strength over specified voltages using the intrinsic dielectric strength of the materials. A crack in the insulation or a flaw not detected by visual inspection would not necessarily lead to an electrical break during the acceptance tests in air given the high dielectric strength of air (typically values over 1 kV/mm). However, it would lead to an electrical failure during operation if Paschen conditions take place. Since the Paschen minimum occurs under vacuum, it is irrelevant for conventional High Voltage (HV) engineering. The application of superconducting technology for plasma fusion devices with a growing size and complexity of magnets has shown Paschen testing as an essential tool. During magnet operation inside the cryostat conditions, if a He leak takes place, critical pressures can be reached locally and Paschen breakdown might occur at voltages significantly lower than the designed ones. The present paper covers an existing gap in International Standards and scientific literature addressing Paschen testing of superconducting magnets and explains why it is needed and how the tests should be done in the most efficient way.

The Quench Detection-Wire-Feedthrough Plug-In of W7-X

The stellarator fusion experiment Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik (IPP), Germany. To allow the steady-state operation, W7-X has a superconducting magnet system. It consists of 50 non planar coils, 20 planar coils, a superconducting bus bar system and 14 current leads. For continuous on-line monitoring of the superconducting state 1624 Quench Detection-wires (QD-wires) are connected at the joints of the coils and the bus bars. The signals of these wires must be fed through the Outer Vessel (OV) shell to the Quench Detection Units. The electrical feedthroughs, wires and wire connections have been designed and qualified for a test voltage of 13 kV under low gas pressure (Paschen test). Other important requirements are a maximum allowed integral helium leak-rate of l/s per QD-dome and a maximum allowed total heat load to the thermal shield of the OV of 140 W per QD-dome. The paper describes the design and qualification of the QD-Wire-Feedthrough-Plug-in, the assembly procedure as well as the tests carried out during and after assembly.

High Voltage Test of ITER Feeder Components Under Paschen Condition

As an important method of quality confirmation for the ITER Feeder system insulated components, high voltage test at Paschen condition need to be carried out once after these components are manufactured. The Paschen test is much more efficient for detecting the micro cracks or defects in solid multi-composite high polymer insulation materials than the conventional high voltage test done in atmospheric condition, and almost without any damage for the qualified components. For this purpose and the task agreement of Feeder R&D with ITER IO, a cryogenic vacuum Paschen test facility, which was modified by the Feeder prototype of Coil Termination Box (CTB), was established in ASIPP (China) in October of 2010, it is suitable to test typical critical high voltage components in Feeder, such as S-bend busbar, superconducting joint, and current lead. The test voltage can be up to 30 kV and provide pressure lower than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . This paper describes the detailed configurations of the insulation mock-ups for Feeder and the test facility, also presents and discusses the test procedure and results acquired up to now.

Reliability of Force Cooled Superconducting Magnets for Fusion

The latest superconducting magnets (SM) for fusion are mostly force-cooled, mainly because it allows reliable electrical insulation of the coils using vacuum pressure impregnation (VPI). SM of this type have many leads, feeders and coolant tubes, located in cryostat vacuum, which must sustain high voltages, induced on them by fast current changes. However vacuum loss can spoil their insulation. A few such cases occurred during the T-15 tokamak coils testing, initially having bare leads relying upon vacuum. But its loss generated a coil quench, a protecting current dump at high voltage, followed by breakdown and arc. Even leads insulation by Teflon and fiberglass tape wrap proved to be insufficient. Nevertheless, similar tape wrap insulation of leads and feeders (ILF) was used in EAST, KSTAR, SST-1 and W-7X. So far, seven breakdowns occurred during their coil tests at operating voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\sim&lt;$</tex></formula> 3 kV. Breakdowns never initiated in the coils, but always on their leads, feeders and sensor lines, indicating that their insulation made by tape wrap were too weak. Instead of ILF improvement some projects undertake Paschen tests. These are planned as the baseline for ITER too. But these tests are valid for the coil with open insulated surface, but are not appropriate for the final tests, when insulation should not be exposed to vacuum. Up to now ILF final tests have been done in all devices at 10–21 kV, but only in good vacuum in spite of the fact that such tests could not guarantee safe operation in case of vacuum loss. We propose to increase ILF strength to the same level, as in the coils, using vacuum-tight grounded stainless steel casings filled up by VPI over magnet leads. This will provide reliable and easily testable solid insulation. Besides, casings would exclude He leaks, providing the second vacuum tight barrier over the ILF. Thus it would increase the magnet reliability and would make it possible to avoid the needs of all single coils test.

Paschen Testing on W7-X Coils and Components in the BNN Test Facility

During the fabrication of the coils for the Wendelstein 7-X experiment the need for pressure dependent high voltage testing-Paschen tests-of the coils and components arose. These tests ensure the safety of the coils in case of an emergency shutdown due to a break in the vacuum of the cryostat. For that purpose the vacuum test facility at Babcock Noell Nuclear was modified in order to perform these tests. It is now possible to test coils and their components up to 17 kV and for pressures below 1.0middot10-3 mbar. Completed coils, winding packs before insertion in the case and also several components have been tested up to now. It has been demonstrated that these Paschen tests are a powerful tool to detect insulation defects. The paper shows the effort which has been taken to set up the test facilities. It will show the preparation of the components under testing as well as present and discuss the results obtained up to now