Thin Film Technology - Student Talk Sophie Chauvin 02/02/16 Leak Detection method for Vacuum Systems 1

Definition of the problem Maintenance of the vacuum pressure in the vacuum chamber Need of a good isolation Tightness specifications required  Find a method to detect leaks in a vacuum system Different possible sources of a leak Leak or no leak, leak size, leak location 2

Problem solution  Method: helium leak detector Why helium? small mass small atom volume low partial pressure in air 3

Problem solution Principle: N. Hilleret, “Leak Detection”, CERN, Geneva-Switzerland pp Leak detection cell 4

Problem solution 5  Direct-flow method K. Zapfe, “Leak Detection”, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany pp Different methods to connect the vacuum system

Problem solution 6  Counter-flow method K. Zapfe, “Leak Detection”, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany pp

Conclusion 7 Advantages of the counter-flow detectors Commercial leak detectors Time-consuming process

Information The leak detection cell is the main part of the helium leak detector. The residual gas is first ionised using electron beam technology. Then the ionised molecules are accelerated with an accelerating electrode. A mass spectrometer is used to filter the ions and a magnetic field enables the separation of the helium ions from the others. The principle of the mass spectrometer is to measure the gas masses in order to separate the different gases. The magnetic field is usually generated by permanent magnets. 8

Information There are different ways to connect the leak detector to a vacuum system. In the direct-flow method, the leak detection cell and the pump are directly connected to the vacuum system. The helium leak rate is defined using the helium partial pressure in the cell and effective helium pumping speed of the high vacuum pump: 9 Therefore if we want to increase the sensitivity, the pumping speed of the tracer gas (helium) has to be reduced but without decreasing the pumping speed of the other gases (which are mostly water).

Information 10 So water needs a high pumping speed and helium a low pumping speed. A liquid-nitrogen trap is used which enables to condense water. The pressure of the cell is lowered by the high pumping speed of the trap so the leak detection can start earlier. This detector is able to detect leaks at Pa.m 3.s -1 for small systems. As the trap needs to be refilled regularly, it is necessary to have a source of liquid nitrogen.

Information 11 In the counter-flow method, the leak detection cell is not directly connected to the vacuum system; it is connected to the inlet of a high-vacuum pump. The compression ratio K of turbomolecular pumps increases quickly with the mass of the pumped gas so injecting the gas from the cell to the pump enables the obtaining of an backstreaming flux enriched for lighter gases. The helium leak rate is defined using the helium partial pressure in the cell and effective helium pumping speed of the high vacuum pump but also with the compression ratio: The helium partial pressure in the detection cell is smaller is the case of a counter-flow method due to the turbomolecular pump.

Information 12 In practice, it will be impossible to have a complete tight vacuum system and it’s not even needed but it has to be small enough to maintain the required pressure in the system. For a high vacuum system:  If leak rate < mbar.l/s : the system is very tight  If leak rate < mbar.l/s : the system is tight  If leak rate < mbar.l/s : the system is leaky

References A. Preglj, M. Drab, M. Moztic, “Leak Detection Methods and Defining the Sizes of Leaks”, NDT.net -February 1999, Vol.4, No2 N. Hilleret, “Leak Detection”, CERN, Geneva-Switzerland pp K. Zapfe, “Leak Detection”, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany pp