Vacuum Technology Part II of the “What exactly do we have down there?” series

What is a vacuum?

Practical concerns limiting vacua

How can we do better?

How does one achieve vacuum? Pumping – Two types –Transfer – relies on moving molecules from low to high pressure regions –Trapping – makes use of chemistry to trap or bury gas particles

Mechanical pump designs Rotary Piston Roots Rotary Vane Kurt J. Lesker Vacuum Technology

Diffusion Pumps Rely on jets of boiling fluid (usually silicone oil) to force air particles out of the region being evacuated. Cold traps prevent back streaming. Kurt J. Lesker Vacuum Technology

Turbomolecular Pumps Similar in design to a jet engine. Alternating rotor and stator blade assemblies turn at 20,000-90,000 rpm to force out molecules. Requires a region of low or medium vacuum behind and in front of pump. Pfeiffer Vacuum GmbH

Ion pumps Main components –Array of parallel stainless tubes –Various charged surfaces –Titanium or tantalum coated surfaces Trap molecules with varying speeds via chemical reactions Varian Scientific Instrumentation, Inc.

Cryopumps Similar in principle to the ion pump but uses a cryogenically cooled surface of activated charcoal or zeolites to condense and trap gas molecules. Kurt J. Lesker Vacuum Technology

Measuring the vacuum Ion gauges – Similar in principle to the triode ion pump –An array of surfaces are held at different potential (collector – grounded, grid ~100V, W or Ir filament ~30V) –Filament emits electrons which circle the grid, bombard with gas molecules to create ions, which are subsequently accelerated toward the collector. –A variation known as a cold cathode uses an electron plasma in crossed E and B fields of ~5kV and 2kG respectively MDC Vacuum, Inc.