6.152J/3.155J Knudson number L= dimension of chamber or reactor k t 1 atm 10-5 102 Flow is viscous: p>1 mT Knudsen No L Pump power >viscosity must transport lg. of molecules λ Molecular ballistic flow: p< 1 mT Pump efficiency critical must attract and hold molecules What does this imply for pumping? Wed, Sept. 10, 2003 Gas flow and pump speed Gases are compressible unlike liquids. so express flow as number of molecules/t, not volume/t [Units of Q(std-p)=> liters/min Conductance of vacuum component: Obm's law →Q=C(P-P [Units of conductance Pp pump 6.152J3.155 Wed, Sept 10, 20036.152J/3.155J 6.152J/3.155J 9 L Flow is viscous; p > 1 mT Molecular, p What does this imply for pumping? Pump power > viscosity; must transport lg. # of molecules must attract and hold molecules. Knudsen N0 ≡ l L l L <1 l L >1 L L l = kB T 2pd 2 p Recall: p l (cm) 1 atm 10-5 10-2 1 mT 10 Wed., Sept. 10, 2003 = dimension of chamber or reactor ballistic flow; < 1 mT Knudson number Pump efficiency critical; 1 Torr 6.152J/3.155J 10 Gas flow and pump speed Gases are compressible unlike liquids… so express flow as number of molecules/t, /t. N µ pV Define throughput, Q: Q = dN dt µ p dV dt ≡ pS Q (std-p I = 1 R Ê Ë Á ˆ ¯ ˜V q t l p pp fi Q = Cp - p ( ) p [Units of conductance µ area/length] Wed., Sept. 10, 2003 not volume [Units of ) => liters/min or sccm] Ohm’s aw: pump chamber Conductance of vacuum component: 5