referring to fig1 to 3 , there is schematically shown a molecular drag pumping stage 1 according to the invention for a turbomolecular pump . pumping stage 1 is a so called molecular drag stage of the gaede type , intended to be embodied into the pump downstream of the “ high ” or turbomolecular stages operating at lower pressures . the invention can however be applied to pumping stages having any kind of rotor discs , either equipped with vanes or smooth , as it will be explained in more detail hereinafter . pumping stage 1 embodies a tangential flow pumping channel 3 , having a c - shaped cross section , defined between a rotor disc 7 , fastened to shaft 5 rotated by the pump motor , and a stator ring 11 coupled with the pump body . an inlet port 13 , communicating with the pumping stage , if any , located upstream of stage 1 or with the suction port of the pump , provides for admitting gas into stage 1 , and an outlet port 15 provides for exhausting gas from stage 1 towards the subsequent stage or the exhaust port of the pump . a baffle or stripper 17 is located between ports 13 and 15 to provide for gas tightness between inlet and outlet regions of channel 3 , through a reduced opening 19 of few tenths of a millimetre between the surfaces of the rotor disc and the stator . pumping channel 3 is radially tapered and has width d 1 at inlet port 13 and width d 2 at outlet port 15 . advantageously , pumping channel 3 is also axially tapered : indeed , the axial distance between rotor 7 and stator 11 varies along the rotor circumference and decreases from a value h 1 at inlet port 13 of pumping stage 1 down to a value h 2 at outlet port 15 of said stage 1 . as better seen in fig3 which is schematical cylindrical cross - sectional view of pumping stage 1 , the pumping channel height progressively decreases along pumping channel 3 between inlet port 13 and outlet port 15 . it is to be appreciated that in the illustrated embodiment the height variation in pumping channel 3 has a linear shape , symmetrical with respect to the rotor disc . yet , a pumping stage with an axially tapered channel could also be provided in which the height of pumping channel 3 varies polynomially , exponentially or according to trigonometrically formula . in this respect , fig3 a shows the development of a pumping stage 1 in which the height of pumping channel 3 decreases between inlet port 13 and outlet port 15 according to an exponentially shape . similarly , a pumping stage could be provided where the channel either is both axially and radially tapered , as in the illustrated embodiment , or is only axially tapered . still further , a pumping stage with a radially and / or axially tapered channel could also be provided , in which said variation is not symmetrical with respect to the rotor disc . in particular , the axial taper could be provided on one or the other disc side only . as known , in case of pumping stages of large diameter , the channel length is excessive and it cannot be wholly exploited since , beyond a given limit distance , pumping becomes ineffective . then , it is advantageous to divide the pumping stage circumference into two or more sections and to form as many pumping channels operating in parallel . referring to fig4 a pumping stage 1 according to a second variant of the invention is shown . that variant is characterised by the presence of three pumping channels 3 a , 3 b , 3 c . each of these channels 3 a , 3 b , 3 c includes a respective inlet port 13 a , 13 b , 13 c and a respective outlet port 15 a , 15 b , 15 c , the inlet ports communicating each with a corresponding channel in the upper stage and the outlet ports communicating each with a corresponding channel in the lower stage . a respective stripper 17 a , 17 b , 17 c is provided at each respective outlet port 15 a , 15 b , 15 c and separates the outlet port of one channel from the inlet port of the subsequent channel . as better seen in fig5 which is a schematical cylindrical cross - sectional view of the pumping stage shown in fig4 where only two of the three pumping channels operating in parallel are shown , the height of each respective pumping channel 3 a , 3 b , 3 c progressively decreases between respective inlet port 13 a , 13 b , 13 c and respective outlet port 15 a , 15 b , 15 c , thereby conferring a saw - tooth circumferential profile to pumping stage 1 . as stated before , the invention can be applied to any pumping stage equipped with a rotor disc . in particular , it can be applied to a pumping stage like that shown in fig6 where rotor disc 7 , instead of being smooth , has peripheral vanes 21 lying in planes perpendicular to the plane of rotor disc 7 . preferably , said vanes are uniformly distributed along the circumference of said disc 7 . using such a rotor disc results in a so - called “ regenerative ” pumping stage : thus , according to the invention , a regenerative pumping stage with axially tapered channel can be made . according to the invention , in any variant thereof , the gas to be pumped enters pumping stage 1 through inlet port 13 and is compressed while travelling inside pumping channel 3 as far as to outlet port 15 , through which the gas reaches the subsequent pumping stage or the exhaust port of the pump . referring now to fig7 pressure difference δp achieved in the pumping stage between inlet and outlet ports 13 , 15 is plotted versus exhaust pressure p fore . in said figure , the performance of a pumping channel according to the invention , with a linear radial and axial taper ( line p 1 ), is compared with that of a pumping channel with uniform cross section ( line p 2 ), said channels having the same height at the inlet port of the pumping stage . as long as the pressure is below 4 mbar , in both cases pressure difference δp linearly increases as exhaust pressure p fore increases , and the two curves substantially overlap . when pressure p fore exceeds 4 mbar , a saturation phenomenon takes place in the uniform height channel and pressure difference δp keeps constant . on the contrary , in case of the axially tapered channel , the linear increase in pressure difference δp as a function of pressure p fore continues , approximately with the same slope , and saturation occurs at a much higher value of p fore , about 10 mbar , and at a value of pressure difference δp that is about 2 . 5 times the saturation value for the uniform height channel . [ 0053 ] fig8 is a graph showing pumping rate v of the pumping stage as a function of exhaust pressure p fore , the inlet pressure being constant . also in this figure the performance of a pumping channel according to the invention , with a linear radial and axial taper ( line v 1 ) and that of a pumping channel with uniform cross section ( line v 2 ) are compared , said channels having the same height at the inlet port of the pumping stage . when the values of pressure p fore are very low , below 2 mbar , pumping rate is slightly higher in the pumping channel with uniform cross section . yet , for the pumping channel with uniform cross section , when pressure p fore exceeds 2 mbar , pumping rate rapidly decreases . on the contrary , in case of the tapered pumping channel , pumping rate keeps constant up to values of p fore close to 6 mbar . the graphs of fig7 and 8 clearly show the advantages in terms of higher exhaust pressure and higher compression ratio afforded by the invention with respect to the traditional channel , the axial and radial size being unchanged . moreover , the axial taper of pumping channel 3 helps in reducing power dissipation , thanks to the higher performance in terms of compression and to the lower tendency to turbulence , what can be expressed by a better control over reynolds number actually , reynolds number is proportional to the pumping channel height and the variation of said height along pumping stage 1 , in particular the height decrease as pressure increases along pumping stage 1 , ensures a better control over reynolds number , especially in case of pressure values exceeding 10 mbar , that is , for pressure values at which the turbulence effects can become important .