fig1 shows the qualitative relationship between the throughput m of superfluid helium - ii flowing in a gap with the gap width s and the pressure drop δp along the gap . accordingly , there are two flow regions indicated by dashed lines for the he - passage through narrow gaps , a narrow , socalled &# 34 ; linear region &# 34 ; with a flat , approximately linear slope of the curve , and an upper , so - called &# 34 ; gorter - mellink region &# 34 ; with a substantially steeper slope of the curve . in the &# 34 ; ideal region ,&# 34 ; only gaseous helium still leaves the gap . in the &# 34 ; gorter - mellink region ,&# 34 ; a mixture of gaseous and liquid helium is discharged . according to the invention described in the following , the gap width s is varied by the phase separator , so that a very accurate control of the throughput in the &# 34 ; ideal region &# 34 ; is possible . the throughput m for helium - ii in the ideal region is : ## equ1 ## where b = gap width transversely to the flow direction , as can be seen from the formula above , the throughput is proportional to the third power of the gap width s and the reciprocal value of the gap length 1 . contrary to the known solution , the present solution therefore utilizes the dependence of the gap width s on the third power with the gap length 1 constant . from this , the following advantages result : if required , the throughput can also be made zero , i . e ., the phase separator can also be shut off without other devices . because the gap width s enters with the third power , the control sensitivity is considerably greater than with a control of the gap length 1 . a disadvantage of the known solution , that when the throughput goes to infinity as 1 goes to zero , is avoided . because the region in which the gap width s is changed is small , a solid state drive , for instance , a piezoelectric drive can be used , in which the adjustments proceed entirely without friction . in the embodiment shown in fig2 the entire phase separator is fastened via a flange section 7 to a tank filled with superfluid helium - ii by means of an appropriately designed tank nozzle 8 . the phase separator comprises essentially a disc element 1 as well as a ring element 2 , the free end face 2 . 1 of the ring element 2 together with an opposite surface 1 . 1 of the disc element 1 forming a radial gap 3 . the disc element 1 is fastened to a control element 4 . 1 of a piezoelectric drive which is in turn connected to the ring element 2 . the electric drive 4 can be a continuously variable drive or a drive operating in individual steps in the order of nanometers , for instance , a so - called &# 34 ; inch - worm ,&# 34 ; which allows an adjustment of the radial gap 3 in the range between 0 and at least 15 μm . the drive can optionally also be arranged outside the he - ii tank at a different temperature level as constant as possible . in this embodiment , the superfluid helium - ii enters radially from the outside into the radial gap 3 and then flows into the interior formed by the ring element 2 . via a heat exchanger 5 connected to the interior , in which possible liquid which might have passed the gap is evaporated in the tank by heat exchange with the superfluid helium - ii , the gaseous helium flows off and can be conducted due to its residual heat , for instance , through the heat insulating system of a cryostat . in the embodiment shown in fig3 only the parts forming the radial gap 3 as well as another drive for setting the gap are shown . to the ring element 2 are fastened , distributed uniformly over the circumference , three setting devices 9 which act on correspondingly associated parts 1 . 2 which are arranged , uniformly distributed at the circumference of the disc element 1 . a control element 9 comprises a yoke 9 . 1 which is firmly connected to the ring element 2 and encloses a drive 9 . 2 in the upper region , and the control element 9 . 3 of which pushes perpendicularly on part 1 . 2 of the disc element 1 . on the side of the part 1 . 2 opposite the control element 9 . 3 , a spring element 10 is arranged which is braced against the yoke 9 . 1 and pushes part 1 . 2 against the control element 9 . 3 . since three such devices 9 are provided , the surfaces 1 . 1 and 2 . 1 forming the radial gap 3 can be adjusted absolutely parallel to each other . the parallelness can be monitored , for instance , capacitively by means of an electrode layer vapor - deposited on one of the gap - forming surfaces . while with the two embodiments described above , only relatively small helium mass streams can be managed , fig4 shows an embodiment , in which a multiplicity of radial gaps can be connected in parallel and the gap widths of which are simultaneously readjusted by a piezoelectric drive . the embodiment shows an arrangement with three radial gaps . the gaps are formed by disc elements 11 . 1 , 11 . 2 , 11 . 3 and ring elements 12 . 1 , 12 . 2 , 12 . 3 . the ring elements 12 . 1 to 12 . 3 are kept at the same spacing by identical spacers 13 . 1 and 13 . 2 . likewise , the disc elements 11 . 1 to 11 . 3 are kept at the same spacing by two spacer sleeves 14 . 1 and 14 . 2 . the spacer rings 13 . 1 and 13 . 2 have radial holes 13 . 11 and 13 . 21 for supplying the helium - ii to the radial gaps 16 and 17 . the disc elements 11 . 2 and 11 . 3 have axial holes 11 . 21 and 11 . 22 for the passage of gaseous helium . the disc elements 11 . 2 and 11 . 3 are sealed by diaphragm elements 18 and 19 against the ring elements 12 . 1 and 12 . 2 , with which they do not form controllable radial gaps . the superfluid helium which enters through the openings 13 . 11 and 13 . 21 , thus cannot circumvent the controllable radial gaps 15 to 17 . the disc elements 11 . 1 to 11 . 3 are axially adjusted relative to the corresponding ring elements 12 . 1 to 12 . 3 via a push rod 20 and a drive , not shown , so that the gap width of the gaps 15 to 17 can be adjusted in the desired range between 0 and 15 μm . as is readily seen , the flow through the radial gaps in the above - mentioned embodiments need not necessarily be from the outside in ; the flow direction of the helium can also be in the reverse direction if the liquid or gas , respectively , is conducted in the appropriate direction . in the foregoing specification , the invention has been described with reference to specific exemplary embodiments thereof . it will , however , be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims . the specification and drawing are , accordingly , to be regarded in an illustrative rather than in a restrictive sense .