Ion pump for cryogenic magnet apparatus

Abstract

Ion pumping modules without ion pump magnets are disposed within evacuated spaces located within the fringing fields of cryostatic housed high field magnets. Improved vacuum conditions are obtainable both within superconducting magnet cryostats and for evacuated auxiliary apparatus proximate the magnet cryostat, particularly where such auxiliary apparatus is sensitive to RF noise.

1 . A vacuum ion pump in combination with a cryogenically cooled magnet, comprising: 
 (a) a non magnetic axially symmetric anode having at least one aperture therein for defining a glow discharge;    (b) a non-magnetic chemically active cathode spaced apart from said anode and having a surface opposite said at least one aperture;    (c) a non-magnetic evacuable housing in gas communication with a region to be evacuated, said housing surrounding said anode and cathode; and    (d) a power source that apply a potential difference between said anode and said cathode establishing an electric field therebetween,    said housing disposed intercepting a portion of the magnetic field intensity of said magnet along a direction substantially parallel to said electric field and said portion characterized by sufficient magnetic field intensity magnitude that operate said ion pump.    
   
   
       2 . The vacuum ion pump of  claim 1 , wherein said magnet is located within a cryostat comprising at least one evacuated volume.  
   
   
       3 . The vacuum ion pump of  claim 2 , wherein said housing comprises said at least one evacuated volume.  
   
   
       4 . The vacuum ion pump of  claim 1 , wherein said region to be evacuated is independent of said cryostat and said region to be evacuated is disposed proximate said cryostat.  
   
   
       5 . The vacuum ion pump of  claim 4 , wherein said another evacuated region is in gas communication with a dewar enclosing an RF coil for use in acquisition of NMR data.  
   
   
       6 . The vacuum ion pump of  claim 5 , wherein at least a portion of said cryostat comprises a bore surrounded by windings comprising said magnet and said dewar is disposed in said bore.  
   
   
       7 . An NMR probe for use in the bore of a superconducting magnet having a substantial fringing field external to said bore, said probe comprising: 
 an RF coil;    an evacuated cryostat housing comprising an annular portion which receipt in said bore, wherein said RF coil is disposed within the annular portion, said annular portion further comprising an a probe bore accommodating a sample of investigation;    refrigeration apparatus disposed in said housing that remove heat from said RF coil;    a tubular anode disposed in said housing, said anode having an axis and said anode in substantial parallel alignment with a local portion of said fringing field; and    a cathode oriented to present a surface having a normal substantially coincident with said axis, said cathode and anode arranged in support a potential difference therebetween.    
   
   
       8 . The NMR probe of  claim 7 , wherein said cathode comprises a chemically active metal selected from the group consisting of titanium, vanadium, tantalum, and zirconium.  
   
   
       9 . The NMR probe of  claim 8 , further comprising a power supply to supply said potential difference.  
   
   
       10 . The NMR probe of  claim 7 , wherein said cathode and anode are disposed in another portion of said housing in open communication with said annular space.  
   
   
       11 . A method of maintaining a vacuum condition within a cryostat housing for thermally isolating a cooled magnet, said magnet having a field distribution consisting of a major portion and a fringe field portion, comprising the steps of: 
 disposing a non-magnetic chemically active anode spaced apart from a nonmagnetic chemically active cathode in gas communication with said housing and within said fringing field;    maintaining an electric field between said anode and cathode, and    orienting said anode and cathode such that said electric field is substantially parallel to said fringe field, whereby residual gasses are sequestered under the combined influence of said fringe field and said electric field.