Extended linear ion trap frequency standard apparatus

• US 5,420,549 A
• Filed: 05/13/1994
• Issued: 05/30/1995
• Est. Priority Date: 05/13/1994
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Prestage, John D.
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US08/246,041
• Time in Patent Office: 382 Days
• Field of Search: 331/3, 331/94.1, 324/304
• US Class Current: 331/94.1
• CPC Class Codes: H03L 7/26 using energy levels of molecules, atoms, or subatomic particles as a frequency reference

Abstract

A linear ion trap for frequency standard applications is provided with a plurality of trapping rods equally spaced and applied quadrupole rf voltages for radial confinement of atomic ions and biased level pins at each end for axial confinement of the ions. The trapping rods are divided into two linear ion trap regions by a gap in each rod in a common radial plane to provide dc discontinuity, thus dc isolating one region from the other. A first region for ion-loading and preparation/fluorescence is biased with a dc voltage to transport ions into a second region for resonance/frequency comparison with a local oscillator derived frequency while the second region is held at zero voltage. The dc bias voltage of the regions is reversed for transporting the ions back into the first region for fluorescence/measurement. The dual mode cycle is repeated continuously for comparison and feedback control of the local oscillator derived frequency. Only the second region requires magnetic shielding for the resonance function which is sensitive to any ambient magnetic fields.

Claims

1. An extended linear ion trap for frequency standard apparatus, said ion trap having two regions axially aligned for dual mode operation,

a first region for preparing atomic ions during a first mode by optical pumping from a ground state level to another energy level, said atomic ions being prepared for atomic frequency comparison to a microwave signal, said signal being derived from a local oscillator,

a second region for comparing said atomic frequency of said prepared atomic ions during a second mode by resonance of said atomic ions with said microwave signal,

voltage controlled means for transporting said atomic ions prepared in said first region during said first mode into said second region for comparison during said second mode, and for transporting said atomic ions back to said first region for a repeat of said first mode, thus completing one of a continuous succession of dual mode operations, and

magnetic shielding of only said second region.

2. An extended linear ion trap as defined in claim 1 wherein said two regions axially aligned comprise

multiple ion trapping rods mutually parallel and equally spaced,

rf voltages applied to said trapping rods for creating electromagnetic fields for radial confinement of said atomic ions to a space between said parallel rods,

dc biased and axially aligned pins at each end of said trapping rods for axial confinement of said atomic ions,

a gap in each of said trapping rods in a common plane normal to said trapping rods, thereby providing dc discontinuity between two regions of each of said trapping rods to provide a linear ion trap having first and second regions axially aligned without interference with said rf voltages applied to said trapping rods for radial confinement of said atomic ions, and

wherein said voltage controlled means for transporting said atomic ions between first and second regions comprises biasing said trapping rods of said first region at a voltage level higher than a voltage level of said second region for transporting said atomic ions into said second region, and switching voltage levels of said two regions for transporting said atomic ions from said second region to said first region.

3. An extended linear ion trap as defined in claim 2 further comprising a plurality of short auxiliary rods straddling said gap plane, said short auxiliary rods being parallel to said trapping rods and radially spaced from the center of said linear ion trap, and means for applying a voltage to said short auxiliary rods to assist moving ions transported from one region further into the receiving region.

4. An extended linear ion trap as defined in claim 3 wherein said voltage is applied to said short auxiliary rods to assist moving ions transported from said first region further into said second region for resonance.