Patent GB-956969-A (GB)
Title: Molecular beam masers and apparatus for transforming the divergence of a molecular beam
IPC: H01J41/06
Filed: 1960-07-12
Abstract: 956,969. Masers. VARIAN ASSOCIATES. July 12, 1960 [Aug. 7, 1959], No. 24322/60. Heading H3B. In a molecular beam maser, the beam is focused into a resonant cavity 6 by an even number of focusing rods 5 which are electrically charged with adjacent rods at different potentials and are arranged symmetrically around the axis of the beam in such a manner that the condition R 1 <SP>2</SP># 1 =R 2 <SP>2</SP># 2  is satisfied. Symbols # 1  and # 2  represent the solid angle divergence of the beam at the entrance to and at the exit from the focuser respectively and R 1 , R 2  represent the cross-sectional radius of the beam at the entrance and at the exit. Alternate rods are connected to a common terminal. The two groups of alternate rods have common terminals differing in potential by V. To meet these conditions, the rods should be parabolic as shown in Fig. 3b but the shapes shown in Figs. 3c, 3d are satisfactory. In Fig. 3c the diameter of the rods varies with the spacing of the rods, the rod diameter being 2R 1  at one end and 2R 2  at the other. Construction of focuser, Fig. 4.-Four tapered stainless steel focusing rods 55 are mounted in a frame comprising three stainless steel washers 51a, 51b, 51c interconnected by glass rods 52 and tungsten springs 53. The springs are fused into the glass rods and soldered to the washers. One pair of focusing rods 55 is attached by fins 56 to washers 51a, 51c and the other pair is attached to washer 51b. To obtain accurate spacing, the rods are held in a jig before they are secured to the washers. Complete maser, Fig. 1.-The pressure in a vessel 23 is maintained at about 10<SP>-6</SP> mm. Hg. by means of a high vacuum sputter pump 25. Ammonia supplied through a pipe 1 streams through a gas effuser 4 (see later) to form a beam containing a mixture of upper-state and lower state molecules. The upper-state molecules are directed by focuser 5 into a resonant cavity 6 where they may be stimulated to drop to the lower state by signals of the appropriate frequency in waveguide 7. The resulting lower state molecules emerging from the cavity are directed by a lower-state focuser 11 (see later) into an ionisation gauge 14 described in Specification 956,970. The gauge measures the intensity of the beam and thereby gives a measure of the radiation output from cavity 6. A discharge in the field of magnet 19 is initiated by a high voltage on anode 18. The current flowing between a grounded housing 16 and titanium cathodes 15 is a measure of the beam strength. Titanium sputtered from the cathodes on to the housing 16 entraps gaseous molecules and keeps the pressure low. A cold trap 26 containing liquid nitrogen may help to keep the pressure low. Construction of gas effuser, Fig. 6.-Tungsten wires (250) of diameter 0.001 inch are plated with nickel to a thickness of 0.0005 inch. The plated wires are sintered together into a bundle of about 0.030 inch diameter. The bundle is sliced into suitable lengths, such as 0.026 inch, and the tungsten is etched out in a bath of hydrochloric acid leaving a honeycomb structure of nickel tubes. Lower-state focuser.-The focuser of Fig. 1 comprises a solid inner conductor 11 to which a high voltage is applied and a transparent outer conductor 9, such as a wire screen, which allows higher-state molecules to diffuse. A lower state focuser may have a central conductor 11, Fig. 7a, tapered to avoid collisions. In a long focuser, the central conductor may be interrupted, Fig. 7b.