Improved maser and superconducting magnet, which operates in vacuum of closed-cycle helium refrigerator, comprise a low-noise, reliable, field-operational receiver input system

Berwin, R. W.

Dachel, P. R.

Weibe, E. R.

English

NASA Technical Reports Server

October 1975	 B75-10281 
NASA TECH BRIEF 
. NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Low-Noise Ku-Band Receiver Input System 
S
The problem: 
Receivers capable of Ku-band operation include a 
traveling-wave maser operating at frequencies be-
tween 14.3 and 16.3 GHz. The required operating 
field for this mase'r must be continuously adjustable 
from 7,000 to 8,000 gauss. Prior masers included 
permanent magnets capable of 2,500 gauss fbr the 
S-band and 500 gauss for the X-band. These magnets 
weigh 180 kg or more. The size of the magnet was 
another limiting factor and prevented the designer 
from increasing the magnetic field while maintaining 
the spatial magnetic uniformity and large gap 
necessary to enclose the closed-cycle refrigerator 
(CCR) shields. A third limitation, sensitivity of the 
permanent magnet to the Earth's magnetic field, 
resulted in phase variations of the received signal 
through the maser as the antenna is rotated. Also, 
instability caused by relative motion between the 
maser and the permanent magnet was difficult to 
eliminate. 
The solution: 
An improved maser for the Ku-band and a 
superconducting magnet, which operates in the 
vacuum of a closed-cycle helium refrigerator, 
comprise a low-noise, reliable, field-operational 
receiver input system. The superconducting magnet 
weighs 63 kg. A superconductive switch, heated via 
radiant energy from an incandescent lamp, improves 
the magnet control-circuit response time. 
How it's done: 
A Cioffi-type superconducting magnet, designed to 
attain magnetic fields of up to 10,000 gauss, provides 
phase stability of the signal with less than 10 variation 
through the maser. Due to the high relative
permeability of the iron in the magnet, the iron acts as 
a shield against external magnetic fields. The 
superconducting magnet is mounted in the same 
4.5-K heat station on the CCR as the maser and 
operates in a persistent mode. The temperature inside 
the CCR is stable to within 0.001 K, eliminating 
temperature compensation of the magnet. Nine 
U-shaped shields are each made of a 50-mm wide by 
0.27-mm thick tape of copper-plated Nb 3Sn material. 
The magnet is wound with 0.127-mm-core NbTi wire. 
The pole pieces and return paths are Hyperco 27 
machined and annealed. 
With a persistent-current superconducting magnet, 
it is necessary that the junction where the winding 
ends are joined has zero resistance below the critical 
temperature for the current necessary to operate the 
magnet. The joint is made by stripping the cladding 
off the wire and winding the exposed superconductor 
into a filament. This filament is then compressed into 
a copper matrix with 12 mm of superconducting wire 
extending through. This type of joint has passed a 
persistent current of 20 A for 3 weeks, indicating an 
upper limit on the junction resistance on the order of 
10_l i ohm. 
The superconducting switch and magnet coil are 
located in the vacuum of the CCR. The switch consists 
of an NbTi superconducting switch-wire shunt that 
passes through a radiation shield containing a 
filamentary light bulb. The lamp provides radiant 
heat to operate the switch by varying shunt resistance. 
While the lamp is glowing, current through the 
magnet can be changed via an external power supply. 
As long as there is a charging current in the magnet 
greater than a certain minimum threshold, the light 
bulb may be turned off, and the induced voltage 
(continued overleaf) 
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(approximately 2 mV minimum) across the magnet 
will keep the switch wire normal. When the current in 
the magnet reaches a stabilized level, the switch wire 
will become superconducting, and the magnet current 
will operate in a persistent mode, at which time the 
external power supply can be turned off. 
The slow-wave structure of the traveling-wave 
maser consists of a comb system with ruby on one 
side. Alumina on the other side also supports the 
isolator material. Radiation at the pump frequency is 
coupled to the ruby through shaped alumina strips. 
The ruby bars are forced against a copper comb by 
pressure from berryllium-copper springs located 
between the bars and the center divider of the 
traveling-wave maser structure. The springs also serve 
to hold the isolators in place. Contact between the 
ruby bars and the comb completes a conductance 
path for heat transfer from the ruby bars, through the 
copper mass constituting the body of the traveling-
wave maser, to a flange which supports the entire 
maser. 
Ruby crystal misorientations on the order of ±10 
can be tolerated without degradation of performance. 
As a result, orientation and assembly procedures need 
not be as closely controlled as with the usual 54.7°
ruby orientation. Crystal quality requirements are 
also readily met by current crystal-growing proce-
dures. Efficient operation of the maser is obtained by 
pumping with a push-push technique; this provides 
larger pump-transition probabilities and higher 
measured-inversion ratios than the prior push-pull 
method. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP75-10281 
Patent status: 
NASA has decided not to apply for a patent. 
Source: Robert W. Berwin, Peter R. Dachel, 
and Ervin R. Weibe of 
Caltech/JPL
(NPO-13645)
S 
E 
Categories: 02 (Electronics Systems) 
B75-10281	 03 (Physical Sciences)


Low-noise K(u)-band receiver input system

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