US3582805A

US3582805A - Amplifier system

Info

Publication number: US3582805A
Application number: US847122A
Authority: US
Inventors: Jans Kliphuis
Original assignee: Jans Kliphuis
Current assignee: JPMorgan Chase Bank NA
Priority date: 1969-08-04
Filing date: 1969-08-04
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01

Abstract

An amplifier system comprising a cooled microwave amplifier in an evacuated chamber with an input waveguide connected by a thermally isolated transition to the input of the amplifier itself. The transition includes a thermally isolated probe extending into the waveguide and a coaxial choke structure connected via a filter to the amplifier. The probe extends from one end of the center conductor of the coaxial structure. The other end of the center conductor is connected to the center coaxial input terminal of the filter and is surrounded by a first coaxial sleeve, one end of which is connected to the outer coaxial terminal of the filter and the other end of which, adjacent the waveguide and the probe, is connected to one end of a second coaxial sleeve. The second sleeve extends back along the first sleeve about lambda /4, where lambda is the wavelength to which the amplifier is tuned. The two sleeves form a lambda /4 transmission line, and the short at one end reflects as an open circuit at the other end of the second sleeve. Surrounding the second sleeve is a third tubular coaxial conductor, which may be a part of the mechanical support structure for the waveguide, and which is also approximately lambda /4 long. The latter conductor is not conductively connected to the other conductor members but the infinite impedance at the end of the second sleeve and the outer conductor facing the amplifier is reflected as a short circuit lambda /4 away, which is the equivalent of a short circuit between the latter end and the adjacent end of the first sleeve directly connected to the waveguide. An enclosure in the waveguide surrounds the probe in a gastight space that communicates with the evacuated chamber and is therefore itself evacuated. Within the chamber is a cooling unit to reduce the temperature of the amplifier. The cooling unit may be, for example, a Peltier effect device supported by a high thermal conductivity part of the chamber, or it may be a refrigerator, for example, a cryogenic device attached, thermally, to the amplifier to reduce the temperature of the amplifier to a few degrees Kelvin.

Description

United States Patent [72] inventor Jans Kliphuis 15 Hartman Hill Road, Huntington, N.Y. 11743 [2]] Appl. No. 847,122

[22] Filed Aug. 4,1969

[45] Patented June 1,1971

[541 AMPLIFIER SYSTEM 12 Claims, 6 Drawing Figs.

Primary Examiner-Nathan Kaufman Attorney-Donald P. Gillette ABSTRACT: An amplifier system comprising a cooled microwave amplifier in an evacuated chamber with an input waveguide connected by a thermally isolated transition to the input of the amplifier itself. The transition includes a thermally isolated probe extending into the waveguide and a coaxial choke structure connected via a filter to the amplifier. The probe extends from one end of the center conductor of the coaxial structure. The other end of the center conductor is sleeve extends back along the first sleeve about M4, where A is v the wavelength to which the amplifier is tuned. The two sleeves form a M4 transmission line, and the short at one end reflects as an open circuit at the other end of the second sleeve. Surrounding the second sleeve is a third tubular coaxial conductor, which may be a part of the mechanical support structure for the waveguide, and which is also approximately M4 long. The latter conductor is not conductively connected to the other conductor members but the infinite impedance at the end of the second sleeve and the outer conductor facing the amplifier is reflected as a short circuit M4 away, which is the equivalent ofa short circuit between the latter end and the adjacent end of the first sleeve directly connected to the waveguide. An enclosure in the waveguide surrounds the probe in a gastight space that communicates with the evacuated chamber and is therefore itself evacuated.

Within the chamber is a cooling unit to reduce the temperature of the amplifier. The cooling unit may be, for example, a Peltier effect device supported by a high thermal conductivity part of the chamber, or it may be a refrigerator, for example, a cryogenic device attached, thermally, to the amplifier to reduce the temperature of the amplifier to a few degrees Kelvm.

PATENTEDJUN HQ?! SHEET 30F a INVENTOR JANS KLIPHUIS AMPLIFIER SYSTEM FIELD OF THE INVENTION This invention relates to cooled microwave amplifiers and particularly to parametric amplifiers operating in an evacuated chamber and cooled to reduce generated noise.

BACKGROUND OF THE INVENTION Cryogenically cooled parametric amplifiers are used in satellite communication equipment as well as radar systems and other installations to improve the signal-to'noise ratio of the received signal. Heat loss through the input signal connection is a major problem in such amplifiers, as described in my US. Pat. No. 3,389,352 and has been attacked heretofore by enclosing a substantial length of the input waveguide or coaxial lines within relatively large evacuated containers. The volume of such containers and the mass of equipment therein, together with the relatively good conductive path by way of the input terminals to the external waveguide system have all contributed to limitations in the operating characteristics, particularly to prolonging the startup time. A substantial time is required to remove all of the gas from a large vessel; periods of 4--8 hours represent the shortest times that have been achieved heretofore in cryogenically cooled amplifiers. Moreover, when the cryogenic machines within such evacuated vessels have been started up, their ability to cryopump the last residual gas has not been very great because of the relatively slow rate of cooling the large mass within the vessel. This leads to a further increase in the startup speed of one of these amplifiers. Low startup times are desirable not only in putting an amplifier into service in the field, where, indeed, fast startup may be of critical importance, but also in factory testing, where the required time of technical operators and use of measuring equipment may involve substantial costs. The amplifier system of the present invention provides directheat transfer from the cold station to the amplifier at the two extremes of the amplifier body, resulting in a substantially uniform temperature with a minimum temperature gradient in the mating surface. Moreover, the configuration of the amplifier system permits substantial reduction in the evacuated volume which reduces cool-down time to less than 1%hours and the radiated heat loss and facilitates cryopumping residual gases into the evacuating chamber. The reduction in size minimizes the surface area in which contaminants could be trapped.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention an amplifier system is provided for connection to an input waveguide. The system includes an evacuating vessel, or dewar, mechanically connected to the waveguide and having a conductive hollow tubular member that forms part of the wall of the dewar and .is joined to the waveguide. Within the waveguide, itself, is a gastight barrier that surrounds the entrance of the tubular member and is sealed to the dewar to be part of the evacuated volume. A probe extends into the waveguide within the barrier to extract a signal from the waveguide. The probe is connected by a slender center conductor to the inner conductor of a coaxial input terminal of the amplifier. Surrounding part of the center conductor is a coaxial sleeve connected to the outer container of the coaxial input terminal of the amplifier. The sleeve extends almost to the waveguide but stops short of thermal or mechanical contact. At the end of the sleeve adjacent the waveguide is a second sleeve directly connected to the first sleeve and extending coaxially back along the first sleeve toward the amplifier for a distance of M4 where k is the operating wavelength of the amplifier. As a result of the short circuit between the first and second sleeves at one end, there is an open circuit between these sleeves at the other end (the :end closer to the amplifier) which is M4 away from the short.

The second sleeve is also coaxially located within, but mechanically and thermally isolated from, a hollow, tubular conductive member that forms the physical channel from the dewar to the waveguide. The conductive member is approximately the same length as the second sleeve and these two elements make up a M4 choke which is open at both ends. The open circuit from the end of the second sleeve closer to the amplifier to the adjacent end of the tubular conductive member is reflected as a short circuit between the latter and the second sleeve at the end remote from the amplifier. Thus the signal effectively sees a direct connection from the first sleeve to the waveguide, but there is complete thermal separation, and heat loss is therefore substantially eliminated.

While the lowest noise figure is obtained by cryogenically cooling the amplifier, this structure may also be used with a cooling unit of much lower cooling capacity than a cryogenic machine. For example, a Peltier effect device may be thermally and mechanically connected to the amplifier and to a good thermally conductive portion of the dewar. The latter portion can then be cooled by a blower to a temperature as low as the ambient so that the additional cooling effect of the Peltier element will reduce the operating temperature of the amplifier substantially below ambient level.

More than one such amplifier may be used within the same dewar. For example, four amplifiers may easily be mounted on a thermal transfer structure surrounding the cryogenic device or attached to the Peltier element. All of the amplifiers may be operated with the same pumping signal in accordance with known parametric amplifier practice, but the clustered configuration of the whole structure permits it to be enclosed within a smaller dewar.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in detail in connection with the drawings in which:

FIG. 1 is a perspective view of an amplifier system constructed in accordance with the invention;

FIG. 2 is a top view of the system of FIG. 1;

FIG. 3 shows the dewar and its included components from the system of FIGS. I and 2 and with part of the walls broken away to show the interior construction;

FIG. 4 shows a top view w of the arrangement of the amplifer around the cryogenic machine in FIG. 3;

FIG. 5 is a cross-sectional view of the input transition used in the system of FIG. 3;

FIG..6 shows a mounting bracket and Peltier cooling unit for a modified form of amplifier system similar to that shown in FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS The amplifier system in FIG. I comprises an input waveguide 11 having a connection flange l2 and seven resonators 13-19 of which only the resonators 13l7 are visible in FIG. I. The waveguide 11 is mechanically attached to the flat upper end 21 of a dewar 22, which also has a plate 23 covering a second opening through which connection may be made for a cold load.'The dewar 22 is divided into an upper portion 24 attached by clamps 25 to a heavy central plate 26 and a lower portion 27 rigidly sealed gastight to the plate 26 and to a heavy bottom plate 28. A frame member 30 also joins the

plates

26 and 28 together and this whole structure is rigidly mechanically attached to a housing 29 that contains electronic components required in the operation of the amplifier system within the dewar 22. The output terminal of the amplifier is a coaxial connection 31 that extends through the plate 26.

A cryogenic refrigerator 32 capable of reaching a temperature of l5 K. extends through the bottom wall of the lower portion 27 of the dewar and is sealed thereto to permit the dewar to be evacuated by way of a vacuum valve 33 connected to a vacuum pump. The temperature of the cryogenically-cooled components within the dewar 22 is measured by a gauge 34 connected into the dewar by a line 36.

FIG. 2 shows the top view of the amplifier of FIG. 1 with the addition of a waveguide connection 37 for a cold load. FIG. 2 also shows an additional support member 38 attached to the

plate

26 and 28 to support the entire assembly. it should be noted that the arrangement ofthe components is such that the dewar 22 and refrigerator structure 32 which are the most rigid and massive components, are the ones that are directly supported by the

members

30 and 38 and that the electronic components within the housing 29 are supported from the dewar 22, rather than the other way around. The location of the dewar 22 in one corner of the electronic housing 29 facilitates this supporting arrangement.

FIG. 2 also shows the upper surface of the waveguide 1]. Near the end of the waveguide is a cap 41 held down by three screws 42. This cap is directly over the input terminal of the amplifier within the dewar 22, as will be explained in greater detail hereinafter. A similar cap 43 held in place by three screws 44 is located on the upper surface of the waveguide 37.

FlG. 3 shows the inside of the dewar 22 and some of the other components. At the top of the dewar is the waveguide 11 with the seven resonators l3-l9 that form a reject filter for transmit frequency leakage power around 6gc. The two resonators l8 and 19 on the bottom of the waveguide 11 match the waveguide to the standard WR 229 waveguide. The waveguide size is reduced WR 187 and the size reduction eliminates the higher order modes normally present at the transmit frequency in the WR 229 and the WR 187 waveguide.

FIG. 3 also shows in some detail the enclosure around the input to the amplifier. Mechanically, this enclosure includes the cap 41 which has an O-ring gasket 46 in the surface facing a support ring 47 that is joined to the upper surface of the waveguide 11. in the lower surface ofthe cap 41 is another ring 48 that serves as a washer to seal, in a gastight manner, the upper open end of a quartz cylinder 49 that surrounds a probe 51. This is the input probe to pick up electromagnetic energy from the waveguide 11 and carry it to the amplifier proper. The waveguide 11 is mounted on a conductive support 52 that forms part of the upper surface 21 of the dewar and has a cylindrical channel 53 therethrough. The conductive member 52 is attached to the flat surface 21 by a plurality of clamps, one of which is indicated by reference numeral 54. An O-ring 56 forms a gastight seal between the member 52 and the plate 21, and since the member 52 is circular and the upper surface 21 is flat, the waveguide can be rotated 360 and can be clamped in place in any orientation that may be desired for ease of connection to other equipment. The center of rotation of the waveguide is the axis of the probe 51.

In order to describe the components within the dewar 22, reference will be made to both FIGS. 3 and 4. The channel 53 includes part of an RF transition 58 which will be described in greater detail hereinafter. The RF transition 58 is connected to the coaxial input of slablinc filter 55 that rejects undesired transmitter frequencies of the usual satellite communication system of which this amplifier system is a part. The slabline filter 55 is connected directly to the input of an amplifier 59 which comprises a circulator unit 61 and a parametric amplifier unit 62. The latter is fed with a pumping signal by way of a waveguide 63, and the output signal ofthe amplifier 59 is connected through a coaxial transmission line 64 to the input terminal 65 ofa second amplifier 66. The latter is identical with the amplifier 59 and comprises a circular unit 67 and an amplifier unit 68 supplied with a pumping signal by way of a waveguide 69. The output signal of the amplifier 66 is connected from a coaxial output terminal 71 through a coaxial transmission line 72 to the input terminal 73 ofa third amplifier 74. The latter is identical with the other two amplifiers and comprises circulator 76 and an amplifier unit 77 supplied with a pumping signal by way of a waveguide 78. Each of the

amplifiers

59, 66 and 74 has three circulators controlled by permanent magnets like the magnets 8183 for the circulators in the amplifier 74. The coaxial output terminal 84 ofthe ampli fier 74 is connected through a coaxial transmission line 86 to the output terminal 31.

The

amplifiers

59, 66 and 74 are cooled to an extremely low temperature of the order of l5 K. or thcreabouts by means of the refrigerator 32, which includes a lower portion 87 having a temperature of about 70 K. and a cold station 88 at the still lower temperature of about 15 K. The

amplifiers

59, 66 and 74 and physically clustered closely about the cold station 88 and are thermally attached to the cold station by means of blocks 89 and 91 of material, such as copper, that has good thermal conductivity. The blocks 89 and 91 are so spaced that the two ends of each of the

amplifiers

59, 66 and 74 are connected to them, thereby resulting in a minimum temperature gradient across the amplifiers and improving the noise figure. The physical arrangement of the amplifiers around the cold station 88 makes it possible to reduce the total amount of space occupied by the amplifiers and this in turn makes it possible to reduce the size of the dewar 22. The reduction in size of the dewar facilitates evacuation of all gases and increases the cryopumping ability of the system whereby the vacuum is improved cryogenically.

A cold load resistor is physically mounted on the amplifi er 74 to make good thermal contact therewith, although it is electrically insulated from the amplifier. Electrical connection from the waveguide 37 in FIG. 2 to the resistor 90 is made by a RF transition 95, which is similar to the transition 58 but does not include a filter. This type of transition permits good elec trical connection between the cold load and the waveguide while maintaining almost complete thermal isolation.

The arrangement of the amplifiers is so that the input of the first amplifier 59 is physically quite close to the transition 58 and the output ofeach of the amplifiers is close to the input to the next amplifier to reduce further the total mass within the dewar 22. The output coaxial line 86 is made relatively long to minimize heat transfer from outside the dewar 22 into the region of the cold station 88. For the same reason, the waveguide 78 and the

other waveguides

63 and 69 are also made longer than necessary for electrical performance. lnsulation of the cold station 88 from radiation is further improved by a screen 92 covered with Mylar film coated with a reflective metal coating. This screen is in the shape of an open cylinder that fits within the upper portion 24 of the dewar 22. The evacuated volume includes the dewar 22 and the small space within the quartz cylinder 49 around the probe 51. The gaskets including the O-ring 56 in the region ofthe upper surface 21 of the dewar 22 have been described. In addition, an O-ring 93 is located in the lower edge of the upper portion 24. This provides a gastight seal when the dewar is evacuated but permits the upper portion to be lifted off when the vacuum is broken in order to get at the inner components for service. The waveguide 78 and the other two

waveguides

63 and 69 have thin gastight windows at their outer ends, such as the window 94 for the waveguide 78.

In order to service the

amplifiers

59, 66 and 74, the refrigerator 32 must be turned off so that the cold station 88 can heat up and in addition, the air must be allowed to get into the dewar 22. The material of which the refrigerator is made would not normally conduct heat rapidly into the interior of the dewar and so a heater, which is in the form of a strap of conductive metal, is placed around the cold station 88 between the blocks 89 and 91, as shown in FIG. 4. This strap 96 has two

wires

97 and 98 connected to it to carry current to heat up the cold station although the strap itself is electrically insulated from the cold station by a band of electrical insulating but heat conducting material 99. A switch 101 may be connected in series with the strap 96 and one of the

wires

97 or 98 to limit the heating of the cold station 88. By virtue of this heating arrangement it would require to bring the temperature of the cold station back up to ambient is greatly reduced. Thus, the down-timc required to service the amplifier system is minimized.

P10. 5 shows the RF transition 58 in greater detail. As may be seen, the probe 51 is connected by a short cylinder 102 of slightly smaller diameter to the central conductor 103 of a coaxial system. This coaxial conductor 103 has a first sleeve 104 surrounding the portion immediately below the cylinder 102 and not far below the probe 51. This sleeve 104 is separated from the inner conductor 103 by a sleeve ofinsulating material 106. The upper end of the end sleeve 104 is connected by an annular ring 107 to the upper end of a second sleeve 108, the length of which is approximately /4, where A is the wavelength to which the

amplifiers

59, 66 and 74 are tuned. The sleeve 104 and the second sleeve 108 comprise a quarter wave transmission line which has a short circuit at the upper end. As a result, an open circuit will be reflected at the lower end 109 of the sleeve 108 so that current cannot pass from the outer coaxial conductor 111 to the lower end 109 of the sleeve 108. The sleeve 108 is eoaxially located within the hollow, tubular, conductive channel 53, which is also approximately A/ 4 long. Conductive channels 53 are connected at either end to the sleeve 108 and, therefore, the open circuit betweenthe lower end 109 of the sleeve 108 and the adjoining lower end 112 of the channel 53 is reflected as a short circuit between the annular ring 107 and the adjacent portion 113 of the conductive member 52. This part of the conductive member 52 is actually a short coaxial transmission line that connects directly with the waveguide 11.

it is important to note that the electrical equivalent of a short circuit between the annular ring 107 and the conductive region 1113 is achieved without any direct metallic contact. Thus the complete heat insulation furnished by the vacuum in the space within the conductive channel 53 is maintained. Because of this the noise temperature of the system is extremely low and is of the order of 13 K or less.

FIG. 6 shows an alternative embodiment that does not include the cryogenic refrigerator. instead, a Peltier cooling element 114 is mounted on a frustum 116, which is'intcgrally formed with the plate 26 of a metal having excellent thermal conductivity and is used to cool the amplifier. Conduction of heat from the amplifiers, of which only the amplifier 66. is shown in broken lines, is achieved by means ofa heavy copper bracket 117 that is mounted on the Peltier element 114 andis provided with mounting screw holes 118 to receive the mounting screws ofthe individual amplifiers.

This structure is enclosed in the upper portion 24 of the dewar 22, which is evacuated for insulating purposes. The plate 26 is cooled by a blower 119 to the ambient temperature and the Peltier element 114 further cools the bracket 117and the amplifiers attached to it to a temperature that may be approximately 50 C. below ambient. All of the advantages of the close configuration of amplifiers are retained with the structure shown in FIG. 6 but the noise temperature, of course, is substantially higher than that of the cryogenically cooled embodiment.

What 1 claim is:

1. An amplifier system comprising:

A. An evacuated vessel;

B. A cooling unit in said vessel;

C. A waveguide mechanically connected to said vessel;

D. An amplifier having two physical ends and located within said vessel and mechanically and thermally connected to said cooling unit and comprising a coaxial input terminal having an inner and an outer conductor;

E. An input transition electrically connecting said waveguide to said input terminal, said transition comprismg:

l. A central conductor directly connected to and extending from said inner conductor and terminating in a probe extending into said waveguide but mechanically separated therefrom,

2. A sleeve extending from said outer conductor toward said waveguide but mechanically and thermally separated from waveguide and eoaxially surrounding said central conductor,

. A second sleeve directly electrically connected at one end to the end of said sleeve remote from said amplifier and eoaxially surrounding said first sleeve, said second sleeve having a length approximately equal to onefourth of a wavelength of the signalamplified by said amplifier, and

4. An outermosting tubular conductor surrounding and coaxial with said sleeve and mechanically and thermally separated therefrom and mechanically connected to said vessel and said waveguide, the length of said outermost conductor being approximately equal to the length of said sleeve; and

F. Enclosure means within said waveguide spaced from said probe and substantially transparent to electromagnetic waves to which said amplifier is tuned and sealed gastight to said waveguide to enclose space communicating with the interior of said vessel so as to be evacuated with said vessel.

2. The amplifier system of claim 1 in which said vessel is cylindrical, and the longest dimension of said amplifier is the dimension between said two physical ends, and said longest dimensions is aligned parallel to the axis of said cylinder.

3. The amplifier system of claim 1 in which said vessel is cylindrical and has a substantially flat top, and said amplifier system comprises, in addition, pivotal mounting means for mechanically connecting said waveguide to said flat top of said vessel.

4. The amplifier system of claim 1 in which said vessel is cylindrical and said cooling unit is a cryogenic refrigerator having a cylinder substantially coaxial with said vessel.

5. The amplifier system of claim 4 in which said cryogenic refrigerator comprises a cold station at one end thereof and said amplifier system comprises, in addition, mounting block means thermally joined to said cold station and to said ends of said amplifier.

6. The amplifier system of claim 5 in which said cold station is cylindrical and said mounting block means comprises separate thermally conductive blocks axially spaced apart on said cold station, said amplifier system comprising in addition, a heater wrapped around said cold station between said blocks.

7. The amplifier system ofclaim 1 in which said cooling unit comprises:

A. A Peltier element;

B. A mounting plate forming one end of said vessel;

C. A thermally-conductive pedestal on said plate and supporting said Peltier element; and

D. A thermally conductive bracket mounted on said Peltier element and supporting said amplifier.

8. The amplifier system of claim 1 comprising, in addition:

A. A reject filter along said waveguide to reject transmitted signals having wavelengths slightly different than the wavelengths of the signal amplified by said amplifier; and

B. An additional reject filter between said amplifier and said coaxial input terminal whereby said additional filter is cooled to the same temperature as said amplifier.

9. The amplifier system of claim 1 in which said vessel is cylindrical, said system comprising, in addition:

A. An additional amplifier having two physical ends, the dimensions between said physical ends of each of said amplifiers beingthe greatest dimension of said amplifiers; and

B. Mounting means for mechanically and thermally connecting both-of said amplifiers to said cooling unit with said greatest dimension of said amplifiers parallel to the axis of said vessel.

10. The amplifier system of claim 9 comprising, in addition:

A. A cold load resistor mechanically and thermally connected to said cooling unit;

B. A second waveguide mechanically connected to said vessel; and

C. A second transition electrically connecting said second,-

waveguide to said cold load resistor but thermally insulating said cold resistor from the wall of said vessel-and from:

said waveguide.

11. The amplifier system of claim 1 in which said enclosure,-

means comprises:

A. a hollow insulating cylinder;

B. Means hermetically sealing one end of said' cylinder to:

the inner surface of said waveguide adjoining said vessel;.

mounting plate attached to and forming a central portion of said vessel, the portion of said vessel on one side of said plate being rigidly attached thereto, and the portion of said vessel on the other side of said mounting plate being separable therefrom.

Claims

1. An amplifier system comprising: A. An evacuated vessel; B. A cooling unit in said vessel; C. A waveguide mechanically connected to said vessel; D. An amplifier having two physical ends and located within said vessel and mechanically and thermally connected to said cooling unit and comprising a coaxial input terminal having an inner and an outer conductor; E. An input transition electrically connecting said waveguide to said input terminal, said transition comprising:

1. A central conductor directly connected to and extending from said inner conductor and terminating in a probe extending into said waveguide but mechanically separated therefrom,

2. A sleeve extending from said outer conductor toward said waveguide but mechanically and thermally separated from waveguide and coaxially surrounding said central conductor,

3. A second sleeve directly electrically connected at one end to the end of said sleeve remote from said amplifier and coaxially surrounding said first sleeve, said second sleeve having a length approximately equal to one-fourth of a wavelength of the signal amplified by said amplifier, and

4. An outermosting tubular conductor surrounding and coaxial with said sleeve and mechanically and thermally separated therefrom and mechanically connected to said vessel and said waveguide, the length of said outermost conductor being approximately equal to the length of said sleeve; and F. Enclosure means within said waveguide spaced from said probe and substantially transparent to electromagnetic waves to which said amplifier is tuned and sealed gastight to said waveguide to enclose space communicating with the interior of said vessel so as to be evaCuated with said vessel.

7. The amplifier system of claim 1 in which said cooling unit comprises: A. A Peltier element; B. A mounting plate forming one end of said vessel; C. A thermally-conductive pedestal on said plate and supporting said Peltier element; and D. A thermally conductive bracket mounted on said Peltier element and supporting said amplifier.

8. The amplifier system of claim 1 comprising, in addition: A. A reject filter along said waveguide to reject transmitted signals having wavelengths slightly different than the wavelengths of the signal amplified by said amplifier; and B. An additional reject filter between said amplifier and said coaxial input terminal whereby said additional filter is cooled to the same temperature as said amplifier.

9. The amplifier system of claim 1 in which said vessel is cylindrical, said system comprising, in addition: A. An additional amplifier having two physical ends, the dimensions between said physical ends of each of said amplifiers being the greatest dimension of said amplifiers; and B. Mounting means for mechanically and thermally connecting both of said amplifiers to said cooling unit with said greatest dimension of said amplifiers parallel to the axis of said vessel.

10. The amplifier system of claim 9 comprising, in addition: A. A cold load resistor mechanically and thermally connected to said cooling unit; B. A second waveguide mechanically connected to said vessel; and C. A second transition electrically connecting said second waveguide to said cold load resistor but thermally insulating said cold resistor from the wall of said vessel and from said waveguide.

11. The amplifier system of claim 1 in which said enclosure means comprises: A. a hollow insulating cylinder; B. Means hermetically sealing one end of said cylinder to the inner surface of said waveguide adjoining said vessel; C. A removable cap on said waveguide opposite said probe; and D. A washer attached to the surface of said cap facing said cylinder to seal the other end of said cylinder gastight.

12. The amplifier system of claim 1 in which said vessel is cylindrical and said system comprises, in addition, a heavy mounting plate attached to and forming a central portion of said vessel, the portion of said vessel on one side of said plate being rigidly attached thereto, and the portion of said vessel on the other side of said mounting plate being separable therefrom.