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i(NA5A-CE-166762) RUGGEL'IZE.D N82 -15297BL?^CTRONOGRAPHIC T UBE DEVEICIML14T FinalProject Report, Oct. 1979 - Oct. 1980 (ITTElectro-Uptical Products Div., Port) 23 p UnclasHC A02/MF A01 CSCL 09A 63/33 08797

&o tt Nevin

ITT Electro-Optical Products Division3700'East Pontiac StreetFort Wayne, Indiana 46803

January 1981

'Final Project Report for Period October 1979 - October 1980

NASA Contract No. NASS-25877

ITT Project No. 11-25900

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Prepared forGODDARD SPACE FLIGHT CENTERGreenbelt, Maryland 20771

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Preface

Electronographic tubes known as Spectracons are permanently

sealed detectors whose signal electrons exit through a thinmica window and penotrate a photographic recording emulsion.

Currently available tubes are of glass ring and glass envelope

construction, suitable p-imarily for laboratory use. Their

photocathode response is limited to the visible spectrum because

graded seals of ultraviolet transmissive materials, such as

magnesium fluoride, to the glass envelope is a difficult

technology.

It is the object of this project to build a, ruggedixedversion of the Spectracon having good ultraviolet sensitivity

and, if possible, a large effective image area.

While the program is uncompleted for financial reasons,

the design and sub-assemblies fabricated appear to meet wellthe requirements, and the chances of building successful operating

Spectracons having the desired characteristics seem very good.

Consequently, it is recommended that the work be completed in

the near future.

. 2 -

Introduction

The Spectracon is a magnetically focused electronographictube whose 40 kilovolt electrons exit a 4 ji ► thick mica window,Tubes described in the literature perform very well, but havesevere limitations that restrict thei-v use. Among the limita-tions are:

o No far ultraviole-'I-, sensitivityo Not rugged enough for rocket launcho, No option for permanent magnet focusing.

Using technology developed. for Generation 11 image tubesand magnetically focused image tubes, these limitations canbe overcome.

Spectracon Design

The performance and mechanical requirements of thisSpectracon design are shown in Table 1. These characteristicsare compatible with the application of the tube.

The design proposed originally is shown in Figure 1.Note that the tube body is made of metallized ceramics withthe accelerating electrodes brazed to it, not hung within it.This is a key feature of the ruggedization of the Spectracon..Although the 4 um thick mica window is delicate, it is of lowmass and if correctly supported it can easily withstand theforces of rocket launch. It is the Spectracon body andelectrodes, the large mass items, that must be designed againstshock and vibration damage.

The photocathode is deposited on an ultraviolet transmittingmagnesium fluoride faceplate in a remote process vacuum system.This permits any desired photocathode, such as bialkal.i, Cs I,or multialkali, and results in uniform sensitivity over thewhole active photocathode area. The faceplate, at-ter photo-cathode processing, is sealed to the ceramic- and- metal tubebody by a indium-bismuth solder seal. This seal tolerates themismatch of the window and body expansion coefficients. It hasbeen used on Many tubes, including the International UltravioletExplorer image diode.

Encapsulation is done with the same potting compounds usedfor 45 kilovolt Generation I imago tube assemblies.

The permanent magnet could be an exotic self-sliielding designof the Raytheon type, but within the volume allotted., a stack ofring magnets, as used on the ITT F4089 magnetic focus image tube,can. be placed and the magnetic ficld trimmed to provide Spectracon.performance meeting Table 1. For the present, such a simplepermanent magnet is proposed. See the Appendix..

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A requirement is for the magnesium fluoride window to havea maximum thickness of 4 um. The I.U.E. image diode had a

k.54 mm thick magnesium fluoride window. Scaling laws wouldmake the Spectracon's larger (70 mm. versus I.U.E.'s 60 mm)faceplate 3.5 mm thick to have no more stress or deflection. Athickness of 3.81 mm was selected, Figure 2.

Before the input window's outside diameter was selectedthe design of the Spectracon's body ceramics was reviewed.The published Spectracon literature notes that McMullan, power.,

and Curtis' had instability problems when they increased the

gaps between their electrodes from 10 mm to 20 mm. They attributethe instability to the charging of insulators exposed to theelectron beam.

The narrow ceramic width of Figure 1 was increased to 3.81 mmto obtain a sufficiently wide metallized surface. Because the

Spectracon assembly has a limited (maximum) outside diameter andneeds potting and leads for 40 kilovolt operation, the ceramic

ring width was attained by mak ag the ring inside diameter smaller

than originally planned. To prevent charging of the ceramic

surfaces, each ring was made only 6.3 mm high, Figure 3.

To accompany the ceramic drawing, a 16-page specification

(ITT VT 10000) was written to ensure metallizing of the ceramicsthat would provide leak tight envelope assemblies. The concern

here can be explained as follows. A Spectracon body is comprised

of over thirty brazed ceramic rings. To have 9 of 10 body

assemblies leak tight means that each metallized ring must be

perfectly brazed 99.67% of the time (299 good in 300 attempts).

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The Spectracon tubs body, Figure 4, required a copper ringat its input end to which the magnesium fluoride window wouldeventually be sealed, anti a U-shaped weld rang at its output endto accommodate the mica window assembly. Each of these electrodeswere scaled from existing designs.

The accelerator rings were designed next. ConventionalSpectracons have simple disk shaped accelerator electrodes,Cone shaped electrodes, Figure S, are more stable in shape.This might be a problem during shock for flat disk electrodes,particularly since copper is dead soft after the braze operation.

The accelerators are lipped in a manner to make theceramic and ring stack self- aligning when it is initiallyassembled at room temperature. Due to the difference in

expansion between the copper and ceramic, alignment rods arealso necessary for the braze fixture.

In using the accelerators, one has a choice regarding theirorientation; they may point toward the photocathode or toward

the mica exit window. Each orientation has advantages and

disadvantages. With the cone-like accelerators pointed towardthe mica window, the electron beam is shielded from the ceramicwall., except adjacent to the photocathode. But it is near thephotocathode that the photoelectrons are most easily influenced

by 'the electrostatic field. Hence, the orientation chosen for

the accelerators is to direct them toward the photocathode, asi

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The voltage on the accelerator nearest the photocathodecan be adjusted to minimize spiral, distortion of the outputimage. This is done, to the specification of Table 1, in theITT F4089 magnetic image tube wherein the same accelerator ringorientation is used. This adjustment is more necessary in thenon-uniform field of permanent magnets than in the field ofgood solenoids.

Despite having made a choice of accelerator orientation,freedom from wall charging and the capability of long-term imageposition stability must be de;,iionstrated in the operating tube.

The mica exit window is specified in Table 1 'is 4 pm thick,

see also Figure 6. The sealing of similar Lenard windows is

described in the literature from at least 1942. ,A 1969 article'by J. D. McGee and D. McMullan specifically addresses the

Spectracon window and suggests sealing the mica to a curvedsurface that approximates the curvature of the mica due toatmospheric pressure, This curved sealing surface tends toreduce stresses at the edge of the window. The article alsonotes that leak-free seals are easily obtained. But fiveyears later, the same group reported' that mica window sealswere indeed a problem--photocathode stability is very sensitiveto small. leaks.

The radius of curvature according to reference 2, equation 2

therein, ideally would be 32 mm, but mechanical constraints leadus to use a 50 mm radius, Figure 7.

The conventional Muscovite mica (thermal expansion

coefficient of 9 to 12 x 10 -6 per oC) window seal is made toSylvania No. 4 metal (9 x 10- 6 per o C) with Corning No 7570glass frit.

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Another combination, with which we have had some experience

is titanium (actually Ti-55A) and Corning No. 7570 PyToceTam

frit. Because the glass sealing steel alloys (Sylvania No. 4,

Carpenter No. 426, etc,) are difficult to buy in small quantities

and because a relatively thick plate, see Figure 7, is desired,

we elected to use titanium (expansion coefficient of

8.5 x 10- 6 per O C) and Pyroceram.

The outside weld ring (4726357) at the exit end of the

Spectracon body, Figure 4, mates with a weld ring (4726558) in

which the titanium support plate is mounted. The two weld

rings are heliarced together as one of the last steps of

tube assembly. Mounting the titanium support plate to the

inside weld ring (made of Carpenter No. 49 steel) presents

somewhat of a problem. Titanium doesn't easily braze to steel;

but it can be brazed if the steel is nickel plated.

Other than specifying the needed braze washers, this

complet6d the design of the Spectracon tube itself. Forthe resistive divider to be potted around the tube, the

resistors used for ITT's F4089 magnetic focus image tubewere selected.

For the tube's 45 kilovolt operation, GE-118 primer andDC-170 encapsulant as used

on Generation I image tube assemblies,

that also operate at 45 kilovolts, were selected. Highvoltage cables 'made by Rowe, Inc., can be accommo dated. within

the package dimensions.

Fixtures were designed for brazing, welding, masking,

evaporation, leak check, processing, etc.

- 15 -

Tube Building

4

After the design effort,materials, and fixtures,

orders were placed for parts,

Mica, in 9 pm, 6 pm, and 4 pm thicknesses was obtained

from Asheville-Schoonmaker Mica Company, The thicker pieces

were to the desired shapes, Figure 6, but the 4 pm thickness

was available only in sheets of varying outlines.

It turned out that titanium stock suitable for the mica

support plate was as elusive to obtain as the Sylvania No. 4

metal we decided not to use, but a source was finally found,

Tests to develop a fritting and firing schedule went ahead

with the thicker mica. pieces and thin titanium sheet stock.

Eventually wrinkle-free and vacuum-tight windows, sealed with

Corning Pyroceram No. 7570 frito were obtained. Certifiedgood windows using the 4 pm thick mica were not quite achieved

during the project, but probably could be made now.

A major offort was being sure that the metallized ceramic

rings were satisfactory. They were 100% visually inspected

when received and sample braze seals made, Some were sent

back for re-work; most were eventually deemed acceptable. Thus,

the quality of the supplied metallized ceramics was pretty

good. Stacks of a few rings were brazed, found to be leak; tight,

and after microscopic examination of the brazed joints the

ceramics were deemed acceptable.

One full size Spectracon stack was brazed and appears tobe leak tight, Figure 8. Because of past ceramic envelopeproblems, this was considered a major accomplishment.

Unfortunately, tube parts and the many fixtures needed to

do the work right resulted in more cost than had been anticipated,

so work had to be stopped.

^ 16 -

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BRAZED, LEAK TIGHT

SFECTRACON BODY

Figure 8

- 17 -

M 9

Status

Presently all parts and materials are on hand to build aSpectracon. One major subassembly, a ceramic body, has beenbrazed together, and it is leak tight. The individual metallizedceramics on hand should provide more good bodies.

The 4 um thick mica window does require some refinementof sealing; technique to be trustworthy, but it is thought thatthe problem is just that, refinement.

We were not successful, during the few trials made, atobtaining good braze seals between the titanium window supportplate and the mating weld rang. Recently we have been workingon another contract with an electron beam welding vendor wholeads us to believe that the titanium could easily be weldedby this technique.

We have no particular concern about photocathode processingor sealing the magnesium fluoride faceplate, but axe curiousabout the possibility of charging at the ceramic wall. Since

the cone--shaped electrodes could be re-oriented fairly easily,

a test tube with phosphor output to evaluate the present designseems like a good experiment prior to building the full scale

Spectracon.

In summary, a good design for a rugged Spectracon has

been brought along through the sub-assembly stage. With or

without a dew verification experiments, a Spectracon is readyto be built.

18 -{

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Appendix

Magnet Design

1

The equation for focus of an electron beam in a magnetic

field is

L = (1.06E-OS)N(V) h B-1

where N is the number of Loops

V is the anode voltage

B is the magnetic field, in Teslas

L is the beam length, in mm

For two loops of focus of a 40 kV beam traveling 212 mm,

the magnetic field needed for focus is 20 mT. This can be .1

generated by a very expensive Raytheon magnet, or by

inexpensive Alnico rings. Using eight rings (a double FW313

four-ring fo(ras magnet, Figure A-1) will provide a permanent

magnet assembly that can be adjusted to 20 mT.

Table A-1 shows the magnet's characteristics and

that of the Raytheon assembly, too.

,f 19 -

Figure A-1

01

FW-313, FW-314IMAGE TUBE FOCUS MAGNETS

EL-_313 4 ring focus magnetAxial field strength : S00 + 100 gaussApplicable tube types : Image Tubes : FW-116 0 FW-132, FW-167

Image Intensifiers : FW-113, FW-117, FW-159Storage Image'T'ubes : FN-231, FW-232

Dimensions:

4-1/414 1/16 5" (± 1/16

+ 1/8EW-314 b ring focus magnet

Axial field strength :

500 + 100 gaussApplicable tube types : Image Intensifiers : FW-152, FW-153,

FW-154, FW-155

Dimensions:

a4-1/411 + 1/16" $" + 1/16"

8-7/16+ 1/8 I

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REFERENCES

1. D. McMullen, J. R. Powell, and N. A. Curtis, Advances inElectronics and Electron Ph sits, Volume 40B,`1976;A.

2. J. D. McGee and D. McMullan, Journal of ScientificInstruments,, Series 2, Volume 2, 1969 0 p

I.

- 22 -