PB 151406

^ecknical ^lote 47

AN IMPROVED HYDROGEN

ATOM BEAM FURNACE

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

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NATIONAL BUREAU OF STANDARDS

technical Ntote

47

MAY 1960

AN IMPROVED HYDROGEN ATOM BEAM FURNACE

Ralph Klein and John Pararas

This research was performed under the National Bureau

of Standards Free Radicals Research Program, supported

by the Department of the Army.

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature. They are for sale by the Office of Technical Services, U. S. Department of Commerce, Washington 25, D. C.

DISTRIBUTED BY

UNITED STATES DEPARTMENT OF COMMERCE

OFFICE OF TECHNICAL SERVICES

WASHINGTON 25, D. C.

Price 50 cents

AN IMPROVED HYDROGEN ATOM BEAM FURNACE

Ralph Klein and John Pararas

An improvement in the construction of the tungsten

furnace for use in a hydrogen atom-beam apparatus has been

effected by fabricating the furnace from a tungsten rod.

This avoids the possibility of leakage of molecular hydrogen which may occur when the tungsten tube is made from

rolled tungsten sheet, the technique used heretofore. The

fabrication of the seamless furnace and the method of

mounting are described.

INTRODUCTION

In many applications of atom beam techniques involving hydrogen, it

is desirable to minimize the molecule-atom ratio of the beam. Several ex- perimental considerations affect this ratio such as the design of the

vacuum system and the construction of the furnace in which the hydrogen is

dissociated. It is the latter problem which will be considered in some

detail.

Experiments in which surface reactions of hydrogen atoms could be

observed with a field emission microscope technique required an apparatus

in which a field emission microscope tube, operating at liquid helium temperature, was attached to a hydrogen atom beam source. Considerable simplification in the interpretation of the field emission patterns would

result if the percentage of hydrogen molecules in the impinging beam were

maintained at as low a level as possible. Further, since the emitting

point presents a target of the order of 10 -8 Sq. cm> a serious problem of

the beam location can be avoided only if the beam cross section is sufficiently large. The intensity need not be high since even a fraction of a monolayer deposited on the emitting tip is easily observable. Considerations of this nature dictate the dimensions of the furnace, and in particular the size of the hole through which the atom effusion occurs.

Lamb and Retherford' describe, in their notable paper on the fine

structure of hydrogen, a hydrogen dissociator made by rolling .004" tungsten sheet in the form of a cylinder. A slot cut near the center provided

the hydrogen outlet. It is pointed out, in the authors 1 description of

this furnace, that there is leakage of molecular hydrogen around the ends

of the cylinder. Fite^ employed a similar technique, using however,

1 Lamb, W. E. and Retherford, R. C, Phys. Rev. 79, 5^9 (1950).

2 Fite, Wade L. , "Ionization of the Hydrogen Atom on Electron Impact",

General Atomics Report GA267, Dec. 20, 1957.
0.001" thick tungsten foil rolled into 3/16" diameter tubes with six layers

of foil making up the tube walls.

It is advantageous to construct the tungsten furnace from a solid

tube with an axial hole and an appropriate central exit slot. That these

have not been employed previously is to be ascribed to difficulties in

fabrication. These difficulties have now been resolved, and the preparation of tungsten tube furnaces can be accomplished.' Fig. 1 is the design

adopted for the application described. The furnace is made as follows:

The starting blank is a tungsten rod, in this case .2/47" diameter. A blind

hole, coaxial with the rod, is drilled with a hole .187" diameter, .062"

deep being provided at the open end. The tube is then ground to give a .005" wall, as shown in the figure. Finally, the effusion hole is bored

through the wal 1

The success of drilling the long, narrow hole in the center of the

tungsten rod depends to a great extent on the skill and experience of the

fabricator. A tungsten carbide drill (carballoy 883) is used. It is

shaped as shown in Fig. 2. The rod is mounted in a lathe and spun at 700

to 900 rpm, the drill being held in the tail stock. As the drilling proceeds, frequent removal of chips with a compressed air blast is required.

It is important that the drill be resharpened frequently. The hole is

made first with a roughing drill and completed with a finishing drill.

About 8 hours are required to complete the operation.

The wall of the tube is ground to a thickness of .005" with an alundum wheel and a sulfur base lubricant. The tube is held with a mandril

of high-speed steel specially ground to accurately fit the hole in the

tungsten rod. The mandril is taper locked for the grinding process. The

effusion hole is drilled through the wall as the final operation on the

tungsten tube. During this process, the high-speed steel mandril is re- placed with a similar one of cold rolled stock. The hole may be of any

size depending on the requirements of the experiment. A fine slot may be

drilled if a rectangular effusion orifice is desired.

The furnace was mounted as shown in figures 3 and 4. Platinum brazing of the tungsten to the molybdenum tube proved quite satisfactory. The

molybdenum tube, surrounded by two concentric cylinders for water cooling,

provides for passage of molecular hydrogen (introduced through a palladium

thimble) to the dissociator. The blind end of the tungsten tube is clamped

by a setscrew in a molybdenum holder. The holder in turn is setscrewed to

the double copper coil as shown. Water circulating through these coils

provides for cooling. The molybdenum holder moves freely in a stainless

steel block, thus providing for the thermal expansion of the tungsten tube.

The tungsten tube furnace is heated with alternating current from a high current transformer, the primary of which is energized by a constant

voltage supply. The leads are attached to the cooling tubes, one end of

The technique was devised by Mr. William Stadler of N. B.S.

2
the furnace befng electrically isolated from the assembly head by the

"alsimag" gasket, as shown in figure 2. A current of about 175 amperes

with a 1-1/2 volt drop provides a steady state furnace temperature of

approximately 2300°C. The temperature has been maintained at this level

for eight hours of continuous operation on several occasions, and the performance of the equipment has satisfactorily fulfilled design expectations,
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U.S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Aslin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

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its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front

cover. WASHINGTON, D.C.

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Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Impedance Standards. Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards. Radio Communication and Systems. Low Frequency and Very Low Frequency Research. High Fre- quency and Very High Frequency Research. Ultra High Frequency and Super High Frequency Research. Modulation Research. Antenna Research-. Navigation Systems. Systems Analysis. Field Operations.