NAT L INST. OF STAND & TECH

AlllDS TbfiflSl

REFERENCE

<^ '^^D S^>^

Stimulating America s Progress 1913-1988 NBS TECHNICAL NOTE 1315

NIST

PUBLICATIONS

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
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'Located at Bouldo-, CO, with some elements at Gaithersburg, MD
Investigation of the

l-iydrogen Source for IVIasers

K.B. Persson

F.L. Walls

Time and Frequency Division

Center for Basic Standards

National IVIeasurement Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

Supported by

Naval Research Laboratories

Washington, DC 20375

S

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1988
National Bureau of Standards Technical Note 1315

Natl. Bur. Stand. (U.S.), Tech Note 1315, 56 pages (Apr. 1988)

CODEN:NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
CONTENTS

Page

1

.

Introduction 1

2 Radio Frequency Discharge Characteristics 2

3

.

Radio Frequency Discharge Circuit 5

4

.

Dissociation of Molecular Hydrogen 6

5

.

Surface Recombination 6

6 Atomic Hydrogen Detector 11

7 . Measurements and Their Interpretation 16

8

.

Lifetime Considerations 24

9 Summary and Recommendations 27

10

.

Acknowledgments 29

11 List of Figures and Tables 30

12

.

References 33

iii

INVESTIGATION OF THE HYDROGEN SOURCE FOR MASERS

K. B. Persson and F. L. Walls

Time and Frequency Division

National Bureau of Standards

Boulder, Colorado 80303

Various engineering and phenomenological aspects of the radio frequency

discharge source for atomic hydrogen used in the masers have been

investigated theoretically and experimentally to provide a base for

minimizing the size of the source and its power consumption. A scheme

and detector have been devised, permitting absolute detection of the

atomic beam, and an atom-atom surface recombination theory was developed,

applicable to the detector as well as to the walls of the discharge

vessel. A self-starting radio frequency circuit, allowing as little as 0.1 W power consumption by the atom source, has been developed.

Key words: atomic hydrogen; atomic hydrogen detection;

dissociation efficiency; hydrogen maser; radio frequency

discharge; surface-recombination.

1 . Introduction

Satellite operation of a hydrogen maser requires long life, small

scale, and low power consumption. The objective of this investigation was to

provide a basis for the design of a small, efficient hydrogen dissociator that

runs on as little power as possible, and is self-starting. To reach this

objective it was necessary to consider the dissociator radio frequency (rf)

discharge, the manner of coupling of the rf power into it to achieve

self- starting, volume production of the atomic hydrogen, loss of hydrogen

atoms by surface recombination, and an absolute technique for measuring the

atom flux emerging from the collimator of the dissociator [1]. For a general

discussion of the plasma problem treated here see [1,2].
2. Radio Frequency Discharge Characteristics

Three different discharge bottles made of Pyrex, using three

different rf electrode configurations as shown in figure 2.1, were used in

the investigation. In all configurations shown, the rf power was coupled

capacitively to the plasma inside the Pyrex bottles. None of the

configurations are easily amenable to a theoretical description. Some

understanding of the behavior of the discharge is obtained by considering

the situation shown in figure 2.2, where the plasma is located between two

plane parallel electrodes with the lateral dimensions large in comparison

with the distance L between the electrodes, and with all dimensions

represented by the area A of the electrodes small in comparison with the

wavelength corresponding to the radio frequency.

Labeling the electric field just inside one of the electrodes with E^

and the field inside the plasma with E , we find, since the current

density J is conserved, that

J = j«€oE, = (ju,€„+a_)E_ (2.1)

where

€„W?

'^P = T^^Tfr (2.2)

is the complex conductivity of the plasma and

^z ^ _iibfi_ (2.3) .2 e'n„

the plasma frequency with n^ the electron density and m the electron mass,

and with u the momentum transfer frequency of the electrons colliding with

atoms and molecules. As a result we find that

and
and

1+^

(j) ^ -

E, 2 2

0) f -v ^ -1 + ^

2 W

(2.5)

2u>e

(j>

p

CO 2

2P

Ml' ' 2

0) 2

r ^2 ^o'^lEa 1^

_E -1 +

i^ 2 a; U)

(2.6)

where Zp is the impedance between the rf electrodes, and P the power

dissipated per unit volume. These three quantities, the plasma impedance Z , the electric field Ep inside the plasma, and the power P dissipated per unit

volume, are strong functions of the electron density for low pressures (i/ comparable to or less than the radio frequency w — 10^ rad/s) . The behavior

of Ep and P as function of the electron density, in terms of the normalized

quantity u)^ /up- , is shown in figure 2.3. The curves have well pronounced

maxima in the neighborhood of plasma resonance (w = w)

.

The momentum transfer collision frequency in the formulas above is

generally energy dependent. Lacking experimental information about collisions

between hydrogen atoms and electrons, we use corresponding information derived

from collisions between electrons and hydrogen molecules when a constant mean

free path S. is an acceptable approximation. From electron mobility data [2] we find that

ip = 3.3 xlO-2 Pa

where c

•^ = 1

(2.7)

(2.8)

and = J 8kT,

Trm (2.9)
is the mean thermal velocity for a Maxwellian energy distribution with the

temperature T^ . The electron temperature is determined by the field EL

and can, in the constant mean free path approximation, be written as

^- =35 J^(^^^) <2-i<»

where k is Boltzmann's constant and M the mass of hydrogen atom or molecule. From the formulas above, we find that the collision frequency

can be written as ^-\l\£ ^

which shows the dependence on the plasma field EL and mean free path i.

The above formulas give a partial description of how the plasma

responds to the applied rf field. To complete the description it is

necessary to discuss how the plasma is generated by the rf field. The

power P, dissipated in the plasma, is primarily fed to the electron gas

which in its turn loses power by elastic, exciting, and ionizing

collisions. The exciting and ionizing collisions involve the high energy

electrons in the tail of an essentially Maxwellian velocity distribution

with the temperature T^ . The electrons and ions are lost to the walls of

the discharge vessel by ambipolar diffusion with a diffusion coefficient

essentially proportional to the electron temperature T^ . In the presence

of the plasma resonance the electron density n^ is determined by the power

density in a very nonuniform and nonlinear manner. The nonuniformities

and nonlinear couplings are tempered by the fact that the electron

temperature is very high (several electron volts) , resulting in high heat

conductivity in the electron gas. The poor thermal coupling between the

electron gas and the walls leads to an almost uniform electron temperature,

although the electron density still is nonuniform.
Most plasmas are nonuniform with the high electron density in the

center of the plasma. At low power and correspondingly low electron density

the plasma resonance starts in the center of the plasma. With increasing

electron density the plasma resonance splits spatially in two parts with each

part moving towards an rf electrode, creating regions with high power

dissipation in front of each electrode. An important phenomenon associated

with an rf discharge is the fact that it is self- limiting in terms of the

power consiamption and the electron density. This is due to the presence of

the plasma resonance and is illustrated by eqs (2.5) and (2.6) as well as figure 2.3. Figure 2.3 shows that when the electron density or correspondingly w^/w^ increases beyond plasma resonance, the field E in the

interior of the plasma decreases drastically relative to the applied field E^^ with a subsequent decrease of power dissipated in the interior of the plasma.

How and for what power this manifests itself depends on the location,

configuration, and design of the rf electrodes, and the discharge vessel.

Maximum electron density is obtained when Wp/w^ — 1, giving n^ =^ 2 10® cm"^

at a frequency of 150 MHz

.

3 . Radio Frequency Discharge Circuit

The circuit used for running the discharge and measuring its power

consvunption is shown in figure 3.1. It consisted of a 150 MHz signal

generator followed by a step attenuator, a power amplifier, a network

permitting the measurement of reflected and transmitted power, and finally

the network coupling the power to the discharge. In case A the output

from the power amplifier was directly coupled to the electrodes which

consisted of copper tapes glued, in opposite positions, to the cylindrical

discharge vessel. The measurements done in this configuration proved very

cumbersome. Every time the hydrogen flux was measured, the discharge had

to be extinguished and reignited. In this configuration the discharge

could be started only with the help of a Tesla coil even at the highest

available rf power.
The discharge could be made self-igniting to as low power as 0.1 W. This was done by using a very low coupling to the autotrans former and tuning it

with a trim capacitance to resonance in the absence of the discharge. When

the discharge was lit it shorted the trim capacitance and presented an inductance in series with the discharge, as load to the power amplifier. The

auto transformer required few turns which could be shaped and located close to or around the discharge bottle, occupying very little space. The only

requirement was that the circuit had a high Q and good capacitive coupling to

the discharge at the high voltage points.

4. Dissociation of Molecular Hydrogen

The dissociation by electron impact of molecular hydrogen and the

various molecular states involved are summarized by Massey and Burhop [3]

The cross section has been measured by Vroom and de Heer [4] and Mumma and

Zipf [4]. The dissociation starts at about 8.8 eV with slowly increasing

cross section, which reaches a maximum of about 10"^^ cm^ around 60 eV.

The rate of production S of atoms per unit volume can be written as

S = In^n^ v^Qd (4.1)

where n^ is the electron density, n2 the molecular hydrogen density, and

VgQp the electron velocity times the dissociation cross section

averaged over the normalized electron distribution function. The

distribution function is particularly complicated for the case of the rf

dissociators used in conjunction with the hydrogen masers. This is due to

bottle and electrode configurations that are difficult to characterize in the

presence of plasma resonance, and the low operating pressures which make

collisions with the walls important. We therefore treat v^Qp as an empirical

function.

5. Surface Recombination

The pressure of hydrogen in the dissociators is too low to consider

volume recombination as a significant loss process for atomic hydrogen,
and it will be neglected. The only loss process for atomic hydrogen is

then surface recombination. To describe this process we introduce the

surface density ng of atomic hydrogen, a capture cross section Q^ , and a sticking time Tg . The surface density ng represents the number of

hydrogen atoms per unit area sticking to the surface at any given time.

The capture cross section is the area around an absorbed atom, within which an atom coming from the gas phase must impact in order to form a molecule. The

steady state equation for conservation of atoms on the surface can be written

as

ni(R)ci = ^ + ni(R)Cin3Q3 (5.1)

where n^ (R) is the atom density just inside the wall, and Cj^ an appropriate

thermal velocity average of the atoms hitting the wall. The left side is the

flow density of atoms from the gas phase to the surface. The first term on

the right side is the flow density of atoms leaving the surface after resting

there the time Tg . The last term represents the loss of atoms on the surface

due to formation of hydrogen molecules. The probability r/, defined as ngQg , that a hydrogen atom hits the surface and forms a molecule can, with the help

of eq (5.1), be written as

ri = HgQg = ^

z . (5.2)

1 + ni(R)ciTgQ3

This formula shows not only that the probability rj depends on Tg and Qg , which are characteristic for the atom- surface interaction, but also that ij depends on the density and temperature of the atom gas in front of the

surface^ and these are partially determined by the rf discharge.

To determine the atom density n^ (R) in front of the surface or the

beam collimating hole, we will use a simplified model of the rf discharge

and the dissociation mechanism. We assume that the discharge bulb is

spherical with the radius R. Using the source function S for generation

of atoms, given by eq (4.1), but introducing a dissociation frequency Uq

defined as
^D = ^e VeQo (5.3)

where Qjj is the dissociation cross section, we can write the steady state

equation for conservation of hydrogen atoms as ^§7 (r^r^) = u^(n,-n,) (5.4)

where F^ is the atom flow density towards the walls, n^, the constant sxjum

of the molecular and atomic hydrogen densities, and n^ the atomic

hydrogen density. The flow density F^ is given by Pick's law

Ti = -Di -|f- (5.5).

where Dj^ is the diffusion coefficient for atomic hydrogen diffusing in

molecular hydrogen. Eliminating the flow density between the two

equations above, we find

where

A| = 2^ . (5.7)

Assuming that A^ can be viewed as a constant we can write the solution to

eq (5.6) as

. sinh X r /c o\ ^1 = n^ - ^1—^ ' '^ " a7 ^^'^^

where the integration constant A^ is determined by the fact that the loss

of atomic hydrogen on the surface must be balanced by the diffusive flow

to the surface. This gives the equations

and
^. , ^ D, af(x) ^ ^1

The spatial distribution of the atomic hydrogen and in particular the

degree of dissociation a(R) in front of the collimator can now be written

as

a(R) = , X = I- (5.11)

1 +^t [k ^'^^H"

Ai

Equations (5,2) and (5.11) show 17 and a(R) mutually depend on each other.

Eliminating »/ between the two equations yields

a(R) 2 J l 2 J ^ ^ (5.12)

where

and

7 = noCiT3Q3 (5.13)

» = ^itif^(fe^"H"- = fr- ^^-^^^

The degree of dissociation o(R) given by eq (5.12) is obtained by

assuming that the concentration of atomic hydrogen is sufficiently small so that the basic equations can be linearized. The result is also applicable

when the relative molecular hydrogen density is small and diffuses in an almost stationary gas of atomic hydrogen. However, it is then necessary to

replace c^ and D^ , characteristic for atomic hydrogen, with Cg and Dj

,

characteristic of molecular hydrogen.

The degree of dissociation, a(R) , in front of the entrance to the

collimator depends on two parameters 7 and i? and is illustrated in figure

5 . 1 where it is shown as the fully drawn curves . The parameter 7
expresses the influence of the wall recombination on the product T^Q,

.

The variable accounts for the influence of the rf discharge through

the dissociation frequency t/p , defined by eq (5.3), and approximately

inversely proportional to the rf power. The degree of dissociation is

bounded by two limiting forms represented by the broken curves. The high

limit, a(R) = 1 , is obtained when the wall recombination is negligible and

generated by letting 7 approach zero. This limit is independent of the rf

power. The lower limit is obtained when any atom hitting the wall is lost

immediately by forming a molecule. It is generated by letting 7 approach

infinity. This limit is a function of the rf power. High degree of

dissociation is obtained at small rf powers provided the wall recombination is

small, but it is also obtained when wall recombination is high provided we use high rf power.

The relation between the variable {) and the dissociation frequency u^

is better illuminated by using approximate forms of Uq . It can be shown that

» - E^ for .„ < % (5.15)

and

«? ~ J52 "2

for i/D > ^ . (5.16)

The dissociation frequency j/p is a very complex function of the properties

of the rf discharge but generally increases with increasing rf power.

The dissociation frequency i/p is approximately proportional to the power

dissipated per particle in the dissociator, atom or molecule, and can be

written as .„"5T^ (5.17)

where P is the total rf power, n^ the particle density, and R the radius

of the spherical container. In the low power limit, corresponding to the

approximation given by eq (5.15), we can write eq (5.12) as

10
a(R) 2 J[i^r^.K^

where K can be viewed as a constant. From this formula we draw the

conclusion that the degree of dissociation a(R) increases with increasing

power and temperature, while it decreases with increasing pressure and radius

of the dissociator. For operating the dissociator at small powers, 7 or the

wall recombination must be very small.

6. Atomic Hydrogen Detector

The design of an absolute detector of the hydrogen atomic flux S that

leaves the exit hole of the collimator is in principle identical with that of

a bolometer used for detection of infrared radiation. A resistive,

temperature-sensitive element, with a surface that encourages atom

recombination, intercepts the atom flux. With a properly prepared surface all

the atoms recombine on the surface, release the recombination energy, and heat

the temperature sensitive element. The relative change of the resistance is

proportional to the atom flux.

The atomic flux detector is best described by using the same theory

that used in the discussion of surface recombination in section 5. The

introduction of a sticking time Tg , a capture cross section Qg , and

conservation of atoms on the surface of the detecting element then leads to

the following expression for ri , the probability of surface recombination

V = n3Q3 = f,^y^^' ^ (6.1) FfFH

where y8 is the solid angle of the atom flux, and r the distance between the

detecting element and the exit hole of the collimator, viewed as a point

source. The power P-^ released to the detecting element then becomes

11
Pi = eSE„1 = , J1 '^'^^

^ (6.2)

where Ep is the recombination energy in electron volts . The highest

sensitivity and the largest range of proportionality between Pj and S are obtained when r is as small as possible and the sticking time Tg is as large

as possible. We have

Pi = SEije (6.3)

provided that

S» -^

(6.4)

Traditionally the atomic hydrogen detector is made of platintim because of its

high and reliable temperature coefficient of resistivity, and because it is a good catalyzer for the recombination process; that is, the sticking time Tg

for platinum is long. The platinum detector element is usually made in the

shape of a very fine wire or strip with a resistance in the range 50 to 150 n,

creating mechanical difficulties in its handling, and giving a very small beam

intercepting cross section. These difficulties were eliminated by providing

the mechanical strength by a thin, high temperature polyimide film (25 /xm

thick) on which platinum was evaporated in layers 15 to 20 nm thick. The

dimensions of the strips are shown in figure 6.1. The ends of the strips were

reinforced with additional evaporated platinum to provide good electrical and

thermal contacts to the strip holders. The strips were clamped to heavy metal

supports with high thermal mass, controlling the end temperatures of the

strip, and incorporated in the bridge illustrated in figure 6.2, with the

platinum facing the exit hole of the collimator. The resistances of the

bridge elements were chosen approximately equal, with R^ , the resistance of

the platinum element at the temperature T^ (room temperature) of the supports,

in the range 70 to 130 Q. The resistance R^ was chosen slightly larger than

Rq and the bridge balanced by adjusting the bridge voltage V^ until the

resistance of the platinum element, due to ohmic heating, was equal to R^ . The

12
additional heating of the platinum element due to recombination of the atoms

on its surface gave rise to an unbalanced voltage AV proportional to the

atomic flux S. The photons generated in the discharge that reach the platinum

element from the collimator should also be included in the power balance of

the bridge. However a crude evaluation of their influence indicates that

their contribution is negligible in so far as this investigation is concerned.

Two calculations must be done in order to obtain the proportionality

constant C between the atomic flux S and the unbalanced voltage AV of the

bridge. The first accounts for the relative change AR/R^ in the

resistance of the platinum element when the bridge first was balanced.

The second calculation accounts for the temperature distribution and the

resulting AR/Rjl caused by the atomic hydrogen flxix hitting the center of

the platinum element. Both calculations are based on the same differential equation

a^T T-Tn P ^ = - Z (6.5)

ax-* A^ a

where P is the dissipated power per unit volume while the boundary

conditions are

T = To for X = ± i (6.6)

with i the length of the platiniom strip with x = in the center, and with

the characteristic length A defined by

a2=|^ (6.7)

where 6 is the thickness of the platiniom strip, a its heat conductivity

and 7 the proper coefficient for the linearized radiation law. The

linearization of the radiation law is justified by the fact that the

platinvun element is only slightly warmer than the environment. The first

term in eq (6.5) represents the heat conduction, the second term the heat

13
loss, and the third term the power dissipated per unit volume in the platinum

element either by ohmic heating or by recombination of the hydrogen atoms

.

In the case of ohmic heating of the platiniom element we assume that

the ppwer is dissipated uniformly and therefore

^= ifdF <«•'')

where P^ is the total dissipated power, and d the width of the platinum

strip. We then obtain the temperature distribution

r. cosh x/a1

[ cosh i/Aj ^-^- = -fik 11- ^^SHJ^l (6.9)

and a relative change in the platinum resistance of

AR

R

= ^P^

fl _ A sinh VA]

27di [ Z cosh i/Aj

<.o.iu;

where a is the temperature coefficient of the platinum resistance. In the

limit A « ^, justified by the design parameters, the relative resistance

change can be written as

AR _ Ri-R. ^ oP^ .. ,,. R R^ 27di '.o.ii;

with the second equality obtained when the bridge is first balanced, and

the power P^ written as

U1+R2J

Turning on the hydrogen beam dissipates a power Pj^ given by eq (6,3) in

the center of the platinum strip. The linearity of the original differential

equation allows us to solve for the resulting temperature distribution,

independent of the ohmic heating created when the bridge was first balanced.

Treating the power source due to recombination of the atoms as a delta

14
function in the center of the platinum element, we find the temperature

distribution

sinh (|^ ± |) AP

T-T = 1 (S 13"i sinh

2X

where the different signs apply to the different sides of the positive

delta function. This temperature distribution gives rise to a relative

resistance change which in the limit A « i can be written as ^ = ^^

(6 14)

Eliminating the group of parameters

a

7di

between eqs (6.11) and (6.14), P^ with the help of eq (6.12), and P^ with the

help of eq (6.3) as well as converting the relative resistance change due

to the recombination power into a voltage response AV of the bridge, one finds that

AV = CS (6.15)

where

C = ^^^"'^^2R^v'"^"^ ^^D ^^-^^^

The consequence of this result is that detailed knowledge of the platinum

element is not needed. The necessary proportionality constant is obtained

from the bridge parameters and the voltage V^ required to balance the bridge

in the absence of the atom flux, provided the design of the platinum element

conforms to the basic assumptions used in the derivation. The first

requirement is that the cross-sectional area of the beam is centered on, and

small in comparison with, the area of the platinum strip. The second

15
requirement is that the characteristic length A is significantly smaller than

the length i of the platinum element. Typical design parameters we used were

Platinxjun Polyimide

d(cm) 0.6 0.6

5 (cm) 2x10"^ 2.5x10-3

i(cm) 2 2

a(W(cmK)-i) 0.71 1.9x10"*

7(W(cm2K)-M -8x10-5 -SxlO'^

A(cm) 2 2

2A/i 9x10"

3

5. 5x10"

3

EdbCJ) 3. 5x10" ^«

Typical values for the bridge parameters and resulting proportionality

constant were

V„ 5.19 V

Ro 61.7 fi

Ri 70

Rg 100 n

C 7.74x10-19 V s"i

In a representative set of measurements we used a 4 mm long collimating

hole with a diameter of 0.4 mm. The pressure in the dissociator was 4.7

Pa with 1.3x10"^ Pa (10"^ T) in the pump chamber and with the platinum strip

located about 5 mm from the exit hole of the collimator. At a rf power of 4 W

we obtained a bridge unbalance of 2 mV corresponding to a recombination power

of 0.91 mW. Assuming that all hydrogen atoms recombined on the platinum

surface, this corresponds to an atom flux of 2. 6x10^ ^ s"^.

7. Measurements and Their Interpretation

The principal measurements were the atomic hydrogen fltix emerging

from the exit of the collimator as function of the rf power P, the

16
dissociator pressure p, the dissociator wall temperature T, and the

intensity of the atomic H„ spectral line. The state of the dissociator,

in terms of impurities, was determined with the help of a high quality

hand spectrometer. No measurements were taken unless only the three lines

H^ , H^ , and H^ were observed. In addition, the degree of dissociation was obtained from the pressure changes generated when the discharge was turned on and off. Measurements were done on the three types of bottles

shown in figure 2.1.

Considerable difficulties in terms of drift and instabilities were

experienced in the early use of the platinum element. Part of the

difficulties were traced to unreliable electrical contacts to the ends of

the platinum element. These difficulties were eliminated by coating the

contact surfaces of the strip holder with indium. The remaining part of the

drift seemed to be associated with slow initial changes in the resistance of

the platintun element and was cured by running it for a few days at a slightly

elevated temperature.

All dissociator bottles were made of Pyrex, and the spectrum from most

of them indicated the presence of molecular hydrogen and impurities . The

degree of dissociation increased and the impurities disappeared from the

spectrum after the dissociator had been run from several hours to many days,

depending on an unknown correlation with the past history of the bottles. All

bottles eventually cleaned up and exhibited the pure line spectriom of atomic

hydrogen and a purplish-red discharge color. Immediately after the discharge

was turned on, the spectrum of two of the dissociator bottles was that of pure

atomic hydrogen. However, the spectrum deteriorated very rapidly and returned

to a pure atomic spectrum only after a rather substantial time . This

observation indicates that the impurities did not come directly from the gas

phase but rather from the walls of the dissociator, where they were stored

either as absorbed surface layers or inside the glass matrix. These

impurities were released by bombardment of the walls by electrons, ions, and

photons generated by the discharge , and removed from bottle by the hydrogen

flow. A conclusion from the observations above was that clean Pyrex, in

^Quilibrium with atomic or molecular hydrogen gas, has a surface with low

17
probability for recombination of atomic hydrogen. Based on this reasoning we decided to process the dissociator bottles in the following manner. The

bottle was made with an inlet and outlet much larger than the collimating hole

and placed in an oven in such a manner that clean hydrogen flowed through the

bottle at the annealing temperature of Pyrex, 550 "*C. After the bottle was annealed for about eight hours, it was attached to the vacuum system as soon

as possible. This processing allowed the impurities to diffuse out of the

glass matrix and be replaced by hydrogen at as high a temperature as possible.

Bottles processed in this manner showed about a factor of 4 decreas in the

time required to reach the state of cleanliness necessary for the

measurements

The first set of measurements was done on bottle A, figure 2.1, the

largest bottle. The rf electrodes consisted of two copper foils 1.5 cm x

3 mm glued to opposite sides of the bottle and coupled directly to the

output of the rf amplifier, scheme A of figure 3.1. The discharge was not

self- igniting within the available power range, causing considerable

difficulties and scattering in the measurements. A measurement consisted

of observing the chan,ge in AV of the bridge, obtained by turning the rf

discharge on, waiting three to four times the thermal time constant of

the platinum element (4 to 5 sonds) , and then turning the discharge off.

In every measurement it was necessary to start the discharge with a Tesla

coil, which upset the dc instruments attached to the bridge. The

measurements that could be done are shown in figure 7.1. These measurements are also displayed in figure 5.1 where they have been displaced to

conform as well as possible in slope and curvature with the theoretical

curves. The displacements are somewhat arbitrary. The agreement with the

theoretical curves cannot be considered evidence for the correctness of

the theory although it lends it plausibility. There are many possibilities for error in the plotted variables. The discussion in section 6 of

the atomic hydrogen detector indicates several possibilities for errors which are all expressed in eq (6.4). The present lack of knowledge of the

Tg and Qg makes it impossible to make a theoretical evaluation of the

errors, and our experimental apparatus was too crude to obtain them

experimentally to better than a factor of two.

18
It is possible to make some evaluation of the measurements by

comparing the atom flux Fj^ , measured by the platinum element, and the total

flux Tq going through the collimator. The total flux was obtained

either by measuring the pump current as a function of the dissociator

pressure, or by measuring the dissociator pressure as a function of time,

after the source of hydrogen was shut off. A simple derivation, based on

long mean free path and geometrical considerations of the collimator

gives

Ti = ^a^ T-( o>j nvTT -75- (7.1)

' i / i>a r(2r>

where a is the radius and i the length of the collimator hole, n the

density and

. = [ w

the average thermal velocity of the gas in the dissociator, and where the

last factor in eq. 7.1 is due to the geometry of the collimator.

Similarly we find that

_ nv 2 1 nv a^ /-7 on 2(1 + -) i>a ° a

with the last factor representing the probability that an atom, making

randomizing collisions with the inside wall of the collimator, leaves

through the exit hole of the collimator. The aspect ratio 2a/i is

generally so small that the approximate forms of the equations are applicable, giving the flux ratio of

h. = i± (7.4)

assuming 100% dissociation within the bulb and that every collision with the

collimator results in recombination.

19
The errors in the derivation of the formulas for the fluxes are of the same nature and likely to be eliminated in the ratio. This ratio is used as a number of merit in the comparison and evaluation of the different

dissociators. With an aspect ratio 1/10 the theoretical flux ratio becomes

0.4. This large ratio and the small solid angle of the collimated beam give a high relative flux density within the collimator beam. When the platinum

element was taken out for inspection, it showed a very distinct image in the

platinum coating corresponding to the expected cross section of the atom beam.

The dissociators were compared at 1 W rf power dissipated in the

discharge . Table 7 . 1 shows the pertinent data for bottle A with a collimator hole radius of 0.2 mm and the aspect ratio of 1/10. The flux

ratio is a factor of 3 to 4 less than expected from theory. This could be

caused by a variety of factors, the most likely associated with the odd

shape of the glass bottle, with the collimator located in a small diameter

appendage

The pressure dependence of the atom fliix from bottle A is shown in

figure 7.2. It indicates a qualitative agreement with the theory illustrated in figure 5.1. For a given 7 and decreasing pressures the degree of

dissociation approaches unity asymptotically. The atom flux becomes

almost proportional to the pressure. With increasing pressure the degree

of dissociation decreases and the atom flux starts to level off. The

dissociator becomes less efficient at higher pressures. Physically this

depends on the fact that at higher atom densities, larger fractions of

the interior dissociator surfaces are covered by atomic hydrogen,

resulting in higher surface recombination rates at higher pressures than

at lower pressures.

Measurements showing the effect of the temperature of dissociator A

on the atom flux are shown in figure 7.3. These measurements were done

after the interior surfaces of the dissociator had been exposed to air.

The measurements were made at a rf power level of about 6 W in the

order A, B, C, and D and started immediately after the spectroscope showed

20
a clean atomic hydrogen line spectrvun. In the first set of measurements

A, the atom fltix showed a rather strong decrease with increasing

temperature, more than expected from a theoretical consideration. The

simple theory predicts that the atom flux is proportional to the density

of the atoms and their average thermal velocity, that is, the relative

change in the atom flux due to a temperature change should be written as

The atom density change due to a temperature change at constant pressure is

less than predicted by simple kinetic theory due to the influence of the wall

recombination on the density change, as reflected in the pressure dependence

shown in figure 7.2. Atom in flux measurements of bottles A initially

violates eq (7.5). The dissociator was then rxin for 72 hours before the B, C,

and D sets of measurements were made (figure 7.3). The results from these

measurements do satisfy equation 7.5. This experience showed that an

apparently clean dissociator could still have impurities on the walls

detrimental to the production of hydrogen atoms, and that a proper processing

of the bottles is essential.

The measurements on the medium size Bottle B, illustrated in figure

2.1, are displayed in figures 7.4 to 7.8. For these measurements the

tuned autotransformer arrangement discussed in section 3 was used, allowing

for self- ignition of the discharge at very low powers. The measured atom

flxix as function of the rf power (fig. 7.4), the pressure (fig. 7.6) and

the temperature (fig. 7.7) behave essentially in the same way as t;he

data obtained with the large bottle A. Two collimator sizes were used,

the large one with a diameter of 0.4 mm and the small with a diameter of

0.1 mm, both with the aspect ratio of 1/10. The data obtained at 1 W

with the large collimator are shown in table 7.2 while the small

collimator data are shown in table 7.3.

Measurements of the flux ratio Tj^/T^ suggest that the small dissociator

was more efficient than the large and that the small collimator was more

efficient than the large collimator. That the largest dissociator was

21
least efficient is most likely explained by the fact that the collimator

was farthest away from the center of the discharge and that the discharge

did not completely fill the dissociator. This would result in lower atom

density in front of the collimator and a smaller atomic flux r^ , while the

total flux Tg would remain unchanged, resulting in a smaller flux ratio

for the larger bottles than for the smaller bottles. The fact that the

large collimator was less efficient than the small collimator has to do

with mean free path considerations. Based on kinetic theory and

cross sections one finds, at the pressure of 6.7 Pa that the mean free

path for the Hg -Hg collisions is about 2 mm while it is about 1 mm for the

H-H collisions. Both these mean free paths are comparable with the

dimensions of the large collimator, in particular for the H-H collisions.

The effect of a mean free path comparable with the collimator dimensions

is to decrease the fluxes relative to the fluxes predicted by the

collision free theory as well as to increase the collimation angle for the

species with the smallest mean free path. As a result the large

collimator will appear less efficient than the collimator which is

small in comparison with the mean free path.

When the spectroscope showed a pure atomic hydrogen line spectrum it was assumed that the hydrogen was close to fully dissociated. This is a subjective judgement, verifiable only through very involved and cumbersome

spectroscopic measurements of the molecular hydrogen concentration, a task

beyond the scope of this investigation. Our conclusion from the

spectroscopic observation was to some degree verified by observing the

relative pressure changes obtained as a function of the power when the

discharge was turned on and off. We can view the dissociator as two,

well communicating volumes V^ and V^^ connected to the rest of the system

through two very small leaks, the inlet leak valve and the outlet collimator.

The voltome V^ is the volume of the dissociator, and V^ the volume created

by the leak valve and the glass tubes connecting it to the dissociator.

If the inlet and outlet leaks are sufficiently small, we can view the

system as closed during a measurement. Simple theoretical considerations

assuming T constant then give

22
2ij:2^ = °^

(7 6)

where p^ and p^ are the pressures before and after the discharge has been

turned on, respectively. A set of measurements of the pressure

changes as function of the rf power is shown in figure 7.8, where the

discharge was turned on and off with a cycle of 44 s. , a time short

relative to the time constant (several minutes) for the pressure changes

due to the inlet and outlet leaks. On the left-hand side of this figure

is the scale for the degree of dissociation a, where it has been assumed

that the two volumes V^ and Vj^ are equal. The data obtained at the

highest pressure, 11 Pa has very little scatter and imply that the

degree of dissociation was unity for rf powers above 0.4 W. The data

obtained at the lower pressures show more scattering with an average

degree of dissociation of about 0.8, lower than anticipated from the

spectroscopic observations and the measurements of the atom flux. This

discrepancy is most likely associated with a nonlinear pressure scale

(thermocouple pressure gauge) . These measurements show that with a proper

instrumentation and development of theory, which takes into account the

effects of the hydrogen flow through the system, the pressure measurements

could be developed into a good tool for measurement of the degree of

dissociation.

The third dissociator, bottle C of figure 2.1, was designed to test

not only a still smaller bottle, but also to illuminate, and possibly

suppress the influence of the plasma resonance. A long, small diameter

bottle with the rf electrodes at the ends decreases the polarization of

the plasma due to the plasma resonance. Plasma resonance is still present

but occurs at a higher electron density. The first set of measurements,

shown in figure 7.9, illustrates how the rf power P necessary to maintain

the plasma varies with the intensity I of the H^^ spectral line.

Measurements at three different pressures are shown, all with pronounced

maxima, which shift to higher intensities I with increasing pressure. The curves bear a striking resemblance to the broken curves in figure 2.3

23
which represent the theory for an idealized configuration around plasma

resonance, and where the normalized rf power is plotted as function of the

normalized electron density. The resemblance between the two sets of

curves is not surprising, considering that the intensity of the H^^ line is

proportional to the electron density. The theory for the dissociator

bottle C and the associated rf electrodes is too complex to include in

this report. Suffice it to say that the presence of the plasma resonance

mechanism generally gives rise to an upper limit for the power dissipated

in the plasma, and that for some configurations this will give rise to two

modes operating at the same power but on opposite sides of the maximum.

The measurements of the atom flux from dissociator bottle C, shown in

figure 7.10, also reflect the presence of plasma resonance. The curves show well developed maxima versus rf power, with the maximum power decreasing

with increasing pressure. The resulting two branches behave differently. In

the lower branch, corresponding to low electron density and low electron

temperature, the atom flux increases with increasing power. In the upper

branch, corresponding to higher electron density and temperature, there is a slight tendency for the atom flux to increase with decreasing power. The fact

that the upper branch is almost independent of the power, implies nearly full

dissociation of the hydrogen. The data obtained at 1 W rf power are shown in

table 7.4. The average, 0.41, of the flux ratios is close to the value 0.40

as predicted by the simple theory represented by eq (7.3) for fully

dissociated hydrogen.

Of the three bottle sizes we tested it appears that the smaller the

bottle is, the higher is the dissociation efficiency. The smallest bottle

appears close to the theoretical limit, in general agreement with the theory

proposed in section 5

.

8. Lifetime Considerations

None of the bottles tested showed any deterioration after they had been

processed until a clean atomic hydrogen line spectrum was observed. The

traditional signs of deterioration like "white out" and "brown out" were not

24
observed. We attribute this to the fact that we used a very clean vacuum

system, well processed dissoclators, and an uncontaminated source of hydrogen

as well as low power. We have no solid experimental evidence on which to base

an estimate of the dissociator lifetime, but will nevertheless advance some points of view we think are important.

In most cases the dissociator had to be run a few days before it was

clean, as judged by the spectrum and the atomic flux. In some cases the

dissociator appeared very clean the moment it was first started, but

deteriorated in a few minutes to a whitish discharge, characteristic for

dirty hydrogen. Running it from a few hours to days restored it to its

original condition with a reddish-purple discharge color, characteristic

of clean atomic hydrogen.

The impurities that appear in the hydrogen dissociator, assuming that

the hydrogen source is clean, have three different origins: the interior

glass surface, the interstitial voids in the glass, and the glass matrix

itself.

The impurities adsorbed on the interior glass surface are weakly bound

and easily detached by bombardment of the wall by ions for the discharge.

These impurities are most likely responsible for the very rapid deterioration

of the discharge, when an originally very clean dissociator is first started.

They represent a finite source and are eventually carried away with the

hydrogen flow through the collimator.

The interstitial voids in the glass are filled with gases originating

in the glass manufacturing process (ovens and flames) . The rate of

removal of these impurities is limited by the slow process of diffusion

through the glass matrix. These impurities should be viewed as a finite

but large source. The rate of their removal increases with increasing

temperature and can cause long term deterioration of the

dissociator if it is run at high temperature. The rate of diffusion into

the bottle may then be larger than the rate of removal through the

collimator, causing a steady increase of the impurity density in the

25
dissociator. This problem can be solved by running the dissociator at low

temperature, decreasing the rate of diffusion, or by removing the

impurities from the glass before the dissociator is attached to the maser. The removal of these types of impurities is traditionally realized by baking

the glass bottle under vacuum at temperatures in the range 400 to 450 "C

(Pyrex) . In the case of the hydrogen dissociator, where hydrogen is not an

impurity, one can obtain a better result by letting clean hydrogen, at

atmospheric pressure, flow through the bottle during the baking, which now

can be done at higher temperatures than during the vacuum processing.

The third source of impurities is the glass matrix itself. It may be

broken down into gaseous components due to bombardment by energetic particles

from the plasma. This source must be viewed as infinite and can be tolerated

only if the rate of production of the impurities is less than the rate of

removal through the collimator. It is a likely cause of a finite life of the

dissociator and related to the design of the rf electrodes. Except for the areas just under the rf electrodes, the bombardment of walls by the electrons

and ions is governed by ambipolar diffusion and a. wall sheath where the ions

may acquire an energy of the order 10 eV. Both the rate of production of ions

and electrons per unit volume and the power dissipated per unit volume in a fully developed, ideal rf plasma (at the maximum of the dissipated power) are proportional to the electron density or the square of the applied frequency.

Both the total dissipated power and the total ion flux hitting the walls are then proportional to the volume of the dissociator. The ion flux density

hitting the walls of spherical bottle is therefore proportional to the radius

of that bottle and that the destruction of the walls decreases with decreasing

radius. A smaller bottle then means longer life. However, the smaller the

dissociator is, the higher is the electric field necessary for starting the

discharge. The requirements that the dissociator is self-starting and has a long life are therefore conflicting requirements, making it necessary to

design the circuit so that, in the absence of the discharge, the electric

field inside the bottle is as high as possible for any given input power as was done in the circuit B shown in figure 3.1

26
The situation is considerably more complex in the area underneath the rf

electrodes. This is the region where the rf fields, and the dc sheath fields

are highest. The high sheath fields are caused partly by high electron

temperature and partly by rectification of the rf fields, both enhanced by the

resonance mechanism. According to other investigators, the first evidence of

white-out shows up in this region. The influence of this source of impurities can be decreased by any means that decrease the effects of plasma resonance.

The small bottle C shown in figure 2.1 represents such an attempt. It is made

long with a small diameter to enhance the resistive part of its impedance

relative to the capacitive part. It has ring electrodes, simulating Faraday

cages, to reduce the ion bombardment of the walls underneath the electrodes.

The overall effect of this configuration is to suppress the polarization of

the plasma and achieve higher electron densities than predicted by the ideal

plasma resonance. The data obtained with the small bottle C show a definite

influence of the configuration and is possibly the most efficient bottle we have tested.

9 . Suimnary and Recommendations

A theory for surface recombination of atomic hydrogen has been developed

which is applicable both to the interior surfaces of the dissociator, where

the aim is to suppress recombination, and to the surface of the atomic

hydrogen beam detector, where the desire is to enhance recombination. The

theory is based on the introduction of a surface sticking time Tg and a surface capture cross section Qg , permitting a theoretical connection between

the probability for recombination on the surface to the degree of dissociation

of the gas just inside the surface. The theory agrees qualitatively with the

experimental data. Better agreement is not expected due to lack of

information about T^ and Qg

.

The atomic hydrogen beam detector was designed by vaporizing a thin layer

of platinum on a film of high temperature plastic, permitting large enough

detector area to completely intercept the collimated atom beam. A theory

developed to apply to this composite element shows that, in spite of its

complex nature, a measurement of the atom flux can in principle be viewed as

27
absolute. Unforttinately, the apparatus using this platinum element was not

flexible enough to determine that all atoms in the collimated beam lead to

recombination on the platintim element. The measurements must be viewed as minimum measurements

,

The atom flux from three different sizes and configurations of

dissociator bottles and rf electrode configurations was measured as fxinction

of the rf power dissipated in the plasma, the pressure, and the bottle's

temperature. An attempt to measure the degree of dissociation by the pressure

changes in the dissociator bottle, caused by turning the discharge on and off, was also made. With some reservations, caused by the uncertainty in the

location of the platinum element, the atom flux measurements versus the rf

power indicated that the smallest dissociator bottle was most efficient in

producing atomic hydrogen, a result supported by the pressure changes obtained

when the discharge was turned on and off. The spectroscopic observations

implied that the gas in the larger bottles with larger collimators also was fully dissociated, a fact not confirmed by the measurements of the flux

ratios. This discrepancy may be explained by the fact the discharge was not

filling the larger dissociator bottles, leading to lower atom densities in

front of the collimator entrance, and by the fact that the mean free path in

atomic hydrogen was comparable with the dimensions of the larger collimator.

Measurement of the atom flux versus the bottle temperature showed that the

flux changes were due to the density changes caused by the temperature as well

as the change in average thermal velocity in the change from molecules to

atoms due to the dissociation process. Larger changes could be associated

with release of impurities from the walls due to the temperature increase.

The theory for surface recombination, the measurements of the atom flxix,

the discussion of various mechanisms that influence the life of the

dissociator, all indicate that a small dissociator bottle, operated at low

powers , is likely to be moire efficient and have longer life than a large

bottle. The choice of size of bottle is however also influenced by the

constraint that the discharge must be self-starting. In general, the

smaller the bottle, the higher the rf field required for self-starting. This

self-starting constraint was not evident for any of the bottles, provided a

28
txined autotrans former was used as coupling element between the rf power

sources and the discharge. This Implies that still smaller bottles could be

used.

The uncertainty in the conclusions based on the results from this

investigation depends to high degree on the fact that the only experimental

apparatus used in this investigation was put together before the pertinent

theories and understanding were developed. With the present knowledge and a redesign of the experimental apparatus, better data could be obtained. The

major uncertainty in the experimental set-up was due to the difficulties in

orienting the platinum element so that the collimated atom beam hits its

center. Any future design should provide a possibility to vary the distance

between the platinum element and the exit hole of the collimator. One can

then measure the product T3Q3 and determine the efficiency of the platinxim

element with regard to recombination, and obtain a better determination of the

useful atom flux. A future investigation should also develop the pressure

measurement method into a more precise method for measuring the degree of

dissociation inside the dissociator bottle so that the recombination loss of

atoms in the collimator can be evaluated. All bottles and collimators used in

this investigation were made of Pyrex. A general conclusion was that the

sticking time of hydrogen on a well cleaned Pyrex surface is short, leading to a small probability for surface recombination. This does not exclude the

possibility that the use of other materials could lead to a better dissociator

both from the atom yield point of view and life. This investigation has also

demonstrated that plasma resonance mechanism, acting through the bottle and rf

electrode configurations, has an influence on the atom yield of the

dissociator, a point that should be considered in future work.

10. Acknowledgments

The authors gratefully acknowledge the support of the Naval Research

Laboratories for this work.

29
11. List of Figures and Tables

Figure Page

2.1 The dissociator bottles. 34

2.2 The plane-parallel case. 35

2.3 The plasma resonance. 36

3.1 The rf circuit. 37

5.1 The degree of dissociation. 38

6.1 The platinum element. 39

6.2 The bridge circuit. 40

7.1 Atom flux versus power. Bottle A. 41

7.2 Atom flux versus pressure. Bottle B. 42

7.3 Atom flux versus temperature. Bottle A. 43

7.4 Atom flux versus power. Bottle B. 44

7.5 Atom flux versus power. Bottle B. 45

7.6 Atom fluix versus pressure. Bottle B. 46

7.7 Atom flux versus temperature. Bottle B. 47

7.8 Pressure change versus power. Bottle B. 48

7.9 Power versus Ha intensity. Bottle C. 49

7.10 Atom flux and plasma resonance. 50

Table

7.1 Dissociation efficiency of bottle A 31

7.2 Dissociation efficiency of bottle B 31

7.3 Dissociation efficiency of bottle B 32

7.4 Dissociation efficiency of bottle C 32

30
Table 7.1 Dissociation efficiency of bottle A

Discharge power is 1 W, collimator is 0.2 nun radius by 4 mm long

Tq is the estimated total flux, and T^ is the atomic flux.

Discharge pressure Tq F^ T^/rQ

s-i

7.8x10^* 0.075

5.4x10^* 0.079

Pa s-i

8.0 1.04x10^6

5.0 6.86x101=

Table 7.2 Dissociation efficiency of bottle B

Discharge power is 1 W, collimator is 0.2 mm radius and i = 4 mm long,

Fjj is the estimated total flux, and T^ is the atomic flxix.

Discharge pressure Fq

Pa s-i

6.4 8.7x1015

5.0 6.9x10^5

2.7 3.6x1015

To Ti/^I-o

s-i

1.14x1015 13

5.4x101* 12

3.4x101* 094

31
Table 7 . 3 Dissociation efficiency of bottle B

Discharge power is 1 W, collimator is 0.005 mm radius by 1 mm long,

Tq is the estimated total flxix, and Tj^ is the atomic flux.

Discharge pressure Tq

Pa

8.0 1.4x10^5

5.0 6.8x101*

3.7 5.1x10^*

Ti/ro

3.6x10^* 0.26

2.3x101* 0.34

1.15x101* 0.23

Table 7.4 Dissociation efficiency of bottle C

Discharge power is 1 W, collimator is 0.05 mm by 1 mm long,

Tq is the estimated total flux, and r^ is the atomic fltix.

,-1

Discharge pressure Tr

Pa s

14.6 1.65x1015

6.9 9.8x101*

3.4 4.3x101*

Ti/ro

7.4x101* 0.45

3.6x101* 0.37

1.75x101* 0.41

32
12. References

[1] Handbook of Physics , edited by E.U. Condon and H. Odlshaw, (second

edition, McGraw-Hill 1967.)

[2] J.L. Park, R.E. Voshall, and A.V. Phelps, "Drift Velocities of Slow

Electrons in Krypton, Zenon, Deuterium, Carbon Monoxide, Carbon Dioxide,

Water Vapor, Nitrous Oxide and Ammonia," Phys. Rev. 127, 2084-2089

(1962).

[3] H.S.W. Massey and E.H.S. Burhop, Electronic and Ionic Impact Phenomena . Clarendon Press, Oxford, England, 1952.

[4] Vroom and de Heer, "Production of Excited Atoms by Impact of Fast

Electrons on Molecular Hydrogen and Deuterium," J. Chem. Phys. 50, 580

(1969).

[5] M.J. Mumma and E.C. Zipf, "Dissociative Excitation of Vacuum Emission

Features by Electron Impact on Moleculor Gases I, H2 and O2 , " J. Chem.

Phys. 55, 1661 (1971).

33
o

U3uo

cd •Haom

CO

0)

I-! Hu

•H

34
ITR. F. ELECTRODE

l^^^^^^^^^t<^^^'^^^^^^^^^M

L V/PLASMA

k^^^^v.^^^v.Sv^^v^^^^^^;:^^

Figure 2.2 The plane-parallel case.

35
o

OS

c

o

CO

T3

Q.

E

<

5 —

4 —

'

' /\ Ea

/ \

2a)EQ P

- / ' W--"

^"^ \ 1 1 1

2 —

0.5 1.0 1.5

a)p\2

2.0 2.5

Figure 2.3 The plasma resonance.

36
bO

•H

37
f do

•HU

cd •HUO

CO

CO •HO

(U

0)

t>0 (U

(U m

0)

00

•H

38
1 cm 1 cm

-2 cm- -

i. ^6mm

J

50 nm

15-20 nm

^—ijyj mil

25 jum Polyimide

Figure 6.1 The platinum element.

39
Figure 6.2 The bridge circuit,

40
10

(0

10

2 —

1 1 1 1 M M

• p = 8 Pa

X p = 5 Pa

1 1 1 II i 1 1

15

•^^^^^^^^ • ••

8

6

4

— t-rti;^ —

— r^ —

H

—4 mm —

— Collimator —

\\\\\N

T 2

0.4 mm —

kWWN

14

1 1 1 1 II II 1 1 1 1 1 1

1 1 0.2 0.4 0.6 1 2 4 6 8

Power (W)

Figure 7.1 Atom flux versus power. Bottle A.

41
en

P (Mm)

1 2 5 10 20 40

_l 1 1 1 1 1 1 1 1 II 11

Power = 6 W

1 1 l_

10^6 Collimator —^^^

|N\\\N| <t) = 0.4 mm —

8 — —

6 _ ^_\^ -*—4 mm .

•

4 — .^^' • —

— ./ —

2 — / —

10^5

• —13.3 Pa ^ —

1 \ 1 1 II II 1 i 1 5 10

P (Pa)

20 40

Figure 7.2 Atom flux versus pressure. Bottle B,

42
4-1

4-1 O

PQ

u

0)

cn

COu

>

iH

•4-1 6

o

4-1 <

CO

0)

00

•H

(Alu) AV

43
w

p = 6.2 Pa

.-«- I p = 5.0 Pa

p = 4.0 Pa -

^^

Collimator ^^^(j) 0.4 mm

4 mm

J \ I I II II 0.2 0.4 0.6 1 2

Power (W)

4 6 8 10

Figure 7.4 Atom flux versus power. Bottle B.

44
CO

1 mm

1 \ I I I I I l|

Collimator —^^<t> 0.1 mm

r.. —p = 7.8 Pa

p = 5 Pa

p = 3.6 Pa

I I I I I II I I 0.4 0.6 1 2 4 6 8 10

Power (W)

Figure 7.5 Atom flux versus power. Bottle B,

45
P (M"^)

20 40 60 100 200 400 600

6 — '

1 1 1 1 1 1 1 1 1 1 1 1 1 M L

4 — - ^

2

1—

1 y

y^

1

lT io15

8

6

y Power = 6 W

— /

/ <:ollimator—^^ <t> 0.4 mm_

4 — —! -•—4 mm _

—

1 1

1 1 1 1 1

1

1 1 1 1 1 1 1 1 ~ 20 40 60 100 200

P(Pa)

400 800

Figure 7.6 Atom flux versus pressure. Bottle B.

46
>

E

>

<

0.6

1 1 1 1 1 1

Collimator-_^ ^^o-i-^"^i

0.5

-•—1 mm

0.4 —

p = 7.9 Pa

3

0.2 — —

0.1

1

1 1 1 10 20 30 40 50 60 70 80 90

Tro

Figure 7.7 Atom flux versus temperature. Bottle B.

47
0.4

0.3

CM Q.

I_ qT

o

Q.

0.2-

0.1

^— O"

o 11 Pa

^ 4.8 Pa

D 2.6 Pa

•o—o or~^ r

Vl = V2

6 ^ a

a

i A A ^ ^ —

1 D'

n

1 mm

r

.^t 0.1 mm

Collimator

44 s r

"onloff r

1 ^OS 0.9^

0.8 3.

0.7 ^

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Power (W)

Figure 7.8 Pressure change versus power. Bottle B.

48
^ 2

o

Q.

o—o,c

p = 3.7 Pa

>

-o^j^

p = 6.9 Pa

VI

o

o\

o^ o,\o^ ^p = 20 Pa

\

8 10 12

I (H^), arbitrary units

Figure 7.9 Power versus Hof intensity. Bottle C.

49
1.0

0.8

> 0.6

E

0.4

0.2

0.1

\ \ I Mill

1 mV-^7.9xlO^'^ H /sec

p = 14.6 Pa —O^—OOo—O-fi

—O'

p = 7.0 Pa

o^

p = 3.6 Pa

• — — a

%

J I I 0.2 0.4 0.6 1 2

Power (W)

Xl

4 6 8

Figure 7.10 Atom flux and plasma resonance.

50
NBS-114A (REV. 2- U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/TN-1315

2. Performing Organ. Report No 3. Publication Date

April 1988

4. TITLE AND SUBTITLE

Investigation of the Hydrogen Source for Masers

5. AUTHOR(S)

K.B. Persson and F.L. Walls

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Naval Research Laboratories

Washington, DC 20375

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sun)nr\ary of most significont information. If docunrtent includes a significant

bibliography or literature survey, mention it here)

Various engineering and phenomenological aspects of the radio frequency

discharge source for atomic hydrogen used in the masers have been

investigated theoretically and experimentally to provide a base for

minimizing the size of the source and its power consumption. A scheme

and detector have been devised, permitting absolute detection of the

atomic beam, and an atom-atom surface recombination theory was developed,

applicable to the detector as well as to the walls of the discharge

vessel. A self-starting radio frequency circuit, allowing as little as 0.1 W power consumption by the atom source, has been developed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

atomic hydrogen; atomic hydrogen detection; dissociation efficiency;

hydrogen maser; radio frequency discharge; surface-recombination.

13. AVAILABILITY

|X~1 Unlimited

I I For Official Distribution. Do Not Release to NTIS ^^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

I I Order From National Technical Information Service'(NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

56

15. Price

* U.S. Government Printins office: 1988—S75-378/85142 USCOMM-DC 6043-P80

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