NASA Contractor Report 17809 1 

SET-UP AND DEMONSTRATION OF A LOW 

ENERGY ELECTRON MAGNETOMETER (LEEM) 

91) SET-UP AND CZMONSZ2APIOB N86-24563 

GY EEECTFCB BBG&EICLETE3 (EEEd) 

f Southero ?lissisei~pi) 46 p 

GSCL 343 Ullclas 

63/35 43353 

Grayson H. Rayborn 

UNIVERSITY OF SOUTHERN MISSISSIPPI 

Hattiesburg, Mississippi 

Contract NAS1-17435 

May 1986 

National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 23665 
TABLE OF CONTENTS 

LIST OF FIGURES ................................. iii 

INTRODUCTIOE ... COKSTRlrCTIOX OF LEEM-2 ............................ 2 

Design of LEE!!-2 ............................ 4 

Materials of construction ................... 8 

Shielding of insulators .................... 11 

Choice of electron gun ..................... 14 

Installation of lateral deflection plates .. 15 

Summary .................................... 17 

SET-UP ASD TLSTIXG OF LEEF.1-2 .................. ***18 

C0:;STRUCTIOK AYD TZSTIKG OF I!ELMMOLTZ COILS ...... 28 

INSTALLATIOR A;:D TESTING OF POSITIOK 

SEF!SITIVE DETECTOR ............................... 38 

FURTIIER POTEYTIAL INPROVEMENTS FOR THE LEEM ...... 41 

LIST OF FIGURES 

Figure l(a) . The LEEM-2 .............................. 19 

(b) . Another view of the LE??\!-2 .............. 19 

Figure 2(a) . A close-up view of the electron gun ..... 20 

(b) . The LEEM-2 disassembled ................. 20 

Figure 3(a) . LEEM-1 and LT:E?I-2 seen toqcther ......... 21 

(b) . LEEM-2 ready for operation .............. 21 

Figure 4 . The beam profile through one stage with 

magnetic shielding ......................... 23 

Figure 5 . The beam profile after one stage without 

magnetic shielding ......................... 25 

Figure 6 . The p. erformance of LEE)I-2 as a 

magne'tometer ............................... 27 

Figure 7 . Beam profile in the second analyzer ........ 29 

Figure 8(a) . Measuring arrangement for 

Helmlholtz coils ........................ 31 

(b) . Close-up of the large coils ............. 31 

Figure 9 . Linearity of field with coil 3 current at center of coils ................. a3 

Figure 10 . Linearity of field with coil 

current on axis off center ................ 34 

Figure 11 . Linearity of field with coil 

current off axis .......................... 35 

Figure 12 . Uniformity of field on axis ............... 36 

Figure 13 . Uniformity of field off axis .............. 37 

Figure 14 . Current from resistive strip 

as beam position varies.. ................. 40 

mmN PAGE iii 
FINAL REPORT 

NASA Contract NAS1-17435 

Set-up and Demonstration of a Low Energy Electron 

Magnetometer (LEEM) 

Grayson H. Rayborn 

Department of Physics and Astronomy 

University of Southern Mississippi 

Hattiesburg, MS 39406-5165 

INTRODUCTION 

The author, with the assistance of graduate student E. 

Darnel1 Moffett, designed, constructed and tested a 

magnetometer whose basis of operation was the deflection of 

of free, focused, monoenergetic electrons by the ambient 

magnetic field. The idea that a reliable magnetometer 

capable of very fast response was needed for a variety of 

tasks both on Earth and in space and that the deflection of 

free electrons was a possible means of achieving a fast, 

sensitive magnetometer was due to Dr. Jag.J. Singh of the 

NASA Langley Research Center. The device was built and 

tested by Nr. Moffett and Dr. Grayson M. Rayborn under NASA 

Contract NAS1-16503. The magnetometer was christened LEEM, 

for Cow gnergy - Electron - Magnetometer, following the usage of 

Dr. Singh et a1 in previous publications that described the 

theoretical behavior of such a device. 1,2 

This first device, constructed principally of stainless 
steel and teflon, and featuring a homemade electron gun 

faithfully obeyed the principles of operation theoretically 

described in the previous and in United 

States Letters Patent No. 4,414,509.~ However, the beam 

currents in the device were weak in typical operation, making 

the LEEM difficult to use and the method of actually 

performing the magnetic field measurement was to restore the 

beam to the Faraday cup manually. This method of operation 

obviated the inherent speed of the instruments. Finally, a 

need for precision IIelmholtz coils was recognized so that the 

capabilities of the instrument could be evaluated properly. 

For these reasons a second contract was undertaken, NASA 

Contract SAS1-17435, to set-up and test an improved version 

of the LEEM. The contract was later modified to require the 

construction of a device capable of producing precision 

magnetic fields and the installation of a position sensitive 

detector. This report describes the performance of these 

tasks. 

CONSTRUCTION OF LEEM-2 

The generally good performance of LECM-1 indicated that 

no radical design alteration was necessary for LEEN-2, so 

EEEM-2 was also designed to be a two-stage, second-order 

focusing, thirty degree, parallel-plate device. This design 
comrcends itself for ease of construction, ease of operation, 

and its stronc focusing properties, Although the basic 

thirty dezree design of LEEM-1 was retained in LEEN-2, 

several major modifications were effected. First, a design 

flaw that placed the slits of the device at other than there 

optimal position was corrected. Second, the materials of the 

device were changed to permit the device to be baked and used 

in ultra-hiph vacuum, and to prevent small, stray magnetic 

fields from forming in the metal plates. Third, a commercial 

electron ;;un with an indirectly heated, oxide coated cathode 

and a non-inductively wound filanent was purchased and 

installed in order to minimize the stray magnetic fields 

associated with the production of the electron beam itself. 

Fourth, shielding plates were introduced into both stages to 

shield the electron beam from stray fields due to the 

accumulation of charges on the insulators. Extensive 

numerical modelling was performed to simulate the electric 

field in the analyzer stages and Laplace's equation was 

solved numerically for the analyzer geometry to guarantee 

that the perturbation of the uniform electric field by the 

shield plates was acceptably small. Finally, performance 

testing of the device mandated two changes whose necessity 

was not forseen: deflection plates were installed between the 

electron gun and the first analyzer stage to permit the 

adjustment of the bean's lateral position and the electron 
detector was totally encapsulated to protect it from the 

general bacl~ground of stray electrons which was found to 

exist whenever the LEEM-2 was operated. 

This section discusses the construction of LEEM-2 with 

these changes, and its set-up and testing. 

Design of LEEM-2 

Each stage of LEEPI-1 and LEE!!-2 is a thirty degree, 

second-order focusing, parallel-plate electrostatic analyzer. 

Parallel-plate analyzers were first described by Yarnold and 

4 Bolton , and soon thereafter by liarrower who gave an 

extensive description of the forty-five degree analyzer.) The 

second-order focusing properties of the thirty degree, 

parallel-plate analyzer were revealed by Green and Proca. 687 

A description suitable for design was given by Poul Dahl in 

his textbook on electron and ion optic^.^ Study of these 

sources revealed that a slight error had been made in 

positioning the slits in LEEM-1. To correct this error and 

to accomodate shield plates to protect the beam electrons 

from stray electric fields originating in charges collected 

on the insulators, LEEM-2 was re-designed. The new design is 

described in the remainder of this sub-section. 

The size of the bell jar in which the LEEM-2 resides for 

4 
testing limits the overall length 06 the device. When it is 

fully assembled with electron gun and full-length shielding, 

for the purpose of preliminary testing prior to actual 

operation, the length of the gun must not exceed thirty 

centimeters in length. This, in turn, limits the length of 

the analyzers and detector to approximately twenty-two 

centimeters. This length determines every other dimension of 

the analyzers except for their width, which was chosen to be 

approximately 4.7 centimeters to insure a uniform electric 

field in the center of the analyzers where the electron beam 

resides. 

The analyzers were constructed with guard rings spaced 

0.605 cm apart to insure that a uniform electric field would 

be present in the active part of the device. These guard 

rings were constructed by cutting the centers out of plates 

which were similar to the deflector plates which form the 

extremities of each analyzer, An eight millimeter wide rim 

was left on each side of each guard plate and a two 

centimeter wide rim was left on each end of each guard plate 

for the purpose of maintaining uniform electric fields. The 

cavities formed by these plates function in conjunction with 

the shield plates described in a later portion sf this 

section of the report and are explained more fully there, 

They do, however, permit the hollow, central portion of each 

analyzer which contains the entrance and exit slits and in 
which the electrons travel, to maintain homogeneous electric 

fields. 

Before entering the first analyzer, electrons from the 

electron gun must pass through a defining slit which is 0.62 

cm in length and is adjustable in width and position. Since 

both this defining slit and the entrance slit to the first 

analyzer, which the electrons next encounter, are maintained 

at ground potential when the LEEM-2 is functioning, the 

region betveen these slits is free of electric fields and the 

clcctrons travel without acceleration for a distance of about 

1.21 cm in this region before entering the analyzer. They 

enter the analyzer through the entrancc slit whose length is 

also 0.62 cm like the defining slit, but whose width is fixed 

at 0.16 cm. When the defining slit is adjusted to its proper 

operating position, the electrons enter the first analyzer at 

angles ranging from 29 to 31 degrees. The electrons follow a 

parabolic path through the first analyzer and exit through a 

slit in thc sane plate as thc entrance slit, identical to the 

entracce slit and 4.192 cm from it. These slits are cut into 

plates which cire 0.127 cm thick in a fashion which allows for 

the side facing the analyzer to re~ain flat while the side 

alcay from thc analyzcr is thinned out in a cup-like manner to 

prevent interference with the path of the electrons. The 

analyzer side of the slits is left flat to minimize the 

disturbance of the uniform electric field in the analyzer. 
After the electrons exit the first analyzer they enter 

another (electric) field-free region which is called the 

separation chanbcr since it serves to separate the two 

analyzers and to provide the field-free region which both 

analyzers require to realize their second-order focusing 

capability. In order to maintain the strong focusing 

characteristics of the device and preserve the feature of 

LEEPI-1 by which the value of the deflection potential on 

either analyzer in volts was made nominally equal to the 

energy of the electrons in electron volts, the thickness of 

the separation chamber must be one-fourth the distance - D 

betveen the analyzer plates. The length of the device is 

determined by the requirement that the distance between 

entrance and exit slits be the square root of three tines the 

plate separation. Thus, the plates are separated by a 

distance of D = 2.420 cm, and the height of the separacicn 

chamber is d = 1.210 cm. The inside length of the separation 

chamber is 5.20 cm and the inside width is 1.22 cm. 

Travelling on a thirty degree slant, the electrons traverse 

approximately 2.42 cm in the separation chamber before 

entering the second analyzer, which is identical to the 

first. The potential on the guard rinos of each analyzer are 

maintained by n string of precision resistors which are 

accurately measured t:o be equal. P~tcntials to the cathode, 

focusing cup, analyzers, and lateral deflection plates are 
supplied by well-rcgulatcd power supplies and precision, tenturn potentiometers. For preliminary testing purposes, a 

magnetic shield to cover the entire LEEN-2 was constructed of 

thin metal sheets of high magnetic permeability, NETIC and 

CONETIC, by trade name. 

Materials for Construction 

The materials employed in the construction of LEEM-2 

were chosen to solve two problems discovered in LEEM-1: the 

presence of small magnetic fields in the plates of the device 

itself, and the inability of the device to be heated to high 

temperatures for the purpose of outgassing and eventual use 

in an ultra-high vacuum. Although all metal parts used in 

LEEM-1 were of "non-magnetic" stainless steel, and any 

residual magnetic fields in the metal itself was below the 

level of detectability using state-of-the-art gaussmeters, it 

was found during the operation of LEEM-1 that, after use, 

small magnetic fields were present near the slits. The alloy 

of stainless steel used, while not itself a ferromagnet, was 

nonetheless an alloy of ferromagnetic metals. It is 

necessary for the operation of the device for intense beams 

of electrons to be generated initially, although their 

intensity is subsequently decreased by energy selection. 

Since even a strong focusing lens will not direct every 
electron through an aperture, a considerable current of 

electrons is incident on the perimeter of some of the slits 

when the device is in operation. Since stainless steel is a 

poor conductor of heat, the electron beam presumably resulted 

in intense heating of the stainless steel in the vicinity of 

its impact. Conceivably, the intense heat may have resulted 

in melting and a different alloy may have formed around the 

point of iepact. It is worth noting, in this regard, that 

intensc electron beams are employed in a similar fashion in 

zone refining of metals. Whatever the reason, magnetic 

fields were detected near the first defining and entrance 

slits of LEEi.1-1 after it was aperated. Although these fields 

were quite weak, their proximity to the actual electron 

trajectory made it possible, if not lilcely, that they were 

interfering with the optimal operation of the device, 

This problem was corrected in LEEM-7 by constructing it 

from a pure, non-ferroniagnet. A11 metal parts in the deviee, 

with the exception of the electron gun and a few brass 

screws, were constructed of oxygen-free copper. Since copper 

is an element, no ferromagnetic alloys c-ould be formed by 

intensc heating. Since copper is a very yo,.>d conductor, the 

heat forncd by the impact of the electron heam is quickly 

spread throughout the device and dissipate+ without 

gencratinzv the str~sses which might c2use parts of ~!Ic. 

device to a. T!~E copper was 1)lateA with qol-d using an 
electroplating technique with gold cyanide in solution. T he 

gold surface has several well-known advantages for an 

electron device. It is an excellent conductor, so that 

impressed potentials are accurately transmitted throughout 

the device. Furthermore, it will not readily oxidize so that 

no insulating spots form on the surface. Finally, gold has a 

very low coefficient for secondary electron emission. Thus, 

the inpact cf clcctrons on the plates around the slits result 

in fever stray electrons, thereby preventing deterioration of 

the cignsl-to-noise ratio of the LEE!!. Despite all this care 

in cGnstruction of the LEYM-2, one additional precnution was 

tnkcn. , cclloidal suspension of graphite in wzter was 

painted arcund all slits in the xagnetometer and pernitted to 

dry. This szne coating \<as used on the back plates of the 

analyzers. Cc6ti1-13 with graphite has been found to result in 

better conducting surfaces when pump oil is present in a 

conventional vacuum system such as was used to test LEEII-2. 

Presumably, the ~olecules of oil are subsumed into the 

graphite structure and prevented from remaining on the 

surfacc whcre they become charged from the presence of 

electrons forming electrostatic potentials which nay 

inadvertently deflect the beam. These coatings were renewed 

periodically as the device was operated. 

LEEM-1 used tef lon where insulators were requircd for 

mounting the metal plates, Although teflon is easily 
machinable and wears well, permitting repeated disassembling, 

its vapor pressure is too high to permit its use in ultrahigh vacuua. Also, it tends to flex excessively under 

bending stress. The decision to employ shielding plates in 

the analyzer sections of LEEN-2 necessitated a longer and 

larger device, thereby placing greater stress on the 

structural components. For these two reasons teflon was 

replaced in LEEN-2 with MACOR, a machinable ceramic 

manufactured by Corning Glass Company. Data provided by the 

manufacturer indicate that MACOR is strong and has a very low 

vapor pressure.' Unfortunately, the ceramic is brittle and, 

although quite copable of being machined, does tend to chip 

and crumble under heavy use and frequent dis-assembling, 

Shielding of Insulators 

A rule of thumb of designers of apparatus using low 

energy ions and electrons is "not to let the electrons (or 

ions) - see an insulator". By "seeingt1 an insulator is meant 

that a straight line path should not exist from the electron 

or ion at any point on its trajectory to an insulating 

surface. If other design considerations prevent this rule 

from being strictly realized, then the elec~rcns should be 

permitted no more than a "peel<" at thc insulator. That is, 

the line-of-sight to the insulator should exist only briefly 
as the electron traverses its path. The reason for this rule 

is that insulators acquire charge by accumulating electrons 

or ions from the background. Since they are not capable of 

conducting these charges away, they remain and give rise to 

spurious electric fields that may disturb the electrons if 

the proper field is not re-established by potentials 

impressed on intervening electrodes. This rule, which is 

very difficult to follow in constructing a parallel-plate 

analyzer, was not observed in the construction of LEEPI-1. To 

observe the rule in the design and construction of LEEM-2, it 

was decided to attach perpendicular plates to each end of 

each guard ring. After computer simulation, as described 

below, the overall height of these plates was chosen to be 

0.55 cm which left a gap of only 0.045 cm between the guard 

rings and the next end plates. This small gap was just 

sufficient to prevent arcing between the plates during the 

operation of the magnetometer. Although the use of end 

plates prevented the occurence of stray potentials due to 

charge collected on insulators, they themselves carried the 

potential of the guard ring to which they were attached, 

thereby distorting the uniform electric field which is the 

basis of the operation of the parallel-plate analyzer. 

To ensure that this non-uniform perturbation was 

sufficiently small as to have no appreciable effect on the 

flight of the electrons, a computer simulation of the 
perturbation was performed, Because the electrons travel on 

or near n plane of symmetry of the device, the potential in 

the three-dimensional analyzer could be validly simulated as 

a two dimensional problem. This was done and typical values 

of the operating potentials were taken for the end plates and 

guard rings of the analyzer. Numerical solution of Laplace's 

equation, using a relaxation technique, showed that the 

potential in the region of interest in the central one-third 

of the device would be disturbed less than one tenth of one 

percent if the height of the end plates was kept less than 

0.60 cm while the length of the guard rings was at least 12.0 

cn. The magnetometer was designed to conform to these 

requirements by making the inside length of the guard rings 

12.7 cm and the height of the shielding end plates 0,55 cn. 

This resulted in a larger magnetometer, as was noted earlier 

in this report, and contributed to the decision to use a 

machinable glass ceramic in its construction. Since the 

portion of the electrons' trajectory that is exposed to the 

field is approximately 4.2 cm in length, the inside length of 

the analyzer is more than three tices the trajectory length, 

ensuring good ho~ogeneity in the field. The presence of a 

slight, known inhonogeneity is preferrable to the possiblc 

presence of a much lc.rger inhomogenejty due to tht> oLtrljr;ior? 

of insu1;tor-s carrying accumulated strap char~c, 
Choice of Electron Gun 

The elcctron gun in LEE!I-1 was a simple, un-coated 

tungsten filanent followed by a custom designed unipotential 

lens. Although custom design permitted a lens specifically 

planned to focus a bcam of electrons on the defining slit, 

actual operation of LEEK-1 revealed that the unipotential 

lens was of slight efficacy. Further~ore, because the 

tungsten filament was not coated and thus had a large itor!: 

function, an enormous filament current was required to heat 

the filanent sufficiently hot to cause copious emission of 

electrons. Such a large current passing through a straight 

conductor (the filament) generates a considerable magnetic 

field of its own; a field which is difficult or impossible to 

shield since it is generated internally and which may well 

interfere with the operation of the device or distort the 

magnetic field measurements made with it. 

To avoid these problems, the decision was made to 

purchase a commercial electron gun for use in LEEM-2. After 

careful analysis of the guns availzblc, the elcctron gun used 

by Central Scientific Company in its laboratory device for 

student ulaeasurenent of the charge-to-fi;ass ratio of the 

electron was purchased, installed and tested. The gun was 

chosen bccausc it was known to produce an intense electron 

bcam, since this experiment requires that the path of the 
electrons be rendered visible by Eluoresence and their impact 

point visible by phosphoresence, and because the gun cannot 

generate appreciable magnetic fields if the students are to 

determine the charge-to-mass ratio accurately. The gun has 

met these criteria well, operating with less than 0.7 amperes 

of filament current while generating intense electron beams. 

In addition, it appears that the cathode is indirectly heated 

by a filsment which is non-inductively wound, further 

decreasing the magnetic field generated by the filament 

current. Thcse characteristics are discussed further in the 

section on set-up and testing of LEEM-2. Unfortunately, the 

gun docs not possess the capability of deflection in either 

of thc transverse directions, and this appeared to be a 

larger defect as testing of LEEl.1-2 continued. 

Installation of Lateral Deflection Plates 

When LEE?4-2 was originally designed, it was believed 

that the slits could be mechanically aligned by precise 

manufacture of the components and the electron gun aligned 

once and for all by optical means. Although it does appear 

that mechanical alignment of the slits in the analyzers has 

been achieved, precise alignment of the electron gun has been 

difficult and maintaining the alignment under thermal stress 

as the gun heated and soolsd in repeated, discontinuous 
operation has been virtually impossible, That alignment is 

definitely a problem has been confirmed both by the operation 

of the magnetometer and by examination of burn marks on the 

graphite around the slits when the device has been disassembled after operation. These burn marks clearly show 

that after operation the LEEM-2 has generated a beam to one 

side of the slit. Sometimes a clear image of the slit shape 

can be seen on the exit slit of the first analyzer, 

coincident with it but displaced to one side. This lateral 

displacement has caused the deterioration of the performance 

of the magnetometer with resulting weak and erratic beams and 

has sometimes prevented the beam from even reaching the 

second analyzer. 

In an attempt to solve this problem lateral deflection 

plates were installed between the electron gun and the 

entrance to the first analyzer as a post-construction 

retrofit modification. Unfortunately, the original design of 

LEEM-2 left very little room for the insertion of lateral. 

deflection plates, and the plates that could be inserted had 

very little extent. Since the deflecting ability of such 

plates depends on the distance the electrons travel between 

then, the efficacy of the plates was minimal. Furthermore, 

since no provision was made for the plates originally, a firm 

sure mounting of them could not be achieved. They tended to 

move as stress was placed on their electrical leads, 
occasionally shorting out to the electron gun mounting plate 

which was held at ground potential. This, of course, totally 

prevented their use, causing them to have to be disconnected 

from their source of potential if the operation of the device 

was to continue in any form. Although these deflection 

plates were used with very little success, the need of plates 

capable of deflecting the beam in both transverse directions 

in future nagnetoneters is manifest. 

Summary 

In summary, then, LEEN-2 was designed to incorporate 

several features not fourid in LEE21-1. A small design flaw in 

thc placement of the defining and entrance slits was 

correctcd in LEEII-2. The material of construction was 

changed to prevent the occurrence of snall magnetic fields 

around points at which the electron beam might impact the 

plates of the devicc. The use of oxygen-free copper with a 

gold plating also increased the electrical conductivity of 

the devicc, presumably resulting in the more accurate 

inposition of electrical fields. The tcflon linsulators were 

replaced with a cachinable glass ceramic to permit the LCEiI-2 

to bc ba!:cd and used in ultra-high vacuua, and also to add 

mechanical stsbility to the device. The anslyzers were redcsic.r,cd an(! crid plates added to shield the electron 1)cal;i 
from any strcy fields cenerated by charge on the insulators. 

h commercial electron gun was purchased, installed and tcstcd 

to decrease the magnetic field generated by the filament 

current and to supply a more copious source of electrons. 

Finally, lateral deflector plates were added in attempt to 

increase the ease alignment of the beam and to permit the 

alignment to be restored after misalignment due to thermal 

strain. The LERM-2 is shown, assembled, from two different 

perspectives in Figure 1. Figure 2(a) shows a close-up of 

the electron gun, while Figure 2(b) exhibits thc device 

largely disassembled. Figure 3(a) compares the LEEI\I-1 (top) 

with LEE:-!-2 (bottom), and Figure 3(b) shows the device 

situated in the bell jar surrounded by one set of Helmholtz 

coils and ready for operation. 

SET-UP AND TESTING OF LEEM-2 

For testing purposes LEEM-2 was set up in a manner 

similar to that indicated in Figure 3(b). To test the 

ability of the electron gun to provide the large beam 

currents for which it was purchased and installed, 

arrangement was made to collect the bean current in the 

field-frce region betwecn the two analyzers. Initially a 

magnetic shield was installed to protect the entire device 
0RSGSNAL PAGE IS 

OF POOR QUALW 

Figure l(a). The LEEM-2. 

Figure l(b). Another view of the LEEM-2. 
Figure 2(a). A close-up view of the electron gun. 

Figure 2(b). The LEEM-2 disassembled. 
Figure 3(a), above. LEEMand LEEM-2 seen together. 

Figure 3(b), left. LEEM-2 

in the bell jar ready for 

operation. 
from the ambient magnetic field until its basic operating 

parameters could be established, This mode of operation 

established that LEEM-2 could indeed provide the enormous 

beam currents for which it had been designed. Figure 4 shows 

the current collected after the first stage as a function of 

the deflection potpntial on the first analyzer for a beam of 

nominal energy of 4.8 x 10-175, or 300 eV. The maximum beam 

current of greater than 0.3 microamperes is particularly 

impressive in view of the fact that this current has 

successfully undergone energy analysis in a dispersive energy 

analyzer. The smooth symmetry of the beam profile exhibited 

in Figure 4 also suggests that the first stage of LEEM-2 is 

faithfully performing according to its design principles. 

The slight flattening at the top of the peak suggests that 

the entire beam may be contained within the exit slit. Since 

the defining slit was deliberately set at maximum width for 

these tests, this would indicate that the focus cup on the 

electron gun was producing a focus wholly within the defining 

slit so that the beam diverging after this focus point did 

not wholly fill the entrance slit to the first analyzer 

stage. This in turn suggests that the focus adjustment on 

this commercial gun is effective in its present application. 

It should be noted, however, that the flattening on the top 

of the profile could also be explained as a widened image of 

the entrance slit of uniform electron density which wholly 
Figure 4. The beam profile through one stage with magnetic shielding. 
encompasses the exit slit. In order to test LEEM-2 more 

strenuously, the magnetic shield was removed and the tests 

repeated with the entire device exposed to the ambient 

magnetic field. (Tests show that the horizontal component of 

the field is of the order of 3 x lo-' T), One would, of 

course, expect deterioration of the performance of LEEM-2 

when it was exposed to the ambient magnetic field. In fact, 

as Figure 5 clearly shows, the performance improved. The 

beam profile is smoother and even more symmetrical and the 

beam current has increased to an incredible 0.43 microamperes. 

The strong beam produced through the first half of the 

LEEM-2 suggests that one stage alone might function as a 

magnetometer, Indeed, on reflection there is reason to 

believe that a single stage device may function bettef' than a 

two stage instrument, Since great difficulty is encountered 

in transmitting the electrons through the second stage even 

with substantial decreases in beam current, a single stage 

device should be more sensitive since the magnitude of the 

current eventually determines the minimum detectable change 

in beam position. If more current than is needed is 

generated the spot size of the beam can be decreased by 

narrowing the defining slit. Since both characteristics 

contribute to instrument sensitivity, more flexibility in 

determining the optimum trade-off would be available. 
Figure 5. The beam profile after one stage without magnetic shielding. 

0.45 

0.40 - 

0.35 - 

0.30 - 

V) 

a 0.25 - 

z 

IG 

ac 

= 0.20 - 3 

m 

3 

0.15 - 

0.10 - 

BEAM PROFILE AnER ONE STAGE 

=?a 

cl 

01 

0 

01 

10 

01 

01 

lfil 

01 

23 

23 

10 

0.05 

0.00 

278 280 290 380 310 

DEFECTION POTENTlAL IN VOLTS 
Furthermore, a single-stage instrument would be more reliable 

than a two stage device. For these reasons the ability of 

LEEM-2 to function as a single-stage magnetometer was 

investigated. Helmlholtz coils were installed around the 

device in such a manner as to produce a field perpendicular 

to the electrons' trajectories. The current to the Helmholtz 

coils was provided by a well-regulated power supply and 

monitored by a digital multimeter. As the current was 

increased the beam was partially, or completely, deflected 

out of the exit slit to the first stage. It was restored to 

the slit by adjustment of the deflection potential on the 

first analyzer. The measurements were made for a variety of 

Helmholtz coil currents with the currents flowing both ways 

through the coils at various beam energies ranging upward 

from 3.2 x 10-175, or 200 eV, to about 4.8 x 10-175, or 300 

eV. Results from one of those runs which denonstrate the 

performance of LEE?!-2 as a magnetometer is shown in Figure 6. 

Here the deflection potential for optimal transmission of the 

beam is plotted as a function of Kelmholtz coil current for 

coil currents flowing in each of the two possible directions. 

The linearity of the plots suggests that the LEEM-2 

magnetometer is a linear instrument. The upper data are fit 

with a straight line of slope 147.9 volts/ampere and an 

intercept of 198.9 volts. The lover data, taken with the 
Figure 6. The performance of LEEM-2 as a magnetometer. 

SINGLE STAGE MAGNETIC DEFLECTION 

8.00 0.02 8.04 0.06 0.08 

COIL CURRENT IN MPERES 
direction of the coil current reversed, are fit by a straight 

line with slope 142.0 voltslampere and intercept 200.6 volts. 

That the LEEM is highly linear can be inferred from the 

correlation coefficients for the two sets of data. The 

correlation coefficient for the top data is r = 0.9968, while 

that for the bottom data is r = -0.9989. The negative sign 

for the second set of data simply indicates that the optimal 

deflection potential decreases with increasing coil current 

when the current flows in that direction, a fact that is 

already well understood. 

The LEEM-2 performs better as a magnetometer using one 

stage rather than two. Part of the reason for this can be 

seen in Figure 7 which shows a beam profile in the second 

analyzer. Notice that three orders of magnitude have been 

lost in comparision to the beam profiles in Figures 4 and 5 

which were made after the beam had left the first analyzer. 

The highly linear response of the device and the large 

currents available through stage one. suggests good 

performance of the LEEM-2 in this mode. 

CONSTRUCTION AND TESTING OF HELMHOLTZ COILS 

In order to provide an ambient environment to test the 

performance of the LEEM-2, it was necessary to provide a 

means of accurately imposing a uniform magnetic field for the 

electrons to experience. This was accomplished by precision 
Figure 7. Beam profile in the second analyzer. 

BEAM PROFILE IN STAGE TWO 
construction of three pairs of square coils which were then 

operated in a modified IIelmholtz arrangement. The two 

smaller coils were constructed for the purpose of controlling 

extraneous components of the ambient magnetic field during 

experiments with LEEM-2. The largest coils were constructed 

to produce the uniform magnetic field in compliance with the 

modified contract. It is these coils which were tested and 

which are described here. 

The coils were constructed from three quarter inch 

channel aluminum by the Mr. Jim Bridges using precision 

milling techniques. They were then wrapped by hand with 

copper magnet wire. The width of the coils is 1.91 cm. The 

length of the side of a square is 47.78 cm inside diameter 

and 51.26 cm producing an average length of 49.52 cm. In 

order to test the coils a transverse probe and Bell Model 620 

Gaussmeter were used. Both the coil current and the Hall 

probe potential were read with digital multimeters. The 

component of the Earth's field along the axis of the 

Helmholtz coils was carefully zeroed out before readings were 

taken. Figure 8 shows the coils situated for the 

measurements. Because varying the coil current and reading 

the Hall probe potential, that is the magnetic field, is a 

better method of avoiding systematic errors than simply 

reading one, or a few, values of the field at various 

locations, we have performed this linear regression analysis 
Figure $(a), Arrangement for measurittig the ut~ifot-mity of the field in 

the Helmholtz coi 1 s. 

Figure 8(b). Close-up of the large coils. 
at some nine Locations in the coils. These locations were: 

the center of the coil and four locations along the axis and 

four locations transverse to the axis from the center of the 

coil. Figures 9, 10 and 11 show some of the results of these 

readings, The measurements were highly linear at all nine 

locations. The correlation coefficient between the coil 

current and the Hall potential was 0,99993 or greater for all 

nine locations. These highly linear relations are shown for 

the center of the coils in Figure 9, for a 2.0 cm axial 

displacement in Figure 10, and for a 2.0 cm lateral 

displacement in Figure 11. The following table shows the 

measured relation between Hall potential and coil currents 

and the calculated correlation coefficients for the nine 

locations. 

Axial Lateral Slope (V/A) Correlation 

Displ (cm) Displ (cm) coefficient 

The data in this table are plotted in Figures 12 and 13. 

Figure 12 shows the variation of field strength with axial 
Figure 9. Linearity of magnetic field with coil current in the center 

of the coils. 
Figure 10. Linearity of magnetic field with coil current two 

centimeters axially displaced from the coil center. 

LINMCRI OF NELO WITH CURRENT 

34 
Figure 11. Linearity of magnetic field with coil current two 

centimeters laterally displaced from the coil center. 

LlNEARrPl OF FIELD WITH CURRENT 

10 20 30 m so so 70 so eo r 

COIL CURRENT (onA) 
Figure 12. Unifomity of the magnetic field along the coil axis. 

AXIAL FIELD UNIFORMIN 
Figure 13. Uniformity of the magnetic field for lateral variations. 
displacement, and Figure 13 shows the variation of lateral, 

or transverse, field strength with lateral displacement. 

Neither graph displays a systematic variation of field 

strength with distance from the center of the coils. Both 

plots show an almost constant magnetic field with small 

random fluctuation presumably due to random errors in the 

measurement. These measurements are deemed to establish 

uniformity of magnetic field consistent with the requirements 

of this contract. Vhile the measure~ents were performed over 

two centimeters in the mutually perpendicular directions, the 

symmetry of the IJelmholtz coils indicates that the results 

are actually applicable for four centimeters in each 

direction and the constancy of the measurements justifies 

extrapolation for the required ten centimeters. 

INSTALLATION AND TESTING OF THE POSITION SENSITIVE DETECTOR 

Because LEEM-2 functioned well when only a single stage 

was employed and because this mode of operation tended to 

produce large beam currents, the decision was made to employ 

a resistive strip as the position sensitive device to be 

installed in the LEEM-2. Resistive strips were obtained from 

TRW and, after testing, one was installed in the first stage 

of the LEEM-2. The strip was run along the entrance plate of 

the first analyzer from just beyond the entrance slit to just 

past the exit slit, covering the exit slit in the process. 
The strip was securcd by mounting it in a teflon mount which 

experienced a compression fit between the entrance plate and 

the first guard ring in the first analyzer. To test the 

strip a beam was generated and a deflection potential imposed 

on the first plate. One end of the strip was grounded 

externally through two 6.3 megohm resistors in series, and 

the current was read from the other end using an 

electrometer. Occasionally, the roles of the grounding 

resistors and the electrometer were interchanged. 

The measured variation of current leaving the resistive 

strip through the electrometer with the deflection potential 

impressed on the first analyzer is shown in Figure 14. 

Although.the data seem to show a smooth decrease with 

impressed deflection potential consistent with the idea that 

the increasing electric field is moving the beam farther from 

the end attached to the electrometer, and despite the fact 

that the filament current was turned off to verify that the 

signal disappeared, the data seem unusually smooth and the 

slight S-shape seen in the curve is sometimes consistent with 

a leakage current rather than a true signal. Further tests 

are needed to confirm this response for the resistive strip. 

Should this behavior of the strip be confirmed, then it would 

indeed form a sensitive detector of the position of the 

electron beam since the current is easily detectable and 

varies substantially with the apparent position of the beam, 
Figure 14. Current from one end of resistive strip as beam position 

varies . 

RESPONSE OF RESISTIVE STRIP 

2.00 

1.90 - 

1.80 - 

m-Y 

U 

B 

1.70 

1.60 - 

1.50 - 

1.40 - 

1.30 - 

1.20 - 

DEFLECIIBN POTENTIAL (V) 
FURTHER POTENTIAL IMPROVEMENTS FOR THE LEEM 

The set-up and demonstration of LEEM-2 has, in fact, 

demonstrated some additional possible improvements. Should 

it be desired to perfect the device, the following changes 

are recommended. 

1) The present electron gun should be replaced with a 

gun from a commercial cathode ray tube. Such a tube 

will have optimally designed deflection plates for 

each of the transverse directions. This feature 

will overcome the most serious defect of the LEI31 by 

pernitting easy adjustment of the beam through the 

device. 

2) The LEL:*I should be reduced to a single stage. The 

stronger signal which would be thus produced would 

more than offset the spreading of the beam spot due 

to chronatic aberration. Thc device would probably 

be greatly simplified. Its operation night even be 

routine. 

3) The larger beam currents would permit the firm 

adoption of thc resistive strip for measuring the 

beam position, avoiding the use of complicated 

mircochannel plates. 
REFERENCES 

18 A Low Energy Electron Beam Magnetometer", Jag. J. Singh, 

George Wood, Frederick !~Jhite, and Grayson H. Rayborn. 

NASA Technical Memorandum 80172 (1979). 

"Improved LEEM Ranges Over Four ~ecades", NASA Tech 

Briefs 5, 469-470, Winter 1980. 

United States Letters Patent No. 4,414,509, "Low Energy 

Electron Magnetometer Using a Monoenergetic Electron 

Beam", Jag J. Singh, George M. Mood, Jr., Frederick A. 

White, and Grayson 11. Rayborn. November 8, 1983. 

Yarnold, G. D., and Dolton, H. C., 

38 (1949). 

J. Sci. Instrum. 

IIarrower, G. A.,Rev. Sci. Tnstrun. 26, 850 (1955). 

Green, T. S., and Proca, G. A., Eev. Sci. Instrum. 2, 

1409 (1970). 

Proca, G. A*, and Green, T. S., 2ev. Sci. Instrum. 41, 

1778 (1970). 

Dahl, Poul, Introduction to Electron and Ion Optics 

(Academic Press, New Yorlc, 1973), Chapter 6. 

Grossman, D. G., Corning Glass Company MACOR Brochure. 
Standard Bibliographic Page 

Set-Up and Demonstratfon of a Low Energy 

Electron Magnetometer ('LEEM) 

1. Report No. 

NASA CR- 17809 1 

6. Performing Organization Code 

7. Author(s) 8. Performing Organization Report No. 

2. Government Accession No. 

Grayson H. Rayborn 

9. 'Performing Organization Name and Address 

3. Recipient's Catalog No. 

4. Title and Subtitle 

Department of Physics and Astronomy 

University of Southern Mississippi 

Hattiesburg, MS 39406-5165 

12. Sponsoring Agency Name and Address 

Nationa1,Aeronautics and Space Administration 

Washington, DC 20546 

5. Report Date 

10. Work Unit No. 

11. Contract or Grant No. 

NAS1-17435 -1 13. Type of Report and Period Covered I 

I 

15. Supplementary Notes 

Technical Monitor: Jag J. Singh 

NASA Langley Research Center 

16. Abstract 

This report describes the design, construction, and test results of a 

Low Energy Electron Magnetometer (LEEM). The electron source is a commercial 

electron gun capable of providing several mixroarnperes of electron beam. 

These electrons, after acceleration through a selected potential difference 

of 100-300 volts, are sent through two 30 degree second-order focussing 

parallel plate electrostatic analyzers. The first analyzer acts as a 

monochromator located in the field-free space. It is capable of providing 

energy resolution of better than 10-3. The second analyzer, located in the 

test field region, acts as the detector for electrons deflected by the 

test field. The entire magnetometer system is expected to have a resolution 

of 1 part in lo3 or better. 

Electron Deflection Magnetometer 

Electrostatic Analyzer 

Monoenergetic Electrons 

Position-Sensitive Detector 

17. Key Words (Suggested by Authors(s)) 

UNCLASSIFIED - UNLIMITED 

Subject Category 35 

18. Distribution Statement 

For sale by the National Technical Information Service, Springfield, Virginia 22161 

NASA Langley Form 63 (June 19R5) 

I 

22. Price 

A03 

21. No. of Pages 

4 6 

19. Security Classif.(of this report) 

Unclassified 

20. Security Classif.(of :his page) 

Unclassified