Two Instruments for Measuring Distributions of Low-Energy Charged ...

NASA

TN D-1035

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TECHNICAL NOTE D-1035

TWO OF

INSTRUMENTS

FOR

LOW-ENERGY

MEASURING

CHARGED

By Michel Bader, and Fred Ames

Moffett.Field,

NATIONAL

AERONAUTICS

WASHINGTON

PARTICLES

Thomas B. C. Wltteborn

Research

AND

DISTRIBUTIONS IN SPACE

Fryer,

Center Calif.

SPACE ADMINISTRATION July 1961

_

• i

i_

_I ¸_iii_i,_ii_

IK NATIONAL

AERONAUTICS

AND

TECHNICAL

TWO

IAVSTRUMENTS

OF

By

NOTE

FOR

LOW-ENERGY

SPACE

D-I035

MEASURING

CHARGED

Michel and

ADMINISTRATION

DISTRIBUTIONS

PARTICLES

IN

Bader, Thomas B. Fred C. Witteborn

SPACE

Fryer,

SUMMARY

Current consists of

estimates indicate that the protons with energies between

of i to iO s particles the energy and density the

standpoint

of

per cm s. Methods distribution of

suitability

for

concluded that electrostatic provide sufficient information and

electrostatic Several

analyzers

instruments

Center for space plasma and data reduction are for operation on solar

and such

space

designed

and

weight_ 14_ m_.

l.l pounds] size_ 2 by 3 by The instrument is designed

tration, the 0.2

energy distribution_ to 20 key range.

payloads.

It

measuring from

is

the energy distribution can experiments. Both magnetic

eventually

be

used.

constructed

measurements_ described. In cell power has

and

instrumentation for a gas are considered

vehicle

analysis of in initial should

bulk of interplanetary gas 0 and 20 key and concentrations

at

the

Ames

Research

and the methods of calibration particular_ the instrument designed the following characteristics:

4 inches; to yield

the

and power information

anisotropy

of

consumption z on the concen-

ion

trajectories

in

INTRODUCTION

The

state

of matter extent on

of

knowledge

in interstellar interpretation

about

space has; of indirect

of plausible theoretical models agreement with this evidence. deduced background hydrogen

that

the

gas

of protons and electrons_ atoms_ and smaller traces

indicated low-energy

that there exist in streams of protons_

generally

associated

composition

probably of

energy

activity.

were in techniques

consists

with a other

the solar sometimes

solar

and

distribution

until recently_ depended to a large evidence_ and on the construction

whose predictions By means of such

interstellar

with

the

of

general it has

small percentage substances. It

system called

been

a thermal of neutral is further

relatively high-density solar windsj which are

Information

on

cosmic

rays_

the higher energy particles_ has been obtained more directly, since many of these particles can penetrate the earth's magnetic field and reach the earth's atmosphere. Order-of-magnitude estimates for the particle populations involved are as follows:

2 Concentration, particles Interstellar neutral gas Interstellar proton

gas

cm-s

i

1,2,3

......

i0 a to

l0 s

i0 -s

Cosmic rays, near earth (gev)

i0 -l°

these

the

References

2,4,5,6

(ev)

Earth' s radiation belts (Mev)

of

Energy flux, gev cm-2sec -1

cm-asec -m ......

10 2

Solar wind, near earth (key)

While

Particle flux, particles

numbers

are

low-energy

10 9 to

i0 s to l0 s

l0 s to

l0 s

7-12

l0 s to

l0 s

13

i

admittedly

component

i0 is

of

the

i

crude,

they

do

interstellar

contains higher numbers of particles by several orders of magnitude than low penetrating power has made them

point

11,12

out

medium:

the

this

importance

portion

and produces higher energy fluxes all other known components. Their impossible to be observed directly

from the earth_ and they are not a biological On the other hand_ they are of prime importance

hazard to space travelers. in questions such as

those of the origin balance in the upper

in and

damage

to

space

With experiments

and distribution atmosphere of

vehicles

the advent yielding

the solar system. energy particles, detection

had

experiments led to the

first were

appropriate

In

but

advanced

a it

by

nuclear

of

the

It the

combinations

of

view

of

the

above

the eventual distributions

problems.

has become possible on the interplanetary

such experiments known to exist and

to perform medium of

have dealt with and for which the cosmic-ray

existing

techniques.

considerations;

can

and be

Specifically; mass analysis; swept to cover

a program

detailed mapping and in accessible regions

the art

highof

physicists. rays_ and the trapped

is in principle possible to obtain important low-energy particles by

can be obtained electrostatically_ The electric and magnetic fields energy and momentum ranges.

objective particle

few

space, the energy possible surface

have increased our information on cosmic discovery by Van Allen and co-workers of

layers about the earth. detailed information on analysis cally. expected

name

of space flight_ direct information

The which

been

- to

of matter the earth,

was

understanding of space.

started In

These they have radiation

very means

of

energy magnetithe

with

of low-energy the following

sections we shall discuss the design principles and feasibility of electromagnetic analysis; and the specific design; construction and calibration of two electrostatic analyzers developed at the Ames Research Center

of

the

NASA

for

space-vehicle

payloads.

PRINCIPAL

Al_ Bl_

A

4

Aa_% Ba3J

curve-fitting

parameters

SYMBOLS

(eq.

(23))

b

half

the

c

half

the width

DTU

digital

telemetry

unit

E

particle

kinetic

energy

Ea

nominal

acceptance

_E

theoretical range of particle energies at any given angle and plate voltage

e

electric

f

acceptance function (For a monoenergetic_ unidirectional f is the ratio of collector current on the entrance slit.)

length

of the of the

entrance entrance

slit slit

0

3

charge

energy

for

current

ICBO

transistor

Ic

collector

Ie

ion

k

Bolt zmann

m

particle

N

total

n

differential

r

radial

ro;rl_r2

mean_ inner; and outer values between analyzer plates

area

plate

voltage

accepted

by

Va analyzer

on a particle

I

leakage

beam of particles_ to current incident

current

current

current

plate

a given

into

entrance

slit

constant mass

particle

concentration particle

distance

from

separation 3 r2 of entrance

slit

concentration center

- rl

or distribution

of curvature of

of analyzer

r; respectively;

function plates for

gap

4

T

absolute

t

slit

u

velocity vehicle

Va

temperature

dimension,

voltage

of

difference

voltage drop and vehicle

v

velocity

of

component

length

particle,

Vs

x,y,z

either

between

rectangular

outer

plasma

particle v

width

undisturbed

across

of

or

as

normal

Cartesian

by

plasma

and

inner

sheath

between

it

enters

analyzer

to

entrance

sheath

around

analyzer

plates

undisturbed

space

entrance

slit

slit

coordinates,

centered

plane

to

on

entrance

slit

(sees etch c6

angle

of

incidence

in

normal

entrance

slit

(see

at

entrance

s etoh values of _ outside slit, respectively transistor

current

vehicle plasma sheath (see sketch (e))

gain

for

a

common

and

emitter

configuration

5

E

_N Ea _a

_N cu/_ve-fitting normalizing _N = kT

parameter

energy chosen for a Nax_ellian

curve-fitting e v

polar

angle

parameter in (24a,b,°)) /it rl

(eq.

parameter of

incidence

trial

angular

(23))

for convenience; distribution

(eq. (see

for

example;

(23)) sketch

distribution

(d)) function

(see

eqs.

P,Pl,P2

_,r

r A ' ,r_

ro

ro

ro

curve-fitting

parameter

(eq.

(23))

¢i

flux

¢2

flux of ions on collector, corrected emission; hence e¢m = Ic

of ions

azimuthal

on

angle

GF_

entrance

of

slit

incidence

DESIGN

(see

for

sketch

secondary

electron

(d))

CONSIDERATIONS

There are t_o important features to be considered in the experiments, in addition to size, weight 3 and power consumption limitations. First, the particles of interest have a relatively large range of energies and momenta; and second, they are likely to reach the vehicle from directions distributed over a large solid angle. In a conventional mass spectrometer, ions are created at essentially thermal energies in a "source." They are then extracted by a system of slits which collimate them into a well-defined beam and accelerate them electrostatically to a well-deflned energy (large compared to thermal). The beam enters a magnetic field, in which particles of different momenta follow paths of different curvature and are hence separated. To detect a range of particle masses, one then has a choice of varying the accelerating electric fields, varying the magnetic field, or using a series of collectors, one for each of the momenta separated by the magnet. The size and power consumption limitations imposed on space vehicle payloads preclude all but the electric field variation. Since 3 in addition, the "source" in space is not of negligibly small energy, a well-defined beam, energywise_ cannot be obtained by electrostatic acceleration alone.

The problem of mass and energy determination is first simplified by requiring that the instrument detect only one mass (say_ protons or alpha particles) as a function of energy. Then, by a floating electrode arrangement, as shown schematically in figure 13 the proper mass current as a function of energy can be obtained with a fixed collector and a permanent magnet. The particle beam is admitted through a slit in the vehicle surface, then accelerated by a variable voltage to a second aperture. It then enters a fixed-voltage, curved-plate, electrostatic analyzer whose mean potential is that of the second aperture. Only the particles with a given energy can get through_ the energy being determined by the voltage across the analyzer plates. These particles then enter the fixed field of a permanent magnet_ so that only those _ith a predetermined

momentum

can get through.

For

a given

fixed

energy

and

U

6

momentum and their acceptance the

only particles of a predetermined mass can reach the collector_ initial energy is the difference between the analyzer energy and the (variable) accelerating voltage. Note that

magnet

must

be

at

the

have been made for such the detection of protons

second

aperture

an instrument in the energy

potential.

Design

calculations

with the following results. For range 0-20 kev_ the mean electro-

static radius of curvature would be I inch; the plate voltage 1.6 kv; the variable supply 3 +i0 kv; and the magnetic field_ The total weight of the device would be about 3 pounds.

flux

It

is

is

concentration

energy mass.

possible

to

simplify times

the

above

velocity_

is inversely proportional Hence it would be a good

the

instrument

as

particle

to the square approximation

difference_ _000 gauss.

follows.

current

at

a

Since given

root of the particle to correct the data

assuming that the particles are 8_ percent protons and 14 percent alphas_ as estimated for cosmic rays_ and forego mass analysis. This procedure is especially attractive to use for the first few experiments in which the maximum probable error so introduced_ when one considers other uncertainties in

about 5 percent_ is unimportant the experiment and the saving

in weight (no magnet needed) . If_ then_ we dispense with the magnet_ the energy of the beam at the electrostatic analyzer exit need no longer be fixed. Hence_ we can leave out the second aperture_ and change the energy accepted by varying the analyzer plate voltages. This should be done in driving

such a way as the inner and

This would voltage to

to keep the outer plates

eliminate large entrance ground_ easing insulation

greatly simplified arrangement been built and tested_ and we design.

ENERGY

T_o

mean potential zero_ that is_ by equal voltages of opposite

easily

cylinders and cross section

machinable

and exit problems.

by sign.

fields_ and minimize We thus arrive at

the the

shown in figure 2. Such instruments turn now to a detailed discussion of

A_ALYZER

shapes

CHARACTERISTICS

for

electrostatic

spheres. For most laboratory of the beam is small and the

defined. Under these conditions to the beam path is insignificant.

have their

the

analyzer

plates

applications_ the incident direction

curvature in the The only important

are

geometric is well

plane perpendicular resulting differ-

ence is that the focal points of a spherical analyzer are i$0 ° apart; while those of a cylindrical analyzer are 127 ° apart, so that a cylindrical analyzer can be made somewhat smaller. Comparable resolutions can be obtained with the two shapes. plates are somewhat easier to m_chine; ones in general usage.

small

For good compared

desirable;

energy resolution to the radius of

a narrow

entrance

Since; in addition 3 cylindrical cylindrical analyzers are the

an analyzer curvature.

slit

is

requiredj

must have a plate separation Since a small instrument is but

a

large

entrance

slit

j!v

V

1

7< i_

7

area is needed %o collect a detectable particle current. Hence we are led %o a long, narrow sli%_ and must consider in more detail the differences in behavior between spherical and cylindrical electrostatic fields_ in particular for large angles of incidence.


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Figure

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number

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NASA

TN

D-I035

National Aeronautics TWO INSTRUMENTS TIONS OF LOW-ENERGY SPACE.

Michel

and Space Administration. FOR MEASURING DISTRIBUCHARGED PARTICLES

Bader,

Thomas

B.

Fryer,

Fred C. Witteborn. July 1961. 42p. OTS $1.25. (NASA TECHNICAL NOTE D-1035) Some

methods

and instrumentation

IN

I. Bader, Michel H. Fryer, Thomas B. IIL Witteborn, Fred C. IV. NASA TN D-1035

and price,

for measuring

(InitialNASA distribution: 6, Astronomy; 16, Cosmochemistry; 17, Communica-

the

energy and density distribution of low-energy (0 to 20 key) ions in space are considered from the • standpoint of suitability for space vehicle payloads.

tions and sensing equipment, flight; 20, Fluid mechanics; 31, Physics, nuclear and particle. )

It is concluded that electrostatic analysis of the energy distribution can provide sufficient information in

NASA

TN

D-I035

National Aeronautics TWO INSTRUMENTS TIONS OF LOW-ENERGY

and Space Administration. FOR MEASURING DISTRIBUCHARGED PARTICLES

SPACE. Michel Bader, Thomas B. Fryer, Fred C. Witteborn. July 1961. 42p. OTS $1.25. (NASA TECHNICAL NOTE D-1035) Some

methods

and

instrumentation

for

(Initial NASA

structed plasma and data

distribution:

6, Astronomy; 16, Cosmochemistry; 17, Communica-

measuring

the

energy and density distribution of low-energy (0 to 20 kev) ions in space are considered from the • standpoint of suitability for space vehicle payloads. It is concluded that electrostatic distribution can provide sufficient initial experiments. Instruments

initial experiments. Instruments designed and constructed at the Ames Research Center for space plasma measurements, and the methods of calibration and data reduction are described.

IN

and price,

I. Bader, Michel II. Fryer, Thomas B. HI. Witteborn, Fred C. IV. NASA TN D-1035

tions and sensing equipment, flight; 20, Fluid mechanics; 31, Physics, nuclear and particle. )

analysis of the energy information in designed and con-

at the Ames Research measurements, and the reduction are described.

Center methods

for of

space calibration

NASA /

Copies

obtainable

NASA

TN

from

SPACE.

OF

Copies obtainable

D-1035

National Aeronautics TWO INSTRUMENTS TIONS

and Space Administration. FOR MEASURING DISTRIBU-

LOW-ENERGY Michel

CHARGED

Bader,

Thomas

PARTICLES B. Fryer,

Fred C. Witteborn. July 1961. 42p. OTS $1.25. (NASA TECHNICAL NOTE D-1035) Some

methods

and

NASA

NASA, Washington

instrumentation

IN

I. II.

Bader, Fryer,

Michel Thomas

III. IV.

Witteborn, NASA TN

Fred D-1035

NASA B. C.

and price,

for measuring

energy and density distribution of low-energy (0 to 20 kev) ions in space are considered from

the the

• standpoint of suitability for space vehicle payloads. It is concluded that electrostatic analysis of the energy distribution can provide sufficient information in

(Initial NASA

distribution:

6, Astronomy; 16, Cosmochemistry; 17, Communicati'ons and sensing equipment, flight; 20, Fluid mechanics; 31, Physics, nuclear and particle. )

TN

from

NASA, Washington

D-I035

National Aeronautics TWO INSTRUMENTS TIONS OF LOW-ENERGY

and Space AdministratiOn. FOR MEASURING DISTRIBUCHARGED PARTICLES

SPACE. Michel Bader, Thomas B. Fryer, Fred C. Witteborn. July 1961. 42p. OTS $1.25. (NASA TECHNICAL NOTE D-1035) Some methods and instrumentation energy and density distribution (0 to 20 kev) ions in space are

for measuring of low-energy considered from

standpoint of suitability for space It is concluded that electrostatic distribution can provide sufficient

initial experiments. Instruments designed and constructed at the Ames Research Center for space plasma measurements, and the methods of calibration and data reduction are described.

initial structed

experiments. at the Ames

plasma measurements, and data reduction

Instruments Research are

and the described.

obtainable

from

NASA i Washington

Copies

obtainable

from

the the

vehicle payloads. analysis of the energy information in designed Center for methods

NASA Copies

IN

and price,

NASA, Washin_on

and space of

I. II. HI. IV.

Bader, Michel Fryer, Thomas Witteborn, Fred NASA TN D-1035

(Initial NASA 6, Astronomy;

B. C.

distribution: 16, Cosmo-

chemistry; 17, Communications and sensing equipment, flight; 20, Fluid mechanics; 31, Physics, particle. )

nuclear

con-

calibration

NASA

and