An ion source using the cyclotron resonance principle is described. A miniaturized ion source device is used in an air gap of a small permanent magnet with a substantially uniform field in the air gap of about 0.5 inch. The device and permanent magnet are placed in an enclosure which is maintained at a high vacuum (typically 10 to the minus 7th power) into which a sample gas can be introduced. The ion beam end of the device is placed very close to an aperture through which an ion beam can exit into the apparatus for an experiment

Anicich, V. G.

Huntress, W. T., Jr.

English

NASA Technical Reports Server

United States Patent 1191 
Anicich et al. 
1541 MINUTURE CYCLOTRON RESONANCE 
ION SOURCE USING SMALL PERMANFlyT 
MAGNET 
[75] inventors: Vincent C. Anicich, Sunland; Wesley 
T. Huntress, Jr., Sierra Madre, both 
of calif. 
[73] Assignee: Celifornia Institute of Technology, 
Pasadena, Calif. 
[21] Appl. No.: !W,970 
[22] Filed: Sep. 11,1978 
[51] Int. Cl .2  ......................... HOlJ l/sO; HOl3 27/00; 
H05H 1/00 
[52] U.S. c1. .................................... 313/36;2; 250/427; 
313/156; 313/363 
[58] Field of Search ............... 313/362, 156, 363, 359; 
250/427 
r56l Referencea Cited 
U.S. PATENT DOCUMENTS 
2,499,320 2/1950 Loevinger ............................ 3131359 
E1 11 4,206,383 
WI Jun. 3,1980 
2,627,034 1/1953 Washburn et al. .............. 313/362 X 
2,743,370 4/1956 McLaren et al. ................ 313/362 X 
3,281,617 1011966 Wood ............................... 313/363 X 
3,283,193 11,4966 Ellison ................................. 313/362 
3<502,863 3/1970 TSUY~IM et al. .......... 250/427 
3,924,134 12/1975 Uman et d. ..................... 313/362 X 
Ptimary Emminer-Palmer C. Demeo 
Attorney, Agent. or Finn-Freilich, Hornbaker, 
Wassetman, Rosen & Fernandez 
An ion souice using the cyclotron resonance principle is 
provided using a miniaturized ion source device in an 
air gap of a small permanent magnet with a substantially 
uniform field in the air gap of about 0.5 inch. The device 
and permanent magnet are placed in an enclosure which 
is maintained at a high vacuum (typically IO-Ttorr) into 
which a sample gas can be introduced. The ion-beam 
end of the device is placed very close to an aperture 
through which an ion beam can exit into apparatus for 
an experiment. 
[57l ABsTRAcr 
8 Claims, 4 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19800018664 2020-03-21T02:57:09+00:00Z
U.S. Patent Jun. 3, 1980 Sheet 1 of 3 4,286,383 
FIG. I 
-3OOOV 
t 
14 
VACUUM 
PUMP 
.S. Patent Jun. 3, 1980 Sheet 2 of 3 4,206,383 
FIG. 2 
SOURCE REGION 
FIG. 3 
U.S. Patent Jun. 3,1980 Sheet 3 of 3 4,206,383 
UNSHIMMED 
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\ I 
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\ 
/ 
I 
I 
/ 
I 
- 
LENGTH, CITI 
FIG. 4 
4,206,3 83 
1 2 
pump for reducing residual gas impurities to less than 
10-7 torr. 
The novel features that are considered characteristic 
of this invention are set forth with particularity in the 
5 appended claims. The invention will best be understood 
from the following description when read in connection 
with the accompanying 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic diagram of the present inven- 
MINIATURE CYCLOTRON RESONANCE ION 
SOURCE USING SMALL PERMANENT MAGNET 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 1o 
(72 Stat. 435; U.S.C. 2457). tion. 
BACKGROUND OF THE INVENTION FIG. 2 is a perspective schematic diagram illustrating 
functional operation of a miniaturized ion source device 
in the present invention shown in FIG. 1. 
FIG. 3 is an end view of the device in FIG. 2 with 
arrows to show the direction of the magnetic and elec- 
tric 
FIG. 4 is a side view of the device in FIG. 2 with 
20 provided in accordance with the present invention as 
compared with the typical magnetic field of the un- 
shimmed maknet. 
DESCRIPTION OF PREFERRED 
EMBODIMENTS 
This invention relates to an ion cyclotron 
mass spectrometer and more particularly to an im- 15 
proved cyclotron resonance ion source. 
spectrometer has come 
into widespread use for the study of ion-molecule reac- 
tiom. with that has considemble development in 
For a comprehensive review of developments, see J. 
Beauchamp, “Ion Cyclotron Resonance Spectroscopy” 
Annual Review of Physical Chemistry, Vol. 22, pp 
527-561 (1971). There the general basis for ion cyclo- 
tron resonance spectrometry is succinctly stated to be 25 
motion of a free charged particle in a uniform magnetic 
field, H. The motion is constrained to a circular orbit of 
angular frequency, W,, in a plane normal to H and is 
unrestricted parallel to H. When an alternating field at 
radio frequency is applied normal to H, absorption of 3o 
energy by the ions can be observed as a decrease in total 
ion current or as a direct power absorption when using 
a marginal oscillator detector. 
One problem, to which the present invention is ad- 
dressed is coupling the ion cyclotron resonance experi- 35 
ment to a non-magnetic experiment or an experiment 
using a different magnetic field. Prior art cyclotron 
resonance mass spectrometers were relatively large, 
heavy and expensive. Typically, the air gap in the mag- 
net is approximately two inches, thus requiring a large 40 
electromagnet consuming a relatively large amount of 
electric power. The problems of such a large electro- 
magnet are then compounded by the difficulty of cou- 
pling the ion source in such a large electromagnet to 
any ion optics, primarily because of the large volume 45 
magnetic field. The large volume has the disadvantage 
of larger residual magnetic fields at several inches dis- 
tance from the cyclotron experiment. 
The ion cyclotron 
and techniques for u s  in such a spectrometer. dotted line CWVes to Show the Shaped magnetic field 
Referring now to FIG. 1 of the drawings, a miniatur- 
ized ion source device 10 is positioned in the air gap of 
a C-shaped magnet comprised of a yoke 11 and ceramic 
(rare earth) magnets 12 and 13 in a vacuum enclosure 14 
of monmagnetic material having an inlet 16 with a con- 
trol valve 18 and an outlet 20, which may also have a 
control valve and which is connected to a high vacuum. 
In operation as a mass spectrometer, a gas to be ana- 
lyzed is allowed to flow through the valve 18 into the 
enclosure 14. Most of the gas then passes out through 
the outlet 20. 
The vacuum enclosure is maintained at an equilib- 
rium pressure of the added gas which is ionized by an 
electron beam in the presence of the magnetic field H of 
substantially uniform flux density as will be described 
more fully with reference to FIG. 4. Ions are formed by 
impact of electrons generated from a hot filament 22 
(FIG. 2) so that motion of the free ions in a plane per- 
pendicular to the field PI is constrained to a circular 
path with the cyclotron resonance frequency 
W,=qh‘/mr, 
where q is the charge of the gas particle, H the magnetic 
50 field strength, m the mass of the gas particle, and c the 
sneed of light. The result is an ion beam which exits the 
OBJECTS AND OF THE 
INVENTION 
-r--- - - ~ 
An object of this invention is to provide an ion source ion Source device 10 and passes directly into ion optics 
that is capable of being coupled directly from an ion 24 (FIG. 1) through an aperture 26 in a plate 26a in the 
beam port of the source into ion optics. enclosure 14. This aperture is electrically isolated from 
These and other objects of the invention are achieved 55 the enclosure, which is maintained at 0 volts, because 
by using a permanent magnet with a narrow gap (ap- the device 10 and magnet 11 can be biased from ground 
proximately 0.5 inches) and a field shaped to be of ap- to several thousand volts, using a ring 27 of dielectric 
proximately uniform strength across a predetermined material around the aperture plate separating it from the 
length to the edges of the gap. A cyclotron resonance enclosure. The ion optics focuses the ion beam into a 
ion source is then placed in the narrow gap with its 60 utilization device 28, such as a mass spectrometer. Ion 
output beam exiting at one end of the gap and its sample beam energies are variable from a thermal velocity 
gas entering generally at the other edge of the shaped distribution to several thousand electron volts. 
magnetic field. An enclosure for the permanent magnet The ion optics and utilization device are conventional 
and ion source is provided with an aperture at the edge in design and operation except that the ion optics are 
of the shaped magnetic field so closely spaced that the 65 placed next to the enclosure directly over the aperture 
ion beam exiting the ion source passes directly into ion 26 for the ion beam. The ion source device 10 is also 
optics. The enclosure is provided with a gas inlet con- somewhat conventional in design and operation, but in 
trol valve at one end and a gas outlet to a high vacuum accordance with the present invention, the ion source 
3 
4,206.383 
device is made two orders of magnitude smaller in vol- 
ume and weight than prior art ion source device using 
the cyclotron resonance principle. Typically, the air 
gap in the magnet required by the prior art devices is 
approximately 2 inches, requiring a large electromagnet 
consuming a relatively large amount of electric power, 
and greatly complicating the apparatus required to cou- 
ple the ion beam to ion optics for the utilization device. 
The ion source device of the present invention is minia- 
turized to a degree that reduces the air gap in the mag- 
net to about 0.5 inches, or less, so that a small permanent 
magnet can be used in place of the large electromagnet. 
Referring to FIG. 2, the miniaturized ion source in- 
cludes six non-magnetic plates, side plates 31-34, a front 
end plate Bo, and the aperture plate 26 (not shown in 
FIG. 2 but shown in FIGS. 1 and 4). These plates are 
electrically isolated from each other as schematically 
indicated in the end view of FIG. 3, and in FIG. 4, by 
dielectric material 266. 
The vertical side plates, 31 and 32, have aligned holes 
35 and 36 for electrons to pass from the hot filament 22 
to an electron collector 37. The collector is maintained 
at a positive potential, such as 20 V, while the filament 
is maintained at a negative potential (0 to -90 V). This 
potential gradient between the filament and collector 
produces a beam of electrons that impact the molecules 
of gas between the vertical plates, thereby ionizing the 
molecules. The vertical side plates are maintained at a 
very low positive potential (0 to 2 V) in order to collect 
electrons not passing through the holes 35 and 36 and 
trap the ions formed. The upper side plate 33 is main- 
tained at a very low positive potential (0 to 2 V) while 
a low voltage AC signal is applied to the lower side 
plate 34 using a bias resistor 38 connected to a very low 
negative potential (0 to 2 V). The AC signal is at some 
radio frequency, typically 307 kHz. 
Positioned over the input end of the ion source device 
is an end plate Bo comprised of a screen in a frame 
maintained at zero volts. Its function is to allow union- 
ized molecules to enter the device 10 and to prevent 
ionized molecules from exiting the device except 
through the output end. In that regard it should be 
noted that although not shown, the space between the 
plates along their edge is sealed with dielectric material, 
such as an epoxy, which also serves to support the 
plates in position relative to each other. 
The motion of the ions formed by electron impact in 
4 
in FIG. 4 over the length of the device 10. The shape of 
the magnetic field over the width of the device is simi- 
larly shaped by this shimming arrangement. Other shim- 
ming arrangements for so shaping the magnetic field 
5 will, of course, occur to others skilled in the art. 
In summary of operation, the miniature ion source 10 
of FIG. 1 uses the cyclotron resonance principle, as in 
the prior art, but uses a permanent uniform magnetic 
field. Relatively low energy ions are produced by 
10 means of electron impact (bombardment with electrons 
generated by a hot filament) in the device chamber. The 
uniform magnetic field causes the ions to move in a 
circular motion (due to the cyclotron principle), and an 
electrostatic field which causes them to exit the cham- 
15 ber as an ion beam. For use as a mass spectrometer, the 
AC field frequency is scanned by the irradiating oscilla- 
tor input E+ It can also be used for studying ion- 
molecule reactions by ion cyclotron resonance methods 
by using the device as an ion source for either a quad- 
20 rapole mass spectrometer or magnetic sector mass spec- 
trometer. In this way the ion source device can also 
serve as a low pressure chemical ionization source. 
What has been disclosed is a small, low-cost ion 
source using the cyclotron resonance principle. Such a 
25 source may be used in a variety of ways, with or with- 
out ion optics. Examples of application include a con- 
ventional mass spectrometer having either a quadrapole 
or magnetic sector, and as a low pressure chemical 
ionization. Consequently, although an exemplary em- 
30 bodiment of the invention has been described and illus- 
trated herein, it is recognized that modifications may 
readily occur to those skilled in the art, particularly as 
to use. Consequently, it is intended that the claims be 
interpreted to cover such modifications and equivalents. 
1. An ion beam source using the cyclotron resonance 
principle comprising an ion source device, said device 
having four electrically isolated electrode plates defin- 
ing a channel having one screened end through which 
40 sample gas molecules enter, and an open end opposite 
said one end through which a beam of ionized mole- 
cules exit, said device including electron bombardment 
means for ionizing said gas molecules as they enter said 
channel at said one end, and means for forming said ion 
45 beam in accordance with said cyclotron resonance prin- 
ciple, said means comprising a magnet having a small 
gap between pole faces extending over the length of 
35 What is claimed is: 
the magnetic field is constrained to a circular path of ;aid device, said magnet being made to provide; sub- 
angular frequency equal to the cyclotron resonance stantially uniform flux density throughout substantially 
frequency Wc, which is independent of the velocity of 50 the full length of said device, with a rapid fall off of 
the ions in a direction perpendicular to the magnetic magnetic flux density at the exit edge of said device, and 
field. The electric field of the AC signal applied perpen- an enclosure for said magnet and ion source device, said 
dicular to the magnetic field can be controlled in ampli- enclosure having an inlet for said gas, an outlet for said 
tude to produce a linear mass spectrum by maintaining gas, and an aperture for said beam out of said device, 
the magnetic field constant and scanning the AC fre- 55 said open end of said device through which said beam 
quency. For that purpose, the permanent magnet is exits being placed directly in front of said aperture 
shimmed (shaped) to produce a uniform field over the whereby said ion beam exiting said device passes di- 
length of the ion source device that is shown in FIG. 4. rectly out of said enclosure. 
A typical unshimmed field is also shown in FIG. 4 for 2. An ion source device for providing ions to an appa- 
comparison. The shims required are shown in FIG. 4 as 60 ratus adjacent said device using the cyclotron reso- 
soft iron caps, 42 and 44 which fit between the magnets nance principle, said device having four electrically 
12 and 13 and the upper and lower plates 33 and 34 of isolated electrode plates defining a channel, said chan- 
the ion source device, each with a hole over about f its nel being open at one end to receive sample gas mole- 
length and width. The substantially rectangular hole is cules, and open at the other end opposite said one end 
provided with rounded comers so that the hole is sub- 65 for providing an ion beam out of said channel into said 
stantially rounded at the ends. The particular shape of apparatus, an improvement comprising magnetic means 
the holes in the shims is selected to provide an airgap at for providing a substantially uniform flux density the 
the central region to reduce the magnetic field as shown full length of said device from said one end to said 
4.206,383 I- 
S 
opposite end, with a rapid fall offof magnetic flux den- 
sity at the ends of said device, means for electron bom- 
bardment of said sample gas in order to ionize said 
sample gas, and an enclosure for said ion source device 
and said magnetic means into which a sample gas can be 
introduced, said enclosure having a wall against said 
device at said other open end with an aperture in said 
wall opposite the channel opening at said other open 
end for said ion beam to pass into said apparatus for an 
experiment. 
3. The combination of claim 2 wherein said means for 
providing a substantially uniform field is comprised of a 
C-shaped permanent magnet having opposing pole 
faces extending just over the length of said device with 
magnetic shims between said pole faces and said device 
to shape the magnetic field to be substantially uniform 
over the length of said device and with rapid fall off at 
the ends of said device. 
4. The combination of claim 3 wherein the gap be- 
tween said pole faces is about 0.5 inches and the length 
of ion source device is about three times the gap. 
5. The combination of claim 4 wherein said ion source 
device is comprised of said four electrically isolated 
electrode plates arranged to provide a rectangular sam- 
6 
which causes them io exit through said other end as an 
ion beam. 
6. The combination of claim 4 wherein said C-shaped 
magnet is comprised of a soft iron yoke with a wide gap 
5 and two ceramic magnet blocks, one on each side of 
said wide gap with shims to close the gap over said ion 
source device and provide said uniform magnetic field. 
7. A miniature cyclotron resonance ion beam source 
having an ion source device with an input end and an 
ion exit end, said device comprising a permanent mag- 
net having a small gap of about 0.5 inches and a substan- 
tially uniform magnetic flux density over the length of 
said gap with rapid drop off of magnetic flux density at 
the ion exit end of said ion source device, a sample gas 
Is enclosure having a wall with an aperture over Said exit 
end at a point before said magnetic flux drops off, and 
means for electron bombardment of gas molecules en- 
tering said device at said input end to ionize gas mole- 
cules entering at said input end, and means for produc- 
ing a d o r m  electrostatic field across the length of said 
device to cause ions, which move in a circular motion in 
accordance with the cyclotron principle, to exit said 
device and enclosure through said awrture. 
ple gas opening at 
said other end, means for electron ~mbardment  of 
sample gas molecules passing through said one end, 
thereby to ionize molecules which move in a circular 
motion in accordance with said cyclotron principle, and 
means for applying voltages to said plates for producing 30 said uniform magnetic field. 
along the length of said device an electrostatic field 
one end and an ion beam e& at 25 8. An ion Wurce device Gdeked-in claim 7 wherein 
said pemment magnet is comprised of a soft iron C- 
shaped yoke with a wide gap and two ceramic magnet 
blocfcs, one on each side of said wide gap with shims to 
close the gap over said ion source device and provide 
* * I * *  
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Miniature cyclotron resonance ion source using small permanent magnet

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