High vacuum cleaning of contaminated surfaces such as hydrocarbon containment films can be accomplished by a plasma cleaning device which includes a plasma discharge housing to permit generation of a plasma in an environment having a higher pressure than the surface which is to be cleaned. A ground electrode and a radio frequency electrode partially surround a quartz plasma tube, for the introduction of an ionizable gas. These electrodes ionize the gas and help generate the plasma. This plasma flows through a non-constrictive aperture, through the plasma discharge housing and then on to the contaminated surface

Gillette, R. B.

Shannon, R. L.

English

NASA Technical Reports Server

United States Patent 
Fletcher et al. 
4,888,926 
[451 May 9,1978 
[54] PLASMA CLEANING DEVICE 
[76] Inventors: James C. Fletcher, administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Roger L. Shannon, 
Federal Way; Roger B. Gillette, 
Auburn, both of Wash. 
[21] Appl. No.: 684,807 
[22] Filed: May 10, 1976 
1511 Int. Cl.2 .............................................. HOlJ 15/02 
L - 4  
[52] U.S. Cl. ................................. 315/111.2; 29/81 C; 
3 13/23 1.3 
[58] Field of Search ............................. 315/111, 11 1.2; 
313/231, 231.3; 29/81 C 
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,209,189 9/1965 Patrick .......................... 315A11.2 X 
3,437,864 4/1969 Kofoid et al. ................. 313/231.3 X 
Primary Examiner-Eugene R. LaRoche 
Attorney, Agent, or Firm-George J. Porter; John R. 
Manning; L. D. Wofford, Jr. 
[571 ABSTRACT 
Apparatus for cleaning contaminated surfaces such as 
hydro-carbon contaminant films in high vacuum envi- 
ronments including a plasma discharge housing for 
allowing a plasma to be generated in an environment 
having a higher pressure than the surface which is to be 
cleaned. A ground electrode and a radio frequency 
electrode partially surround a quartz plasma tube, for 
the introduction of an ionizable gas therein. These elec- 
trodes ionize the gas and help generate the plasma. This 
plasma flows through a non-constrictive aperture, 
through the plasma discharge housing and then on to 
the contaminated surface. 
7 Claims, 2 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19780019970 2020-03-20T13:37:44+00:00Z
USPatent May 9, 1978 4,088,926 
FIG. I .  
FIG. 2 .  
4,088,926 
1 
PLASMA CLEANING DEVICE 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 5 
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 
(72 Stat. 435; 42 U.S.C. 2457). 
FIELD OF THE INVENTION 
The present invention relates to a cleaning device 
utilizing a plasma and has particular application in 
cleaning contaminated surfaces contained in a high 
vacuum (low pressure) environment, such as optical 15 
surfaces in space, thereby restoring the reflectance of 
both radiation-damaged white paint and organic-fdm- 
contaminated telescope mirror surfaces. 
10 
BACKGROUND OF THE INVENTION 2o 
Existing plasma cleaning devices depend primarily on 
the production of a plasma in a gas environment sur- 
rounding the contaminated surface. The contaminant is 
then removed as the active plasma species collide with 
the surface. At low gas Dressures Cless than about 25 
torr), either the gas-dksity is too low for a plasma 
discharge to be established, or the plasma density is too 
low to produce significant contaminant cleaning. Con- 
sequently, existing plasma cleaning devices are not op- 
erable at high vacuum conditions, since these devices 
must produce the plasma in the high vacuum environ- 
ment. In the context of this application, the terms “high 
vacuum environment” and “low pressure environment” 
are interchangeable. 
A solution to this problem is to generate the plasma in 
a confined higher pressure region and allow it to flow 
into the high vacuum area. Prior attempts to implement 
this solution utilized a flow restriction to separate the 
high and low pressure regions. These attempts failed 
because the active plasma species were destroyed dur- 
ing passage through the flow restriction. The present 
invention implements a solution to this problem by 
generating the plasma at the point immediately before 
the confined gas enters the high vacuum region. 
U.S. Pat. No. 3,264,508 issued to Lai et a1 shows a 
plasma torch used for cutting, welding or for flame 
spraying tasks involving fine or delicate work which 
generates a hot plasma in a manner similar to the present 
invention. It is significant to note that all these opera- 
tions are performed in regions of normal atmospheric 
pressures and not in a high vacuum environment. The 
torch contains a source of highly ionizable gas which is 
passed through a tube surrounded by a wire coil con- 
nected to a radio frequency generator. When the power 
is applied to this r.f. generator, the gas within the region 
of the coil ionizes and is then passed through a constric- 
tive opening before leaving the tube. 
Other representative patents relating to plasma gener- 
ators are U.S. Pat. No. 3,139,509 issued to Browning; 
U.S. Pat. No. 3,192,427 issued to Sugawara et al; and 
U.S. Pat. No. 3,903,891 issued to Brayshaw. 
SUMMARY OF THE INVENTION 
30 
35 
40 
45 
_ _  
2 
the environment directly surrounding the contaminated 
surface have been eliminated by generating the plasma 
in a higher pressure environment than the contaminated 
surface and then ejecting the plasma into the high vac- 
uum region through a non-constrictive aperture. Addi- 
tionally, the present invention utilizes cold, low inten- 
sity plasma ensuring that the surface being cleaned is 
not damaged. 
Very low gas flow rates‘are needed to meet the high 
vacuum requirements for vacuum chamber applications 
because of pumping limitations. Low flow rates are also 
called for in space applications to conserve gas. The 
high vacuum condition requires either the plasma dis- 
charge to occur at low pressure or to have a flow re- 
striction between the discharge region and the high 
vacuum region. The earlier development attempts using 
the flow restriction technique have been found to be 
inadequate. The plasma was destroyed in passage 
through the the restriction by collisions with the wall. 
This plasma attenuation depends on the size of the re- 
striction and the mean free path of the plasma species. 
As the mean free path length approaches the diameter 
of the restriction, most of the plasma particles will col- 
lide with the wall before passing through the restriction. 
The importance of pressure in this process is seen from 
the following order of magnitude table of mean free 
path length versus pressure. 
PRESSURE (TORR) MEAN FREE PATH (CM) 
loo0 
100 
10 
1 
lo-’ 
10-2 
10-3 
10-4 
10-5 
10-6 
10-5 
10-4 
10-3 
10-2 
10-1 
1 
10 
Id 
At low gas flow rates the pressure at the downstream 
end of the restriction approaches that of the surround- 
ing vacuum and the gas molecules have many wall 
collisions before entering the vacuum. The present de- 
vice generates the plasma at relatively low pressure by 
using an axial r.f. field so as to reduce wall effects which 
limit the establishment of a discharge. By generating 
this plasma near the downstream end of the plasma tube, 
a significant portion of the plasma species enter the 
vacuum without suffering wall collisions. 
According to the present invention, high vacuum 
50 cleansing can be accomplished by a plasma cleaning 
device containing a plasma discharge housing separat- 
ing a plasma generating mechanism contained in the 
interior of the housing from the high vacuum enviorn- 
ment. The plasma generating mechanism is constituted 
55 by a plasma tube connected to a source of ionizable gas. 
A pair of electrodes partially surrounding the tube is 
connected to a r.f. generator creating an axial r.f. field in 
the region between the electrodes and thereby generat- 
ing a plasma therebetween. This plasma then flows 
60 through a non-constrictive aperture into the high vac- 
uum region and then onto the contaminated surface. In 
this manner, no contact is ever made between the elec- 
trodes and the plasma thereby preventing contamina- 
It is the primary object of the present invention to tion of the contaminated surface. 
BRIEF DESCRIPTION O F  THE DRAWING 
The above and additional objects and advantages 
inherent in the present invention will become more 
overcome the defects of the prior art by producing a 65 
plasma cleaning device capable of adequately cleaning a 
contaminated surface present in a high vacuum region. 
The problems associated with producing the plasma in 
4,088,926 
3 
a high temperature resistant dielectric material such as 
quartz, effectively separating the interior of the quartz 
tube 18 from the high vacuum environment 30 contain- 
ing a contaminated surface 46 which must be cleansed. 
The interior of the shield 18 contains a substantially 
cylindrical inner quartz conduit or tube 16 having an 
admitting end section 48 in direct communication with 
a source 12 of ionizable gas such as argon or oxygen, an 
emitting section 50 and a nozzle portion 44. The tube 16 
is partially encircled by a cylindrically-shaped ground 
electrode lead 20, a ring-shaped upstream ground elec- 
trode 52 and a downstream ring-shaped radio frequency 
electrode 22 connected to a radio frequency generator 
32 by means of electrical leads 34 and 36, respectively, 
said leads being insulated by lengths of quartz tubing 
while within the tube assembly. The leads are con- 
nected to the plasma tube by standard coaxial cables. 
The upstream electrode 52 is brazed to lead 20 which 
also acts as a shield for the upstream gas. Both elec- 
trodes 22 and 52, as well as lead 20 can be constituted of 
a copper material. Since tuning the r.f. power supply for 
proper operation may require observation of the dis- 
charge, light pipes (not shown) can be included for 
transmitting light from the discharge end of the device 
to the outside of the plasma tube assembly. 
The end wall 38 of the plasma cleaning device 10 
contains an aperture 40 which has the same or greater 
cross-sectional area and diameter as the inner tube 16 in 
the section between the ground electrode 20 and the 
end wall 38, including nozzle portion 44. This nozzle 
portion 44 of the tube 16 and end wall 38 is a single 
integral unit, the nozzle portion 44 terminating at aper- 
ture 40 thereby allowing the plasma generated therein 
to be directed into the high vacuum region 30 contain- 
ing the contaminated surface 46. 
In operation, the gas 14 flows through the inner tube 
16 toward the nozzle area 44. When power is applied to 
the r.f. generator 32, the gas contained between the 
electrodes 20 and 22 is ionized and a plasma is generated 
in regions 24-26, allowing a visible, conical plume 28 to 
flow into the high vacuum region 30. To provide 
plasma to the high vacuum environment, the discharge 
must occur close to the plasma tube exit 26 as well as in 
region 24. If the discharge occurs only in region 24, 
then the plasma is attenuated by wall collision effects 
before reaching the plasma tube exit. 
The production of the plasma plume 28 requires a 
proper combination of inner tube diameter, gas flow 
rate, radio frequency electrode geometry, radio fre- 
quency and input power. Although the exact values are 
not crucial, it has been found that a very good plume of 
apparent by reference to the description of an illustrated 
embodiment of the drawing thereof in which: 
FIG. 1 is a partially sectional view of an embodiment 
of the plasma cleaning device according to the present 
invention; and 5 
FIG. 2 is an end view of the present invention show- 
ing the non-constrictive aperture. 
DETAILED DESCRIPTION OF THE 
PREFERRED EMBODIMENT 
The plasma discharge device 10 illustrated by FIGS. 
1 and 2 comprises a generally cylindrical and substan- 
tially hollow conductive shield or tube 18 constituted of 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
4 
trode 22 and the end 38 of the shield tube 18 can be 
varied between 0 and 3 cm. It was also found that the 
r.f. frequency can be varied between 50 and 200 MHz 
with the r.f. power input kept below 50 watts. Elec- 
trodes 52 and 22 are adjustably mounted upon the hol- 
low conduit 16 for facilitating the production of an 
optimum performance. 
While this device has been described with particular 
reference to the figures, it should not be construed to be 
limited to exactly to what is shown in these drawings or 
described in the specification. It will be obvious for 
those who possess ordinary skill in the art to make 
changes and modifications to this device without de- 
parting from the scope of the invention. 
What is claimed is: 
1. A method for cleaning contaminated surfaces in a 
high vacuum environment using a cold plasma dis- 
charge apparatus having an outer shield separating the 
interior of the discharge apparatus from the high vac- 
uum environment, said method comprising the steps of: 
introducing an ionizable gas into a conduit disposed 
within the outer shield of the plasma discharge 
apparatus, said conduit partially encircled by two 
radio frequency electrodes; 
generating a radio frequency field within the conduit 
thereby ionizing the gas contained therein to create 
a cold plasma within said conduit between said first 
and second electrodes as well as between a non- 
constrictive nozzle and said second electrode; 
ejecting the plasma at a low flow rate from the 
plasma discharge apparatus through said non-con- 
strictive nozzle and aperture into the high vacuum 
environment while directing the plasma toward the 
contaminated surface. 
2. A plasma cleaning device for cleaning contami- 
nated surfaces present in a high vacuum environment 
comprising: 
a substantially hollow housing member having an 
outer wall, and an end wall containing an aperture, 
said outer and end walls separating a higher pres- 
sure region within the interior of said housing from 
the high vacuum environment outside of said hous- 
ing; 
a hollow conduit, having an admitting end section 
and an emitting end section disposed within said 
housing member; 
supply means connected to the admitting end section 
of said conduit for supplying highly ionizable gas 
therethrough; 
first and second electrodes, each electrode partially 
encircling said conduit, said first electrode interme- 
diate of said gas supply means and said second 
electrode; 
a non-constrictive nozzle connected to the emitting 
end section of said conduit and disposed adjacent 
to said aperture; and 
a radio frequency generator connected to said first 
and second electrodes generating a cold plasma 
between said electrodes as well as between said 
second electrode and said non-constrictive nozzle, 
said plasma flowing into the high vacuum environ- 
ment through said non-constrictive nozzle means 
and said aperture. 
3. A plasma cleaning device in accordance with claim 
cold plasma can be produced using a 4 mm inner diame- 65 2 wherein said first and second electrodes are adjustably 
ter tube 16 at gas flow rates of 0.05-5.0 STD cc/minute. mounted upon said hollow circuit. 
The spacing between the electrodes 52 and 22 can be 4. A plasma cleaning device in accordance with claim 
varied from 2-5 cm, and the spacing between r.f. elec- 2 where said nozzle means, the section of said conduit 
4,088,926 
5 6 
between said fmt electrode and said end wall, and said 
aperture all have the w e  cross-sectional area. 
4 wherein said fust electrode is a ground electrode and 
said second electrode is an r.f. electrode. 
with claim 
5 wherein the spacing between said f j t  and second 
electrodes is in the range of 2-5 cm, the spacing be- 
tween said second electrode and the end wall of said 
housing member is in the range of 0-3 cm, the gas flow 
rate in the range of 0.05-5.0 STD cc/minute, the r.f. 
duit has an diameter of -. 
7. A plasma cleaning device in accordance with claim 
6 wherein the power Of SBme radio frequency generator 
is less than 50 watts. 
5. A plasma device in accordance with frequency is in the range of 50-200 MHz, and said con- 
5 
6, A plasma device in 
* * * * *  
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Plasma cleaning device

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