A source of hydrogen ions is disclosed and includes a chamber having at one end a cathode which provides electrons and through which hydrogen gas flows into the chamber. Screen and accelerator grids are provided at the other end of the chamber. A baffle plate is disposed between the cathode and the grids and a cylindrical baffle is disposed coaxially with the cathode at the one end of the chamber. The cylindrical baffle is of greater diameter than the baffle plate to provide discharge impedance and also to protect the cathode from ion flux. An anode electrode draws the electrons away from the cathode. The hollow cathode includes a tubular insert of tungsten impregnated with a low work function material to provide ample electrons. A heater is provided around the hollow cathode to initiate electron emission from the low work function material

Mirtich, M. J., Jr.

Roman, R. F.

Sovey, J. S.

English

NASA Technical Reports Server

United States Patent [191 
Mirtich, Jr. et al. 
[111 4,218,633 
[45] Aug. 19, 1980 
[54] HYDROGEN HOLLOW CATHODE ION 
[75] Inventors: Michael J. Mirtich, Jr., N. Olmsted; 
James S. Sovey, Strongsville; Robert 
F. Roman, Brookpark, all of Ohio 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
SOURCE 
[73] Assignee: The United States of America as 
[21] Appl. No.: 953,391 
[22] Filed: Oct. 23,1978 
[51] Int. ( 3 . 2  .............................................. HOlJ 17/26 
[52] U.S. C1. .................................. 313/362; 313/231.4 
[58] Field of Search ..................... 313/231.4, 362, 361, 
3 13/359 
1561 References Cited 
U.S. PATENT DOCUMENTS 
3,113,427 12/1963 Meyer .................................. 313/359 
3,238,715 3/1966 Reader et al. ................... 313/361 X 
3,345,820 10/1967 Mask  .............................. 313/362 X 
3,728,861 4/1973 Pawlik et al. .................... 313/204 X 
Primary Examiner-Robert Segal 
Attorney, Agent, or Firm-Norman T. Musial; John R. 
Manning; James A. Mackin 
P I  ABSTRACT 
A source of hydrogen ions is disclosed and includes a 
chamber having at one end a cathode which provides 
electrons and through which hydrogen gas flows into 
the chamber. Screen and accelerator grids are provided 
at the other end of the chamber. A baffle plate is dis- 
posed between the cathode and the grids and a cylindri- 
cal baffle is disposed coaxially with the cathode at the 
one end of the chamber. The cylindrical baffle is of 
greater diameter than the baffle plate to provide dis- 
charge impedance and also to protect the cathode from 
ion flux. An anode electrode draws the electrons away 
from the cathode. 
The hollow cathode includes a tubular insert of tung- 
sten impregnated with a low work function material to 
provide ample electrons. A heater is provided around 
the hollow cathode to initiate electron emission from 
the low work function material. 
6 Claims, 2 Drawing Figures 
f 25 
14 
https://ntrs.nasa.gov/search.jsp?R=19800024678 2020-03-21T03:15:38+00:00Z
U.S. Patent Aug. 19, 1980 4,218,633 
I 
18 
-27 
- 29 
FIG. 2 
4,218,633 
2 
HYDROGEN HOLLOW CATHODE ION SOURCE 
ORIGIN OF THE INVENTION 
This invention was made by employees of the United 
States Government and may be manufactured and used 
by or for the Government without the payment of any 
royalties thereon or therefor. EMBODIMENT 
DESCRIPT1oN OF THE 
FIG. 1 is a half axial section of a hydrogen ion source 
FIG. 2 is an axial section of the cathode of FIG. 1 
built in accordance with the invention. 
showing the construction thereof in greater detail. 
5 
DESCRIPTION OF A PREFERRED 
BACKGROUND OF THE INVENTION 10 Referring to FIG. 1 there is shown a hydrogen ion 
source 10 comprised of a cylindrical shell 11 having an 
annular magnetic pole piece 12 at one end and a second 
annular pole piece 13 at the other end. A back plate 14 
1. Field of the Invention 
This invention relates to ion sources and is directed 
more Particularly to a high current density ion SXrce of stainless steel or the like is attached to the pole piece 
suitable for heating plasmas in controlled thermonu- 15 12. 
clear reaction experiments. Ion source modules which 
produce ion beams at 40 kv and ion current densities of 
At the downstream end of ion 10 there is 
0.5A/cm2 have advanced to a rather sophisticated state 
of development. However, in the future, fusion test 
reactors will require multimegawatt neutral beams, 
each comprising a multiplicity of ion source modules. 
These modules will be exposed to the radioactive envi- 
ronment of the fusion reactor. Thus, highly reliable ion 
sources operating at high duty cycles will be required in 
order to expedite and simplify remote maintenance, 
handling and adjustment. In order to satisfy the neutral 
injection requirements for high beam currents, dis- 
charge currents ranging from 10 to 40 times the beam 
current will be required. 
2. Prior Art 
Prior art ion sources for neutral injection achieve the 
necessary discharge current by employing a relatively 
large number of filament cathodes which operate at 
temperatures approaching 3000" K. The lift of such 
cathodes is limited by high thermal stress, sublimation 
and ion sputtering. Furthermore, heat radiated from the 
filament may cause undesirable thermal loads on critical 
components such as the ion optics. 
Hollow cathodes have been used for ion thrusters and 
in inert gas ion sources. 
OBJECTS AND SUMMARY OF THE 
INVENTION 
It is an object of the invention to provide a high 
current density ion source having greatly extended life 
as compared to the prior art. 
It is another object of the invention to provide an ion 
source which operates at high temperatures with no 
liquid cooling required. 
Still another object of the invention is to provide an 
ion source which provides the high current density 
beam of hydrogen or deuterium. 
It is yet another object of the invention to provide an 
ion source having baffles which reduce heating of the 
cathode resulting from ion bombardment. 
An additional object of the invention is to provide an 
ion source wherein the geometry of the baffles coacts 
with the magnetic field and the flow rate of the hydro- 
gen or deuterium to control the discharge electrical 
impedance. 
In summary, the invention provides a high current 
density hydrogen ion source utilizing a hollow cathode 
and suitably al;mged baffles to reduce self heating of 
the cathode and to effect the electron and ion flow in a 
manner such as to desirably aid in controlling discharge 
impedance. 
provided a screen grid 15 and an accelerator grid 16 
insulated from one another by a spacer 17 which is 
preferably mica. The grids 15 and 16 together with the 
2o mica spacer 17 are supported on the annular pole piece 
13. 
A hollow cathode 18 through which hydrogen gas is 
injected into the ion source is disposed in an aperture in 
the back plate 14. 
An igniter electrode 19 also extends through the back 
plate 14 adjacent the hollow cathode 18. When pulses of 
high voltage are applied between the cathode 18 and 
the igniter 19, an electrical breakdown occurs between 
3o electrodes 18 and 19 and subsequently a discharge is 
established between cathode 18 and cylindrical anode 
20. The anode 20 is at a positive voltage with respect to 
the cathode 18 and is disposed radially inwardly of the 
shell 11. The hydrogen atoms which lose electrons thus 
To the end that ions produced as just described will 
be directed through the grids 15 and 16 with maximum 
efficiency and to the end that the ion source will have a 
useful life many times greater than those of the prior art, 
4o a baffle plate or disc 21 is disposed on the axis 22 of the 
ion source 10 and a cylindrical ring 23 coaxial with the 
axis 22 is attached to the back plate 14. The inside diam- 
eter of cylinder 23 is substantially greater than the diam- 
eter of baffle 21 leaving an annular space between them. 
45 The baffle 21 and 23 serve to prevent ions from bom- 
barding the cathode 18 which would seriously shorten 
its life and, further, affect the discharge impedance of 
the ion source as will be discussed presently. 
At the downstream end of the ion source 10 an annu- 
50 lar ring 24 extends radially inwardly from pole piece 13 
to serve as an ion optic mask. This mask adjusts the 
diameter of the ion beam. 
In order to enhance ion utilization efficiency of the 
hydrogen ion source 10, a plurality of elongated elec- 
55 tromagnets 25 are disposed outwardly of shell 11 ex- 
tending between the pole pieces 12 and 13. These elec- 
tromagnets with the pole pieces 12 and 13 produce a 
magnetic field inside the cylindrical shell 11. 
Referring now to FIG. 2, there is shown the hollow 
60 cathode 18 of FIG. 1 in more detail. As shown, hollow 
cathode 18 comprises a cathode tube 26 having at one 
end an annular radiation flange 27. Disposed coaxially 
in tube 26 is a porous tungsten tube 28. An orifice plate 
29 is nested in radiation flange 21. The space between 
65 the tube 28 and the tube 26 is filled with an electrically 
conductive, refractory material as, for example, molyb- 
denum foil to position the tube 28 and to provide good 
electrical conductivity between tubes 26 and 28. 
25 
35 become positive charged ions. 
4,218,633 
3 4 
The tungsten tube 28 is impregnated with a low work 
function material to provide electrons when cathode 18 
is heated. Barium calcium aluminate is an example of 
one suitable low work function material. 
While the ion source 10 as described utilizes hydro- 
gen to generate hydrogen ions, deuterium can a h  be 
wd. 
It will be understood that changes and modifications 
To heat the cathode to provide electrons for starting 5 may be made to the above-described ion source without 
the hydrogen ion source, a swaged heater 31 i s  disposed departing from the spirit and scope Of the invention as 
set forth in the claims appended hereto. 
What is claimed is: around the outside of cathode tube 26 extending from radiation flange 27 to the end Of tube 28 Opposite the 1. In a hydrogen ion of the type comprising a orifice plate* To 
tube 28, radiation shields are placed outwardly of the 
heater. These shields are preferably tantalum foil al- 
An arrow 33 indicates the flow of hydrogen gas into 
heat the porous tungsten 10 chamber with an electron emitting cathode at the closed 
end thereof, an accelerator grid at the other end and an 
anode between the cathode and the accelerator grid, a 
though other refractory metals could be used. screen grid being disposed between the anode and the 
accelerator grid, the improvement comprising: 
the hollow cathode 18. The gas will exit through the 15 a cylindrical metal baffle disposed coaxially around 
orifice plate 29 into the ion source 10. said cathode abutting said closed end; 
Referring back again to FIG. 1, the shell 11, the a baffle disc positioned on the axis of said cathode, 
anode 20 and the pole pieces 12 and 13 are covered with said disc being of smaller diameter than said cylin- 
refractory metal foils which serve as radiation shields drical baffle, said disc being centered in said cylin- 
not shown. This construction allows the magnets to 20 drical baffle at the end opposite said closed end of 
operate temperatures much lower than the anode or the ion source; 
shell whose temperatures exceed 1m" C. during con- said cathode comprising a porous tungsten tube im- 
tinuous operation. The geometry of the baffles 21 and pregnated with a low work function material and 
having an orifice plate at one end inwardly of the 23, the flow rate of the hydrogen through the hollow closed end of said ion source; cathode 18 and the magnetic field strength in the cham- means for heating said tungsten tube to cause electron 
emission comprising heating coils swaged onto the ber coact to determine the discharge electrical impe- 
outside of said cathode tube and a plurality of iso- dance of the ion source. 
lated layers of refractory metal foil disposed The shell 11 with the cathode 18 and screen grid 15 
operate at high positive potential. Anode 20 is electri- 30 around the outside of said heating coils to effii- 
cally positive with respect to cathode 18 so that elec- ciently heat the porous tube; and wherein hydro- 
trons emitted from low work function material impreg- gen flows from a hydrogen supply through said 
nated into the tube 28 of cathode 18 will be drawn along cathode into said ion source; 
with electrons torn away from the hydrogen atoms a set of electromagnets to adjust discharge impedance 
flowing into the ion source through cathode 18 to the 35 and ionization efficiency and means for ionizing 
anode 20. The accelerator grid 16 is operated at nega- hydrogen flowing into said hydrogen ion source 
tive potential to extract the beam and repel electrons in through said cathode. 
the exhaust. For ion energies exceeding about 10 Kev, 2. The ion source of claim 1 wherein said baffle disc 
staged ion optics consisting of 3 or grids would is from about 50% to 75% the diameter of said cylindri- 
generally be employed. 
is evacuated and the 3. The ion source of claim 1 wherein said tungsten 
cathode 18 is heated by the swaged heater 31 shown in tube is inside a refractory metal cathode tube with a 
layer of electrically conductive refractory metal there- FIG. 2. Hydrogen gas is then directed into the ion between, said cathode tube having an annular radiation Source as at  33 in 
which has been impregnated with a work function ma- 4. The ion wherein said low work 
terial such as barium calcium aluminate. Positive volt- function material is barium calcium aluminate. 
age pulses are applied between the igniter 19 and the 5. The ion Source of claim 1 wherein said cylindrical 
cathode 18 causing ionization of the hydrogen as flow- baffle, said baffle disc and said anode are of refractory 
ing through hollow cathode 18 and subsequent dis- 50 metal and are each thermally isolated from the electro- 
charge ignition between cathode 18 and anode 20. The magnets by a plurality of layers of radiation shielding. 
ions produced by the discharge drift to the screen grid 6.  The ion source of claim 1 wherein said radiation 
and then are accelerated by the screen grid 15 and the shielding is by separated layers of tantalum foil. 
accelerator grid 16. * * + * *  
25 
40 cal baffle. 
In operation, the ion SOUCe 
and passes through tube 28 45 flange at one end and surrounding said orifice plate. 
of claim 
55 
60 
65 


Hydrogen hollow cathode ion source

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