A cathode constructed of a thin, directly heated strip of LaB_6 is described. The cathode is simple to construct, requires modest heating power, has high current emission capability and is quite rugged. Construction details will be given and cathode performance data presented. The cathode has been used in tokamak dc current injection experiments

Bellan, P. M.

Brown, M. R.

Siegrist, K.

English

Caltech Authors - Main

Downloaded 11 Dec 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp
Simple, high current LaB6 cathode 
K. Siegrist, M. R. Brown, and P.M. Bel!an 
California Institute ofTechnology, Pasadena, California 91125 
(Received 29 August 1988; accepted for publication 17 January 1989) 
A cathode constructed of a thin, directly heated strip ofLaB6 is described. The cathode is simple 
to construct, requires modest heating power, has high current emission capability and is quite 
rugged. Construction details will be given and cathode performance data presented. The cathode 
has been used in tokamak de current injection experiments. 
Recently, there has been much interest in the development of 
LaB6 cathodes due to their high current emission capability. 
LaB6, however, is fragile and highly susceptible to fracture 
under the stress of thermal cycling. Previous cathode de-
signs1--3 have circumvented this problem by indirectly heat-
ing the cathode, which requires high input power and a com-
plex configuration, or by directly heating specially designed 
LaB6 filaments. Neither method has achieved the high cath-
ode temperatures necessary for efficient, high current den-
sity emission. This paper describes a simple, inexpensive, 
extremely high current density LaB6 cathode which requires 
comparatively low power and is impervious to thermal cy-
cling. The maximum current emitted by the cathode was 17 5 
A, for an average current density of 140 A/cm2 , with input 
power of only 250 W. The essential design elements which 
enabled high current output with low power input were the 
use of very thin directly heated filaments with filament 
holders also made ofLaB6 which eliminated fracture during 
thermal cycling. 
The cathode, shown schematically in Fig. 1, was built 
directly on a vacuum flange with high current feedthroughs 
(Ceramaseal804B2779-0l-W, 150 A, 12 kV). Boron nitride 
was painted on the feed through base to prevent arcing, while 
the use of graphite rods prevented surface contamination of 
the filament. Parallel slots were filed in the rods to accom-
modate LaB6 filament holders which were secured with 
shims ofO.l-mm graphite sheet. 
Both holders and filaments were cut and ground to size 
by a local petrographics firm from LaB6 disks. Holders with 
less than a 3-mm-square contact area did not provide enough 
surface area for good electrical contact. Filaments were 
about 25 mm long, 3 mm wide and 0.4-0.5 mm thick. Some 
were ground to be 0.02-0.07 mm thinner at the ends to give 
more uniform heating across the filament. Shims made from 
thin slices ofLaB6 etched in a nitric acid solution secured the 
filament in the holder. When the electrical contact was suffi-
cient for high-temperature operation, the filament was ob-
served to heat from the middle rather than from the contact 
points. Poor electrical contact resulted in overheating and 
melting at the point of contact. 
The filaments were heated directly with ac power by 
means of a 5-V 115-A transformer controlled with a variac. 
The feedthrough temperature was never more than 100 •c 
indicating localized heating at the filament. The filament 
was typically operated in argon with the chamber function-
ing as the anode. Due to power supply limitations, the dis-
charge voltage was pulsed and we were unable to run de 
discharges. 
A number of experimental tests of the cathode have been 
performed in a large (25 t) vacuum vessel. The cathode has 
been operated continuously in 150 mTorr of argon for over 
60 hat 1800 oc (measured with an optical pyrometer) with 
pulsed emission current of 125 A ( 1% duty cycle), corre-
sponding to 100 A/cm2• The discharge current was mea-
sured with a transformer-type current monitor. The input 
heating power was 240 Wand discharge voltage 140 V for 
this experiment. The higher sublimation rate at the central 
(hottest) part of the filament and attendant local increase in 
heating ultimately caused the filament to melt there. Thus, 
filament lifetime has been determined to be a strong function 
of operating temperature but we were unable to ascertain the 
effects of de ion bombardment on filament lifetime. 
In Fig. 2, (a) typical current and (b) discharge voltage 
pulses are shown. The current pulse was flat and noise-free 
up to the highest emission obtained. A 1 00-A discharge from 
the cathode in 1 SO mTorr of argon produced a plasma with a 
space potential of =30 V, density= 1010 cm- 3, and Te =3 
eV, as measured by a Langmuir probe 20 em from the cath-
ode in the afterglow plasma. We can estimate the plasma 
density at the cathode sheath from the relations4 J, ~ (11.11 I 
LAB6 CATHODE 
SIDE-VIEW: 
LAB6 FILAMENT HOLDER 
10mm x 6.4mm x 3mm 
FIG. 1. Schematic drawing of LaB,. cathode. The LaB, filament holder 
serves as a thermal anchor eliminating fracture during thermal cycling. 
964 Rev. Sci. lnstrurn. 60 (5}, May 1989 0034-6748/89/050964-02$01.30 @ 1989 American Institute of Physics 964 
Downloaded 11 Dec 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp
(a) 
1:1'1 
~100 
<t 
0 
(b) 
1:1'1 
-0100 
> 
50 
0 20 40 80 100 
( 1-L sec} 
FIG. 2. Typical traces of (a) emitted current and (b) applied bias voltage for 
a 60-ps shot. 
me) l/ 2 J; and J; G?!.nec_., where J; is the ion current to the 
cathode and c, is the ion sound speed. For a discharge cur-
rent of J'" = 100 A/cm2 we find ll= wn em 3. Evidently, 
dense plasma produced at the cathode expands to fill the 
large vacuum vessel. 
A plot of emitted current versus heating power is shown 
in Fig. 3 for discharge voltages of 75-140 V. Note that we 
are evidently operating in a thermally limited (as opposed to 
current limited) regime. Experiments performed at fixed 
voltage showed that saturated emission current (i.e., the 
emission level at which current no longer increases with ei-
ther discharge voltage or pressure) followed the Richardson 
equation for temperature limited current emission, 
J = AT2e- eNki; where A = 120 A/cml T 2 for LaB6 • The 
effective work function, ¢ (determined from a best fit to our 
data), was 2.86 eV which is consistent with Lafferty's5 ac-
cepted value of2.67 eV. 
The LaB6 cathode has been used to inject a small toroi-
dal current into Caltech's Encore tokamak [similar to ex-
periments performed on CDX (Ref. 6)}. When the cathode 
was operated at a fill pressure such that the emitted current 
was maximized (about 100 mTorr), then the current chan-
nel was dissipated by collisions with neutrals before it made a 
single toroidal transit. At lower fill pressures ( -10- 4 
Torr), the cathode current was about 20 A and the current 
channel made five toroidal passes for a total plasma current 
of 100 A. If high levels of emission at low tokamak fill pres-
965 Rev. Sci. lnstrum., Vol. 60, No.5, May 1989 
100 200 
Power (Watts) 
300 
FIG. 3. Plot of emitted current as a function of heating power for two bias 
voltages (75 and 140 V). Note that the cathode is thermally limited over 
most of the operational range. Current limited operation is only evident at 
relatively high powers. 
sures ( -w-·1 Torr) are to be obtained, then a hollow tube 
cathode configuration will be necessary. For that experi-
ment, the cathode is encased in a housing with a gas inlet and 
small aperture to the main chamber. Goebel, Crow, and For-
rester1 have found that a large pressure differential can be 
maintained between a small aperture hollow tube cathode 
and the exterior chamber. This should allow the cathode to 
be maintained at the high-pressure necessary for high-cur-
rent emission, while the chamber remains at the desired low-
er pressure. In this way, the cathode might be efficiently used 
to inject large toroidal currents. 
This work was performed under DOE Grant No. DE-
FG03-86ER53232. One of us (KS) was supported by Cal-
tech's Summer Undergraduate Research Fellowship Pro-
gram. M.R.B. is a U.S. DOE Fusion Energy Postdoctoral 
Research Fellow. 
1D. M. Goebel, J. T. Crow, and A. T. Forrester, Rev. Sci. Instrum. 49,469 
( 1978). 
~D. Mo Goebel, Y. Hirooka, and T. A. Sketchley, Rev. Sci. Instrum. 56, 
1717 (1985). 
'K. N. Leung, P. A. Pincosy, and K. W. Ehlers, Rev. Sci. Instrum. 55, 1064 
(1984). 
4F. W. Crawford and A. B .Cannara, J. Appl. Phys. 36, 3!35 ( !965). 
'J. Mo Lafferty, J. Appl. Phys. 22,299 (l95t). 
'M. Ono, G. J. Greene, D. Darrow, C. Forest, H. Park, and T. H. Stix. 
Phys. Rev. Lett. 59,2165 (1987). 
Notes 965 


Simple, high current LaB_6 cathode

Downloaded 11 Dec 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp
Simple, high current LaB6 cathode 
K. Siegrist, M. R. Brown, and P.M. Bel!an 
California Institute ofTechnology, Pasadena, California 91125 
(Received 29 August 1988; accepted for publication 17 January 1989) 
A cathode constructed of a thin, directly heated strip ofLaB6 is described. The cathode is simple 
to construct, requires modest heating power, has high current emission capability and is quite 
rugged. Construction details will be given and cathode performance data presented. The cathode 
has been used in tokamak de current injection experiments. 
Recently, there has been much interest in the development of 
LaB6 cathodes due to their high current emission capability. 
LaB6, however, is fragile and highly susceptible to fracture 
under the stress of thermal cycling. Previous cathode de-
signs1--3 have circumvented this problem by indirectly heat-
ing the cathode, which requires high input power and a com-
plex configuration, or by directly heating specially designed 
LaB6 filaments. Neither method has achieved the high cath-
ode temperatures necessary for efficient, high current den-
sity emission. This paper describes a simple, inexpensive, 
extremely high current density LaB6 cathode which requires 
comparatively low power and is impervious to thermal cy-
cling. The maximum current emitted by the cathode was 17 5 
A, for an average current density of 140 A/cm2 , with input 
power of only 250 W. The essential design elements which 
enabled high current output with low power input were the 
use of very thin directly heated filaments with filament 
holders also made ofLaB6 which eliminated fracture during 
thermal cycling. 
The cathode, shown schematically in Fig. 1, was built 
directly on a vacuum flange with high current feedthroughs 
(Ceramaseal804B2779-0l-W, 150 A, 12 kV). Boron nitride 
was painted on the feed through base to prevent arcing, while 
the use of graphite rods prevented surface contamination of 
the filament. Parallel slots were filed in the rods to accom-
modate LaB6 filament holders which were secured with 
shims ofO.l-mm graphite sheet. 
Both holders and filaments were cut and ground to size 
by a local petrographics firm from LaB6 disks. Holders with 
less than a 3-mm-square contact area did not provide enough 
surface area for good electrical contact. Filaments were 
about 25 mm long, 3 mm wide and 0.4-0.5 mm thick. Some 
were ground to be 0.02-0.07 mm thinner at the ends to give 
more uniform heating across the filament. Shims made from 
thin slices ofLaB6 etched in a nitric acid solution secured the 
filament in the holder. When the electrical contact was suffi-
cient for high-temperature operation, the filament was ob-
served to heat from the middle rather than from the contact 
points. Poor electrical contact resulted in overheating and 
melting at the point of contact. 
The filaments were heated directly with ac power by 
means of a 5-V 115-A transformer controlled with a variac. 
The feedthrough temperature was never more than 100 •c 
indicating localized heating at the filament. The filament 
was typically operated in argon with the chamber function-
ing as the anode. Due to power supply limitations, the dis-
charge voltage was pulsed and we were unable to run de 
discharges. 
A number of experimental tests of the cathode have been 
performed in a large (25 t) vacuum vessel. The cathode has 
been operated continuously in 150 mTorr of argon for over 
60 hat 1800 oc (measured with an optical pyrometer) with 
pulsed emission current of 125 A ( 1% duty cycle), corre-
sponding to 100 A/cm2• The discharge current was mea-
sured with a transformer-type current monitor. The input 
heating power was 240 Wand discharge voltage 140 V for 
this experiment. The higher sublimation rate at the central 
(hottest) part of the filament and attendant local increase in 
heating ultimately caused the filament to melt there. Thus, 
filament lifetime has been determined to be a strong function 
of operating temperature but we were unable to ascertain the 
effects of de ion bombardment on filament lifetime. 
In Fig. 2, (a) typical current and (b) discharge voltage 
pulses are shown. The current pulse was flat and noise-free 
up to the highest emission obtained. A 1 00-A discharge from 
the cathode in 1 SO mTorr of argon produced a plasma with a 
space potential of =30 V, density= 1010 cm- 3, and Te =3 
eV, as measured by a Langmuir probe 20 em from the cath-
ode in the afterglow plasma. We can estimate the plasma 
density at the cathode sheath from the relations4 J, ~ (11.11 I 
LAB6 CATHODE 
SIDE-VIEW: 
LAB6 FILAMENT HOLDER 
10mm x 6.4mm x 3mm 
FIG. 1. Schematic drawing of LaB,. cathode. The LaB, filament holder 
serves as a thermal anchor eliminating fracture during thermal cycling. 
964 Rev. Sci. lnstrurn. 60 (5}, May 1989 0034-6748/89/050964-02$01.30 @ 1989 American Institute of Physics 964 
Downloaded 11 Dec 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp
(a) 
1:1'1 
~100 
<t 
0 
(b) 
1:1'1 -0100 > 
50 
0 20 40 80 100 
( 1-L sec} 
FIG. 2. Typical traces of (a) emitted current and (b) applied bias voltage for 
a 60-ps shot. 
me) l/ 2 J; and J; G?!.nec_., where J; is the ion current to the 
cathode and c, is the ion sound speed. For a discharge cur-
rent of J'" = 100 A/cm2 we find ll= wn em 3. Evidently, 
dense plasma produced at the cathode expands to fill the 
large vacuum vessel. 
A plot of emitted current versus heating power is shown 
in Fig. 3 for discharge voltages of 75-140 V. Note that we 
are evidently operating in a thermally limited (as opposed to 
current limited) regime. Experiments performed at fixed 
voltage showed that saturated emission current (i.e., the 
emission level at which current no longer increases with ei-
ther discharge voltage or pressure) followed the Richardson 
equation for temperature limited current emission, 
J = AT2e- eNki; where A = 120 A/cml T 2 for LaB6 • The 
effective work function, ¢ (determined from a best fit to our 
data), was 2.86 eV which is consistent with Lafferty's5 ac-
cepted value of2.67 eV. 
The LaB6 cathode has been used to inject a small toroi-
dal current into Caltech's Encore tokamak [similar to ex-
periments performed on CDX (Ref. 6)}. When the cathode 
was operated at a fill pressure such that the emitted current 
was maximized (about 100 mTorr), then the current chan-
nel was dissipated by collisions with neutrals before it made a 
single toroidal transit. At lower fill pressures ( -10- 4 
Torr), the cathode current was about 20 A and the current 
channel made five toroidal passes for a total plasma current 
of 100 A. If high levels of emission at low tokamak fill pres-
965 Rev. Sci. lnstrum., Vol. 60, No.5, May 1989 
100 200 
Power (Watts) 
300 
FIG. 3. Plot of emitted current as a function of heating power for two bias 
voltages (75 and 140 V). Note that the cathode is thermally limited over 
most of the operational range. Current limited operation is only evident at 
relatively high powers. 
sures ( -w-·1 Torr) are to be obtained, then a hollow tube 
cathode configuration will be necessary. For that experi-
ment, the cathode is encased in a housing with a gas inlet and 
small aperture to the main chamber. Goebel, Crow, and For-
rester1 have found that a large pressure differential can be 
maintained between a small aperture hollow tube cathode 
and the exterior chamber. This should allow the cathode to 
be maintained at the high-pressure necessary for high-cur-
rent emission, while the chamber remains at the desired low-
er pressure. In this way, the cathode might be efficiently used 
to inject large toroidal currents. 
This work was performed under DOE Grant No. DE-
FG03-86ER53232. One of us (KS) was supported by Cal-
tech's Summer Undergraduate Research Fellowship Pro-
gram. M.R.B. is a U.S. DOE Fusion Energy Postdoctoral 
Research Fellow. 
1D. M. Goebel, J. T. Crow, and A. T. Forrester, Rev. Sci. Instrum. 49,469 
( 1978). 
~D. Mo Goebel, Y. Hirooka, and T. A. Sketchley, Rev. Sci. Instrum. 56, 
1717 (1985). 
'K. N. Leung, P. A. Pincosy, and K. W. Ehlers, Rev. Sci. Instrum. 55, 1064 
(1984). 
4F. W. Crawford and A. B .Cannara, J. Appl. Phys. 36, 3!35 ( !965). 
'J. Mo Lafferty, J. Appl. Phys. 22,299 (l95t). 
'M. Ono, G. J. Greene, D. Darrow, C. Forest, H. Park, and T. H. Stix. 
Phys. Rev. Lett. 59,2165 (1987). 
Notes 965 


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Simple, high current LaB_6 cathode

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