High-power rf generation in a hollow-cathode - Physics@Technion

High-power radio frequency generation in a hollow-cathode discharge
D. Arbel, Z. Bar-Lev, J. Felsteiner, a) A. Rosenberg, and Ya. Z. Slutsker
Physics Department, Technion-Israel Institute of Technology, 32000 Haifa, Israel
�Received 23 September 1994; accepted for publication 10 January 1995�
High-power rf oscillations are obtained in a resistive load installed in the external electrical circuit
of a low-pressure magnetic-field-free hollow-cathode discharge. These intense rf oscillations have
been shown to be due to a collisionless instability of the cathode sheath. A very simple device based
on this effect is shown to serve as a convenient and powerful rf oscillator. Typical results first
achieved without any optimization, for frequencies of 20–30 MHz and load resistances of several
tens of ohms, are as follows: power of about 5 kW and efficiency �25%. © 1995 American
Institute of Physics.
We have obtained high rf power in the resistive load of a
low-pressure hollow-cathode discharge. The rf oscillations in
the discharge current have been shown to be due to a
cathode-sheath instability in the discharge. 1 This instability
causes current oscillations with a frequency slightly lower
than the ion plasma frequency near the cathode-sheath edge.
The instability appears when, for a certain cathode-fall voltage,
the time taken for the ions to cross the sheath is approximately
equal to the period of the rf oscillations. A similar
instability is known for the electrons crossing the sheath near
a positively charged electrode. 2,3 In this letter we present
experimental results showing that a device based on this phenomenon
can serve as a powerful rf oscillator.
The experimental apparatus is shown in Fig. 1. We used
a discharge tube composed of an open-ended cylindrical
cathode, 10 cm long, having an inside diameter of 8.6 or 6.2
cm. The anode shape, its diameter, its location, or its material
had no influence on the experimental results. Typically, the
anode was located at one of the ends of the cathode cylinder.
The cathode and anode were mounted inside a glass vacuum
vessel filled with He. The operating pressure P had no significant
influence on the experimental results provided that
PD�3.0 Torr cm �here, D is the cylinder diameter�. 1 Typical
operating pressures were 0.1–0.2 Torr. The discharge was
driven by a pulse generator with a pulse duration of 2 �s.
The discharge current studied was in the range 5–50 A. The
oscillations in the current appeared when the discharge current
was above 6 A. In order to separate the average and rf
components of the discharge current a simple LC filter was
used. A variable load resistor R was placed in series with the
capacitor C �see Fig. 1�. The cutoff frequency of the filter
was chosen to be �5 MHz, so that it was transparent to the
2 �s pulse. The rf oscillations �which were above 20 MHz�
were coupled through the capacitor C to the load R. The rf
current through R was measured by a Rogovsky coil. A typical
scope trace of the rf current is shown in the insert to Fig.
1. The voltage across the tube was measured using a resistive
divider. In Fig. 2 we show the dependence of the tube voltage
V a on the average discharge current I a . It is seen that
V a grows up monotonically with I a . For the range of I a
between 5 and 50 A, the voltage V a varied just between 350
a� Electronic mail: phr01ya@technion.technion.ac.il
and 600 V. The voltages measured for different tube diameters
and pressures were approximately the same.
The dependence of the amplitude of the rf current I R
through the load on the discharge current I a for two different
cathode diameters and two different values of R are shown in
Fig. 3. It is seen that I R grows up monotonically with I a and
under certain conditions, namely a low value of R and a large
cathode diameter, I R becomes closer to I a . When R is
shorted, I R becomes comparable to I a , which is the same as
the situation described in Ref. 1. In Fig. 4 we show the
dependence of I R on the load resistance R for particular values
of I a and cathode diameter. It is seen that I a decreases
with increasing R. For different values of I a and cathode
diameters we obtained a similar behavior. From the data in
Fig. 4 it is easy to estimate the internal resistance R i of the
present hollow-cathode rf generator device. We find
R i�120 � for I a�38 A and 8.6 cm cathode diameter. The
value of R i does not depend significantly on either I a or the
cathode diameter. For example, for I a�15 A and 6.2 cm
cathode diameter we get R i�100 �.
It has been shown 1 that the oscillation frequency increases
either when the discharge current goes up or when
the cathode diameter is reduced. We note that the value of
the load resistance had no effect on the oscillation frequency.
However, for a high value of the load resistance a slight
increase in the discharge current was needed to start the os-
FIG. 1. Experimental apparatus. �1� Cathode cylinder with 8.6 or 6.2 cm
diameter; �2� anode; �3� 2 �H inductor; �4� 660 pF capacitor; �5� load
resistor; �6� Rogovsky coil; �7� typical scope trace of the rf current passing
through the load resistor �sensitivity 5 A/div, time base 40 ns/div�.
Appl. Phys. Lett. 66 (10), 6 March 1995 0003-6951/95/66(10)/1193/3/$6.00 © 1995 American Institute of Physics
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FIG. 2. Dependence of the tube voltage on the average discharge current.
FIG. 3. Dependence of the peak rf current through the load on the average
discharge current: �1� for 8.6 cm tube, 23 � load; �2� for 8.6 cm tube, 82 �
load; �3� for 6.2 cm tube, 23 � load.
FIG. 4. Dependence of the peak rf current through the load on the load
resistance for 8.6 cm tube and 38 A discharge current.
FIG. 5. Dependence of the rf power in the load �a� and of the efficiency �b�
on the average discharge current: �1� for 8.6 cm tube, 82 � load; �2� for 8.6
cm tube, 23 � load; and �3� for 6.2 cm tube, 23 � load.
FIG. 6. Dependence of the rf power in the load �a� and of the efficiency �b�
on the load resistance for 8.6 cm tube and 38 A discharge current.
1194 Appl. Phys. Lett., Vol. 66, No. 10, 6 March 1995 Arbel et al.
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cillations. For example, when R�80 �, the oscillations
started from Ia�10 A, compared to Ia�6 A for R�10 �.
Knowing the peak rf current IR through the load and its
resistance R, it is easy to calculate the output rf power; WR �I R
2 R/2. A graph of WR as a function of I a is shown in Fig.
5�a� for two different cathode diameters and two different
values of R. As it is seen, the rf power grows up monotonically
for both diameters and for both values of R. From Fig.
2, for each value of I a one can obtain the dc power W
�V aI a which drives the discharge. The ratio W R /W, which
is the efficiency of this device as an rf generator, is shown in
Fig. 5�b� as a function of I a for the same values of R and
diameters. It is seen that this ratio decreases as I a grows up.
Figs. 6�a� and 6�b� show, respectively, the output rf power
and the efficiency versus the load resistance, for certain values
of discharge current and cathode diameter. It is seen that
both the rf power and the efficiency are maximal at about
120 �. Similar curves, with a maximum at about the same
place, were obtained for different values of discharge current
in the investigated range of 10–45 A. Note that the maximal
power occurs when the value of the load resistance R is close
to that of the internal rf resistance R i .
To conclude, the present very simple device can be used
as a new kind of a pulsed rf power generator, with an output
power of 4 –5 kW and efficiency of about 25%. This efficiency
is comparable to that of conventional rf power generators.
The present results have been obtained without optimization
of the device so more efficiency and power can, in
fact, be expected. The advantages of this device are simplicity
and very low cost. Finally, we also note that this device
has an instantaneous starting which is an advantage over
conventional powerful hot-cathode electron tubes.
This research was supported in part by Technion V. P. R.
Fund-Harry Werksman Research Fund.
1
D. Arbel, Z. Bar-Lev, J. Felsteiner, A. Rosenberg, and Ya. Z. Slutsker,
Phys. Rev. Lett. 71, 2919 �1993�.
2
R. L. Stenzel, Phys. Rev. Lett. 60, 704 �1988�; Phys. Fluids B 1, 2273
�1989�.
3
Yu. Ya. Brodskii, S. I. Nechuev, Ya. Z. Slutsker, A. M. Feigin, and G. M.
Fraiman, Fiz. Plazmy 15, 1187 �1989� �Sov. J. Plasma Phys. 15, 688
�1989��.
Appl. Phys. Lett., Vol. 66, No. 10, 6 March 1995 Arbel et al.
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