STUDY OF A PLASMA-FILLED X-BAND BACKWARD-WAVE OSCILLATOR

We present experimental studies of a plasma-filled X-band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement of a factor of 7 in the total microwave power emission. The bandwidth of ·the microwave emission increased from <;0.5 to 2 GHz when the BWO was working at the rf power enhancement pressure region. The rf power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) as compared to a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output · compared to the vacuum BWO over a broader region of magnetic guide field strength.

(Received 17 December 1991; accepted for publication 2 March 1992) We present experimental studies of a plasma-filled X-band backward wave oscillator (BWO). Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement of a factor of 7 in the total microwave power emission. The bandwidth of · the microwave emission increased from <;0.5 to 2 GHz when the BWO was working at the rf power enhancement pressure region. The rf power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) as compared to a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output · compared to the vacuum BWO over a broader region of magnetic guide field strength.
Vacuum_slow wave devices have been attractive highpower microwave sources since the early 70s. Introduction of plasma into a vacuum microwave device can have several beneficial effects. Background plasma can neutralize beam charge, allowing beam propagation in the slow wave device well above the space-charge limited current, yielding higher power microwave output. Plasma can also enhance the interaction efficiency, by allowing greater bunching through neutralization of space-charge effects. The first plasma-filled backward wave oscil1ator (BWO) demonstrated a factor of 3 in microwave power enhancement over it's vacuum counterpart. 1 Recent experiments 2 have achieved a factor of 8. These plasma-filled BWO experiments concentrated on producing high-power microwaves. In a recent theoretical description of the plasma-filled BW0 3 it was shown that the addition of plasma results in a frequency upshift in the output of the BWO, the amount of shift depending on the plasma density, nP. An upshift up to 2.5 GHz was predicted for a plasma density of 8X 10 11 cm-3 for the BWO geometry considered. However, the calculation gave no information about how bandwidth changes with plasma density. High-power, frequency tunable and high-power, broadb'and microwave devices have some useful applications. The goal of this work was to study the effect background plasma had on the microwave frequency and bandwidth of the output radiation in a highpower plasma filled BWO.
Our experimental setup (Fig. 1) uses a hollow electron beam (650 kV, 1.5-3 kA, 500 ns pulse duration) with l.8 cm outer diameter, and 0.2 cm thickness, produced by field emission from a graphite cathode. Beam current and voltage were monitored near the cathode, with down stream current measured by a shunt resistor. Plasma was produced by electron beam impact ionization of background neutral helium gas which filled the BWO and the diode. To measure the plasma density, we used a 9.6 GHz X-band magnetron as the rf source. In order to avoid strong Xband TMo 1 radiation and to not modify the oscillator, we replaced our BWO with a stainless-steel tube of 10 mm radius, keeping anode and cathode geometry the same. Since the X-band radiation consistently arrived 140 ns ( ± 10 ns) into the beam pulse, we could correlate the microwave signal with the plasma density measurement at the turn on of the microwaves. The magnetron rf was fed through the plasma by X-band waveguide, carrying information about np(t) in its phase change ll<f>(t). The probe rf was then mixed with rf from a IocaJ oscillator so that a mixed frequency of -100 MHz was obtained.
6.cp(t) occurs in the frequency difference f M(t) = f magnetron -I oscillator as a frequency change 6.f.'oef(t). 6..cp(t) can be obtained by comparing this new frequency to the upsbifted mixed frequency f MO=f M(t<to.to is the beam start time), Mo· With no neutral gas· in the system we detected no shift in the frequency diffe rence A/ MO• for up to 3 µs. The beam voltage pulse was synchronized with the stable portion of the mixed frequency.
The BWO is a cylindrical waveguide with a periodically varying wall radius, R(z), sinusoidally rippled about the mean radius, R 0 , such that R(z) =Ro+hcos. (koZ) and k 0 =21Tlzo, where h=0.45 cm is the ripple amplitude, z 0 = 1.67 cm is the period, and Ro= l.45 cm. This is a copy of the system described in Ref. 3. The BWO was immersed in a 6-18 kG guiding magnetic field. An X-band horn placed 2 meters from the end of the drift tube received the microwave emission from the BWO. The microwaves were guided through 20 meters of X-band waveguide into a screen room. Here the microwaves were measured by a crystal detector and an  X-band 8 channel microwave spectrometer, covering 8.2 GHz <f < 12.4 GHz. Each channel had a bandwidth of approximately 0.5 GHz. The spectrometer filters had 50 dB stop-band insertion loss with 0.9 dB passband insertion loss. Figure 2 (a) shows the beam voltage and the 8 channel X-band microwave spectrometer signals from the vacuum BWO for 3 kA beam current. Microwave output was always in the first channel of the spectrometer ( 8.2 to 8. 725 GHz,8.46 GHz center frequency). Maximum power emission was about 80 to 100 MW with microwave pulse duration of 50 ns (FWHM). This is a very short pulse, considering our beam pulse was 500 ns. However, when the BWO operated at lower level powers, the microwave emission lasted longer. For example, microwave radiation with power output 30-40 MW lasted between 130 to l 50 ns. As the background neutral helium gas pressure increased in the BWO, we observed both a microwave frequency upshift and a bandwidth increase. At low gas pressure (under 10 mTorr) and beam current of 3 kA only channel 1 and 2 of the spectrometer detected micro.wave signal. However, in the pressure region where the enhancement in rf power was observed (between 50 to 80 mTorr), signals appeared in the first four channels of the spectrometer. It is interesting to note that the signal in channel 1 was never less than the vacuum BWO output level, and actually its output was the largest among all channels when the background helium pressure was below 25 mTorr. When the helium pressure was between 25 and 50 mTorr, rf signals appeared in the first three channels of the spectrometer, and the signal amplitude in the second and third channel increased to the same level as the first channel. At the pressure where maximum power was obtained. the third channel detected the largest signal. This indicted a frequency upshift of 1 GHz. Figure 2(b) shows a shot taken at 60 mTorr with 3 kA beam current. If we sum the signal output from all channels of the spectrometer in Fig. 2(b) and compare to the signal in Fig. 2(a), it is more than 7 times larger. Comparison also indicated the bandwidth of the microwave emission increased from <0.5 GHz in vacuum to 2 GHz. We observed similar frequency upshift and bandwidth in- crease for the 1.6 kA beam case as well. Microwave emission bandwidth as a function of the background helium pressure is shown in Fig. 3. The percentage band width l:::.flfo changed from .;;;;5% in vacuum to 25% at the pressure which gave maximum power enhancement.
When 3 kA of beam current was injected into the helium filled BWO, we observed a microwave power enhancement for helium pressure between 10 to 100 mTorr. The maximum microwave power emission appeared at 60 mTorr. When the beam current was reduced to 1.6 kA, the microwave power enhancement began at -80 mTorr, peaked at 110 mTorr, and then declined. The plasma filled BWO microwave power output data is shown in Fig. 4. The output is the summation of all the signals from channel 1 to channel 4 of the spectrometer (frequency range from 8.