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Effect of non-uniform slow wave structure in a relativistic backward wave oscillator with a resonant reflector

Physics of Plasma

November 2013
20(11):3113-

DOI:10.1063/1.4835335

Authors:

Renzhen Xiao

Northwest Institute of Nuclear Technology, Xi'an, China

Jun Sun

Tsinghua University

Show all 8 authorsHide

This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in a relativistic backward wave oscillator (RBWO) with a resonant reflector. Compared with the uniform SWS, the reflection coefficient of the non-uniform SWS is higher, leading to a lower modulating electric field in the resonant reflector and a larger distance to maximize the modulation current. Moreover, for both types of RBWOs, stronger standing-wave field takes place at the rear part of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO strongly depends on transit time effects. Thus, the matching condition between the distributions of harmonic current and standing wave field provides a profound influence on the beam-wave interaction. In the non-uniform RBWO, the region with a stronger standing wave field corresponds to a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been obtained, compared to that of 3 GW in the uniform RBWO.

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Effect of non-uniform slow wave structure in a relativistic backward wave oscillator

with a resonant reflector

Changhua Chen, Renzhen Xiao, Jun Sun, Zhimin Song, Shaofei Huo, Xianchen Bai, Yanchao Shi, and Guozhi

Liu

Citation: Physics of Plasmas (1994-present) 20, 113113 (2013); doi: 10.1063/1.4835335

View online: http://dx.doi.org/10.1063/1.4835335

View Table of Contents: http://scitation.aip.org/content/aip/journal/pop/20/11?ver=pdfcov

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Effect of non-uniform slow wave structure in a relativistic backward wave

oscillator with a resonant reflector

Changhua Chen, Renzhen Xiao, Jun Sun, Zhimin Song, Shaofei Huo, Xianchen Bai,

Yanchao Shi, and Guozhi Liu

Science and Technology on High Power Microwave Laboratory, Northwest Institute of Nuclear Technology,

Xi’an 710024, China

(Received 8 October 2013; accepted 13 November 2013; published online 26 November 2013)

This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in

a relativistic backward wave oscillator (RBWO) with a resonant reﬂector. Compared with the

uniform SWS, the reﬂection coefﬁcient of the non-uniform SWS is higher, leading to a lower

modulating electric ﬁeld in the resonant reﬂector and a larger distance to maximize the modulation

current. Moreover, for both types of RBWOs, stronger standing-wave ﬁeld takes place at the rear part

of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO

strongly depends on transit time effects. Thus, the matching condition between the distributions of

harmonic current and standing wave ﬁeld provides a profound inﬂuence on the beam-wave

interaction. In the non-uniform RBWO, the region with a stronger standing wave ﬁeld corresponds to

a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a

diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been

obtained, compared to that of 3 GW in the uniform RBWO. V

C2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4835335]

I. INTRODUCTION

The relativistic backward wave oscillator (RBWO) is

one of the most promising high power microwave generators

and has been investigated intensively because of its virtues

such as high power, high efﬁciency, and high repetition

rate.

1–27

To increase the beam-wave interaction efﬁciency,

improved structures, such as non-uniform slow wave struc-

ture (SWS),

5–12

sectional SWS,

13,14

coaxial SWS,

11,15–17

and

the introduction of resonant reﬂector,

9,18–27

modulation

cavity,

23–26

and extraction cavity,

23–27

are adopted. In partic-

ular, the non-uniform SWS has been proven to be effective

to increase the efﬁciency about several decades ago and is

still widely used now.

5–12

The physics of the non-uniform SWS is generally inter-

preted from the point of view of the coupling impedance and

phase velocity.

5–11

The coupling impedance between the

slow space charge wave on the electron beam and the surface

harmonic of the backward electromagnetic mode can be

modiﬁed by varying the ripple amplitude, or by varying the

magnetic ﬁeld distribution within the tube. The phase veloc-

ity of this harmonic can be varied by gradually changing the

period of the ripples. The variations in the coupling imped-

ance or phase velocity affect the interaction between the

electron beam and electromagnetic modes along the length

of the tube and increase the beam-to-wave efﬁciency. As an

exception, Moreland et al. suggests that prebunching the

electron beam in the initial section of the RBWO with two-

stage non-uniform amplitude SWS results in increased

microwave generation efﬁciency.

The RBWO with a resonant reﬂector is characterized by

efﬁcient electron beam premodulation in the reﬂector region.

Theoretical and experimental studies have demonstrated the

feasibility of increasing the microwave power and energy,

mechanically tuning the oscillation frequency, and enhanc-

ing the efﬁciency in this kind of RBWO.

9,18–21

This paper

will provide a fresh insight into the mechanism of increased

efﬁciency using a non-uniform SWS in the RBWO with a

resonant reﬂector. The goal of this study is to explore the

physics of the non-uniform RBWO with a resonant reﬂector,

which may reveal methods of further improving the efﬁ-

ciency of microwave generation in a RBWO.

II. MODULATION OF BEAM CURRENT BY THE

STRONG FIELD IN THE RESONANT REFLECTOR

The velocity of an electron beam will be modulated after

it propagates past the resonant reﬂector of a RBWO. With

the assumption that the beam electrons are guided by an

inﬁnitely strong axial magnetic ﬁeld, the relativistic electron

equation of motion is given by

dcm0v

ðÞ

dt ¼eE0sin xt

ðÞ

;(1)

where eand m0are the electron charge and rest mass, E0and

xare the amplitude and angular frequency of the modulating

ﬁeld, respectively, vis the electron velocity, and

c¼1=ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

1v2=c2

p,cis the light speed in vacuum.

Since the axial electric ﬁeld in the resonant reﬂector is

very strong, the classical velocity perturation method is not

appropriate for this case. Thus, we introduce the momentum

perturation method. Assuming the electron enters the reso-

nant reﬂector at time t0and velocity v0, then it can be

derived from Eq. (1)

cb c0b01eE0dM1

c0m0c2b2

sin xt0þhd



;(2)

1070-664X/2013/20(11)/113113/5/$30.00 V

C2013 AIP Publishing LLC20, 113113-1

PHYSICS OF PLASMAS 20, 113113 (2013)

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where b0¼v0=c,hd¼xd=v0is the dc transit angle, dis the

reﬂector width, and M1¼sin hd



=hd

2is the coupling

coefﬁcient.

Next, we will calculate the rf current after the resonant

reﬂector. For simplicity and a qualitative analysis, the modu-

lating electric ﬁeld and the space charge ﬁeld in the drift

space and the SWS are not taken into account. Thus, the

electron phase can be expressed as

h¼xt0þxz

v:(3)

Then the fundamental harmonic current amplitude is given by

I1z

ðÞ¼I0

pð2p

eihdh0;(4)

where I0is the dc current and h0¼xt0is uniformly distrib-

uted from 0 to 2p.

As an example, for E0¼750 kV=cm;d¼1cm;and f¼

9:7GHz;the distribution of fundamental harmonic current

amplitude after the electron beam with energy of 920 keV

and current of 13.2 kA propagates past the resonant reﬂector

is plotted in Fig. 1. The harmonic current reaches its maxi-

mum at the distance of about 8.4 cm from the center of the

resonant reﬂector. For comparison, the harmonic current dis-

tributions for two other modulating ﬁelds, E0¼700 kV=cm

and E0¼800 kV=cm;are also shown in Fig. 1.Itisobvious

that a larger modulating ﬁeld leads to the appearance of the

peak current at a smaller distance. Consequently, for ﬁxed

beam energy, the peak location of modulation current can be

changed by modifying the amplitude of modulating ﬁeld to

match the distribution of axial electric ﬁeld in the SWS. In

Sec. III, we will demonstrate that the introduction of a non-

uniform SWS decreases the modulating ﬁeld in the resonant

reﬂector.

III. EFFECT OF NON-UNIFORM SWS ON REFLECTION

COEFFICIENT AND FIELD DISTRIBUTION

The uniform SWS consists of seven homogeneous peri-

ods, and the non-uniform SWS comprises three periods with

increasing amplitude and four homogeneous periods, as

shown in Fig. 2. The reﬂection coefﬁcients for two types of

SWSs are illustrated in Fig. 3. Apparently, compared with

the uniform SWS, less power is transmitted from the right

port to the left port for the non-uniform SWS at the fre-

quency of 9.7 GHz. Therefore, the electric ﬁeld in the reso-

nant reﬂector of the non-uniform RBWO is smaller, as

indicated in Fig. 4. In addition, the axial electric ﬁeld distri-

butions in most parts of the two SWSs, especially in the

homogenous regions (19.4–25 cm), are similar.

FIG. 1. Fundamental harmonic current distribution after an electron beam

propagates past a resonant reﬂector for different modulating ﬁelds.

FIG. 2. Calculation model for the uniform SWS (a) and non-uniform SWS

(b). A TM

mode is injected from the right port.

FIG. 3. Reﬂection coefﬁcients for the uniform SWS and non-uniform SWS.

FIG. 4. Axial electric ﬁeld distributions for the uniform RBWO and non-

uniform RBWO when the electron beam is absent.

113113-2 Chen et al. Phys. Plasmas 20, 113113 (2013)

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IV. EFFECT OF NON-UNIFORM SWS ON

FUNDAMENTAL HARMONIC CURRENT DISTRIBUTION

AND MICROWAVE GENERATION

Particle-in-cell (PIC) code UNIPIC

is used to simulate

the two types of RBWOs, as shown in Fig. 5. In Fig. 5(a),

the SWS is uniform, and it is non-uniform in Fig. 5(b).In

addition, the lengths of drift sections between the resonant

reﬂector and the SWS are slightly varied to adjust the fre-

quencies of the two RBWOs to the same. The pictures of

both RBWOs operation derived from simulations indicate

that the microwave owes its origin to random oscillation, and

then the dominant frequency component determined by the

electron beam parameters and the SWS begins to increase.

With the elevation of modulating ﬁeld in the resonant reﬂec-

tor, the harmonic current rises, and its peak position shifts

from the collector end to the cathode end until the micro-

wave achieves saturation. Figure 6displays the fundamental

harmonic current distributions after saturation. The values of

peaks for two harmonic currents are almost the same, but the

locations differ substantially. For the uniform RBWO, the

peak locates at about 17.4 cm, far away from the positions

where the larger axial electric ﬁeld appear (Fig. 4). Whereas

for the non-uniform RBWO, the peaks locate at 17.7 cm and

18.9 cm (the distances from the center of the resonant

reﬂector are 6.8 cm and 8.0 cm, close to the results obtained

in Sec. II), much nearer to the maximum axial electric ﬁeld.

Moreover, at the point where the maximum electric ﬁeld

occurs (22.2 cm), the harmonic current is much larger in the

non-uniform RBWO (8.9 kA) than that in the uniform

RBWO (7.3 kA).

It should be noted that in the two RBWOs, the operation

mode is TM

mode, near ppoint, as shown in Fig. 7. The

beam interaction with the SWS produces a TM

mode that

consists of backward traveling surface and volume harmon-

ics with almost the same amplitude. The backward volume

harmonic is reﬂected by the resonant reﬂector producing a

forward traveling volume harmonic and forming standing

waves in the SWS. The observation of the electric ﬁeld dis-

tributions at different instants obtained from the PIC simula-

tions proves this. Thus, besides Cerenkov effects, the energy

conversion process will strongly depend on transit time

effects. That is to say, the matching condition between the

distributions of the harmonic current and the standing-wave

ﬁeld will provide a profound inﬂuence on the beam-wave

interaction. A typical instantaneous distribution of the beam

current and axial electric ﬁeld is shown in Fig. 8. Obviously,

the matching between the beam current and axial electric

FIG. 5. Phase space plots for the uniform RBWO (a) and non-uniform RBWO (b).

FIG. 6. Fundamental harmonic current distributions for the uniform RBWO

and non-uniform RBWO.

FIG. 7. Dispersion curves of the uniform SWS. Also shown is the beam line

vb¼0:934ccorresponding to a beam energy of 920 keV, k0¼2p=z0;z0is

the length of the SWS period.

113113-3 Chen et al. Phys. Plasmas 20, 113113 (2013)

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ﬁeld in the non-uniform RBWO is more beneﬁcial to the

energy exchange.

Figure 9shows the output microwave powers for the

uniform RBWO and non-uniform RBWO. With a diode volt-

age of 1.02 MV and beam current of 13.2 kA, the generated

microwaves are 3.0 and 4.0 GW, giving the beam-to-micro-

wave conversion efﬁciencies of 22% and 30%, respectively,

and the microwave frequencies are both 9.7 GHz. The

shorter starting time in the uniform RBWO can be attributed

to the larger modulating ﬁeld in the resonant reﬂector.

V. CONCLUSION

In conclusion, the radiation enhancement in the non-

uniform RBWO can be elucidated with the better matching

between the distributions of the fundamental harmonic cur-

rent and the axial electric ﬁeld. Compared with the uniform

SWS, the reﬂection coefﬁcient of the non-uniform SWS is

higher, leading to a lower modulating electric ﬁeld in the res-

onant reﬂector and a larger distance to maximize the modu-

lation current. Moreover, for both types of RBWOs, stronger

standing-wave ﬁeld takes place at the rear part of the SWS,

and the energy conversion process strongly depends on

transit time effects. In the non-uniform RBWO, the region

with a stronger standing wave ﬁeld corresponds to a higher

fundamental harmonic current distribution. Therefore, when

the diode voltage is 1.02 MV and beam current is 13.2 kA, a

microwave with power of 4 GW has been obtained, com-

pared to that of 3 GW in the uniform RBWO.

VI. FUTURE WORK

Generally speaking, the stronger ﬁeld in the RBWO

with a resonant reﬂector appears at the rear part of the tube,

so it is favorable to comparatively decrease the axial electric

ﬁeld in the reﬂector to shift the peak of the harmonic current

to the collector end. For this purpose, some higher-order

mode reﬂector, such as TM

021

(Ref. 24)orTM

022

reﬂector,

where the average modulating ﬁeld is weaker than that in the

conventional TM

020

reﬂector, may be used. Adding an

extraction structure either between the reﬂector and the SWS

or at the end of the SWS is potential to decrease the modulat-

ing ﬁeld in the reﬂector. Moreover, the amplitude and phase

of axial electric ﬁeld distribution can also be modiﬁed to

meet the requirement of effective beam-wave interaction by

the introduction of a middle cavity between the SWS and an

extraction cavity at the end of the SWS.

23–27

Existing and

ongoing work suggests that further increasing the power con-

version efﬁciency to exceed 50% by combining the advan-

tages of aforementioned methods is possible.

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Sep 2023

The frequency hopping technology is one of the most significant research directions for high-power microwave (HPM) devices. This paper presented a novel HPM oscillator with frequency hopping across C, X, and Ku bands based on magnetic field tuning. A coaxial transit time oscillator (TTO) is nested onto the outer conductor of the hollow relativistic Cherenkov microwave oscillator, which forms a dual electromagnetic structure with a single-annular cathode. When the electron beam is guided by gradient magnetic fields, it interacts with TTO to produce Ku-band HPMs. If the gradient magnetic field changes into the uniform magnetic field, the electron beam would enter the relativistic Cherenkov microwave oscillator, and the frequency of generated microwaves decreases, which are decided by the strength of the magnetic fields according to the cyclotron resonance absorption theory. In the particle-in-cell simulation, when the diode voltage and gradient magnetic field are 580 kV and 0.5 T, respectively, a Ku-band HPM output with a frequency of 13.9 GHz and a power of 2.09 GW is obtained, corresponding to power efficiency of 42%. When the magnetic field transforms into uniform, the device produces an X-band HPM output with a frequency of 9 GHz and a power of 2.4 GW at a diode voltage of 683 kV and a magnetic field of 0.7 T. When the voltage and magnetic field strength are increased to 699 kV and 1.5 T, respectively, the device generates a C-band HPM output with a frequency of 4.5 GHz and a power of 2.1 GW. The corresponding conversion efficiency of the X-band and C-band Cherenkov microwave oscillators is 35% and 30.7%, respectively.

Efficiency improvement by a beam filtering ring in a relativistic backward wave oscillator at low magnetic field

Article

Full-text available

Apr 2022

This paper presents a design method of the relativistic backward wave oscillator at low magnetic field, which can improve the efficiency by 29% in the particle in cell simulation. The core of this method is to introduce a beam filtering ring. The beam filtering ring takes the characteristic of the radial position change as the electron oscillates. The structure manipulates the axial current, so that a large proportion of the electrons expected to be in the accelerated phase in the slow-wave structure is absorbed by the structure. It greatly enhances the bunching of the beam in the RF field and improves the beam-wave conversion efficiency significantly. The particle in cell simulation results reveal that at a permanent magnet with a magnetic induction intensity of 0.68 T, the output microwave power of the relativistic backward wave oscillator with a beam filtering ring is 5.9 GW, and the conversion efficiency can be up to 54% when the diode voltage is 890 kV and the beam current is 12.2 kA.

Study of SGEMP Field-Coupling inside and outside Reentrant Cavity

Article

Full-text available

Mar 2022

This article studies system-generated electromagnetic pulse (EMP) and coupling in a reentrant cavity—a simplified spacecraft model with the annular slit and internal shaft, numerically by using the three-dimensional conformal particle-in-cell method. The results indicate that when the electromagnetic fields arrive at the annular slit, obvious field-coupling will be induced that EMP may enter or escape from the slit without many restrictions at the low-irradiation fluence. In this case, the total field can be acquired by linearly summing the EMPs derived from the moving electrons released separately by the external and internal emitting surfaces. The reentrant cavity is to form a repetitive-oscillating capacitor model, while the electromagnetic waves of damped oscillation with comparatively large amplitude are found near the annular slit and the internal shaft. The frequency of the damped oscillation is mainly influenced by the cavity geometrical factor and is going to converge to the natural frequency of the reentrant cavity. Furthermore, the reason accounting for the change of both the frequencies is revealed.

Experimental Study on a Moderately Overmoded Ka-Band Cherenkov Oscillator Operating With Low Magnetic Field

Article

May 2020

This article presents the results of the numerical and experimental study of a Ka -band Cherenkov oscillator with the average diameter of the slow wave structure (SWS)

{D}= {2.4}\lambda

, where

\lambda

is the wavelength. The operating point on TM ₀₁ is set close to the

\pi

-point to enhance the electric field in SWS. Meanwhile, by optimizing the structural parameters, the influence of TM ₀₂ on the mixed output mode is minimized. Then in the optimized structure with a straight output waveguide, the proportion of TM ₀₁ in the mixed output mode reaches as high as 97%, achieving the target of mode selection. The experimental researches are carried out with a high-current SINUS881 accelerator. A solenoid magnet provides the guiding magnetic field of 0.9T. In the range of voltages 552–585 kV and currents of 7.3–7.85 kA, the generation of 7–10 ns pulse at the frequency 29.93–29.96 GHz and 277–312 MW output power was realized. The radiation mode is investigated by the neon lamp luminescence and the radiation pattern of power density in far-field, which both verified that the output mode is mainly composed of TM ₀₁ , demonstrating the practicability of the proposed method in solving the mode competition problem.

Calculation of Characteristic Time of Space Charge Limited Effect of SGEMP

Article

Apr 2020

When the X-ray beam illuminates a material surface, the backscattered photo-electrons are emitted from the surface, and stimulates the system generated electromagnetic pulse (SGEMP). As the X-ray flux rises, the space charge limited (SCL) effect occurs and a relatively thin layer develops near the emitting surface. The characteristic time is defined as the occurring time of the SCL effect. In this paper, an accurate analytical equation is derived by applying the momentum conservation equation for the mono-energetic electrons, and verified by the PIC (particle-in-cell) simulation. But for the electrons with velocity distribution, the analytic equation using the average velocity approximation instead of the exponential distribution is not accurate as the interaction between the low energy and high energy electrons is ignored. PIC simulation results show that the characteristic time of the SCL effect is longer by the PIC simulation than the average energy approximation when the average energy of the electrons is relatively low, and vice versa.

Design and Simulation of Dual-Band Nonuniform Relativistic Backward Wave Oscillator Using a Bragg Structure as Its RF Circuit and Reflector-Cum-Mode Converter

Article

Mar 2020

A relativistic backward wave oscillator (RBWO) operating in dual (

{C}

and

{X}

) band is designed and studied using finite-difference time domain (FDTD) particle-in-cell (PIC) simulations implemented in MAGIC. It consists of a sinusoidally corrugated, moderately overmoded nonuniform slow wave structure (SWS) in the downstream section and a single fold two-way helically corrugated Bragg structure in the upstream section. A single intense relativistic electron beam (IREB) having ~5.1 kA of current is predicted for 600 kV of diode voltage. An RF output power ~720 MW in

{X}

-band and ~510 MW in

{C}

-band are predicted in TE ₁₁ mode with a guiding magnetic field of ~2.5 T.

Improved fundamental harmonic current distribution in a klystron-like relativistic backward wave oscillator by two pre-modulation cavities

Article

Full-text available

Apr 2013

In a klystron-like relativistic backward wave oscillator, the velocity modulation is mainly obtained from the resonant reflector. By introducing two pre-modulation cavities between the input cavity and the resonant reflector, the amplitude and phase of density modulation can be adjusted relatively independently, to ensure an improved fundamental harmonic current distribution. Two peaks of harmonic current with high modulation coefficient of 1.2 appear in the second slow wave structure and the dual-cavity extractor and result in large beam energy losses in both regions. Particle-in-cell simulations show that a microwave with power of 11.5 GW and efficiency of 57% can be obtained.

Suppressing RF breakdown of powerful backward wave oscillator by field redistribution

Article

Full-text available

Jan 2012

An over mode method for suppressing the RF breakdown on metal surface of resonant reflector cavity in powerful backward wave oscillator is investigated. It is found that the electric field is redistributed and electron emission is restrained with an over longitudinal mode cavity. Compared with the general device, a frequency band of about 5 times wider and a power capacity of at least 1.7 times greater are obtained. The results were verified in an X-band high power microwave generation experiment with the output power near 4 gigawatt.

High-power relativistic microwave sources based on the backward wave oscillator with a modulating resonant reflector

Article

Full-text available

Aug 2008
TECH PHYS+

Results of theoretical and experimental investigations into a relativistic backward wave oscillator with a modulating resonant reflector are generalized. The modulating resonant reflector is used to reflect a counter propagating wave and guide it toward an electron collector. It is shown that premodulation of the electron beam near the reflector may have a significant effect on the starting conditions of oscillation; selective properties of the oscillator; and its efficiency, which may reach 40% when a high-current beam is transported by a strong magnetic field. In the reduced magnetic fields that were employed in the pulsed-periodic regime and were 1.5-2.0 times lower than those at which cyclotron resonance with the counter propagating wave is observed, the oscillator efficiency (30-35% at a wavelength of 8 mm) is limited by position and velocity spreads of particles. Mechanical pulsewise frequency tuning within about 10% at a repetition rate of 1-50 Hz and a multigigawatt microwave power, as well as a rise in the power and energy of microwave pulses via an increase in the cross-sectional dimensions of the slow-wave structure, are demonstrated to be feasible.

INVESTIGATION OF A RELATIVISTIC CERENKOV SELF-GENERATOR.

Article

May 1981

The results are presented of an experimental investigation of various regimes of a relativistic Cerenkov self-generator at a wavelength of 3. 2 cm (the TM//0//1 mode) as a function of the strength of the external longitudinal magnetic field. It is shown that when the magnetic field is strong, the power radiated in the fundamental mode of the TM waves which are generated increases significantly with H//0 approximately equals 1. 6 T, the radiated power reaches approximately 1 GW, corresponding to a transformation coefficient of approximately 30% from the flux of energy in the electron beam into the flux of radiation. At the same time, an abrupt decrease in the level of power radiated in the fundamental mode of the TM waves is observed when the magnetic fields are such that omega //H approximately equals gamma omega .

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structure and depth-tunable extractor

Article

Feb 2013

A high efficient relativistic backward wave oscillator with coaxial nonuniform slow-wave structures (SWSs) and depth-tunable extractor is presented. The physical mechanism to increase the power efficiency is investigated theoretically and experimentally. It is shown that the nonuniform SWSs, the guiding magnetic field distribution, and the coaxial extractor depth play key roles in the enhancement of the beam-wave power conversion efficiency. The experimental results show that a 1.609 GHz, 2.3 GW microwave can be generated when the diode voltage is 890 kV and the beam current is 7.7 kA. The corresponding power efficiency reaches 33.6%.

Investigation of an improved relativistic backward wave oscillator in efficiency and power capacity

Article

Oct 2012

Investigation of relativistic backward wave oscillator with high efficiency and power capacity is presented in this paper. To obtain high power and high efficiency, a TM{sub 021} mode resonant reflector is used to reduce the pulse shortening and increase power capacity to about 1.7 times. Meanwhile, an extraction cavity at the end of slow wave structure is employed to improve the efficiency from less than 30% to over 40%, through the beam-wave interaction intensification and better energy conversion from modulated electron beam to the electromagnetic field. Consistent with the numerical results, microwave with a power of 3.2 GW, a frequency of 9.75 GHz, and a pulse width of 27 ns was obtained in the high power microwave generation experiment, where the electron beam energy was configured to be {approx}910 kV and its current to be {approx}8.6 kA. The efficiency of the RBWO exceeds 40% at a voltage range of 870 kV-1000 kV.

Experimental study of a compact P-band coaxial relativistic backward wave oscillator with three periods slow wave structure

Article

Aug 2012

A compact P-band coaxial relativistic backward wave oscillator with three periods slow wave structure was investigated experimentally. The experimental results show that the frequency of the P-band coaxial relativistic backward wave oscillator is 897 MHz and the microwave power is 1.47 GW with an efficiency of about 32% in the case in which the diode voltage is 572 kV, the beam current is 8.0 kA, and the guide magnetic field is about 0.86 T. In addition, the device can generate a 3.14 GW microwave radiation as the guide magnetic field increases to 1.2 T at the diode voltage of 997 kV and the beam current of 15.3 kA. The experimental results are in good agreement with those obtained earlier by numerical simulations.

High efficiency X-band relativistic backward wave oscillator with non-uniform slow wave structures

Article

Aug 2012

Backward wave oscillators (BWOs) driven by intense relativistic electron beams are very efficient means of producing high-power microwaves. However, the efficiency of conventional BWO is lower than 30%. An X-band oversized BWO with non-uniform slow wave structure is designed to improve RF output characteristics. In particle-in-cell simulation, a high power microwave with a power of 8.0 GW and efficiency of 40% is obtained, compared with that of 30% obtained in a conventional relativistic BWO. © 2012 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

Recent Advance in Long-Pulse HPM Sources With Repetitive Operation in S-, C-, and X-Bands

Article

Jun 2011

Recent experimental results of three kinds of long-pulse high-power microwave (HPM) sources operating in S-, C-, and X-bands are reported. The difficulties in producing a long-pulse HPM for the O-type Cerenkov HPM source were analyzed theoretically. In S- and C-bands, single-mode relativistic backward-wave oscillators were designed to achieve long-pulse HPM outputs; in X-band, because of its shorter wavelength, an O-type Cerenkov HPM source with overmoded slow-wave systems (D/λ ≈ 3) was designed to increase power capacity. In experi- ments, driven by a repetitive long-pulse accelerator, both S- and C-band sources generated HPMs with power of about 2 GW and pulse duration of about 100 ns in single-shot mode, and the S-band source operated stably with output power of 1.2 GW in 20-Hz repetition mode. The X-band source generated 2 GW microwaves power with pulse duration of 80 ns in the single-shot mode and 1.2 GW microwave power with pulse duration of about 100 ns in the 20-Hz repetition mode. The experiments show good per- formances of the O-type Cerenkov HPM source in generating repetitive long-pulse HPMs, especially in S- and C-bands. It was suggested that explosive emissions on surfaces of designed eletrodynamic structures restrained pulse duration and operation stability.

Backward wave oscillators with rippled wall resonators: Analytic theory and numerical simulation

Article

Sep 1985

In this paper, a comprehensive theoretical treatment is developed for backward wave oscillators composed of a relativistic electron beam guided by a strong magnetic field through a slow wave structure consisting of a cylindrical waveguide with a sinusoidally varying wall radius. This analysis, equally applicable to traveling wave tube operation, includes both a linearized theory of small-amplitude perturbations and numerical simulations of the saturated, large-amplitude operating regime. The variation of device operating characteristics with system parameters is examined in detail. Comparisons of the analytic and numerical results with experiments and additional calculations show excellent agreement and justify a high degree of confidence in the validity of the theory.

Effect of non-uniform slow wave structure in a relativistic backward wave oscillator with a resonant reflector

Abstract

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