Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz 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) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm₀₁ mode prevailed when the background plasma density was below 6×10¹¹ cm^-3 . At a critical background plasma density of ≃8×10¹¹ cm^-3 power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse     

Experimental results from a tandem BWO-TWT system used to generatehigh-power microwaves

A relativistic backward wave oscillator (BWO) in tandem with a traveling wave tube (TWT) amplifier has been used to generate relatively long pulses of high-power X-band microwaves. In these experiments, a BWO is used to modulate the annular relativistic electron beam, which subsequently drives a TWT producing high-power microwave radiation. A special RF sever located between the two structures cuts off microwaves generated in the BWO from the TWT. Peak powers in excess of 100 MW are observed with overall beam-to-microwave efficiencies as high as 35%. By operating the BWO below saturation levels, pulse-shortening effects are minimized so that microwave pulses of duration comparable to that of the beam (100 ns) are possible. The operating frequency of the tandem system is tuned from 11 to 12 GHz by varying the effective energy of the beam     

Development of a 2 MW relativistic backward wave oscillator

In this paper, a high power relativistic backward wave oscillator (BWO) experiment is reported. A 230 keV, 2 kA, 150 ns relativistic electron beam is generated using a Marx generator. The beam is then injected into a hollow rippled wall metallic cylindrical tube that forms a slow wave structure. The beam is guided using an axial pulsed magnetic field having a peak value 1 T and duration 1 ms. The field is generated by the discharge of a capacitor bank into a solenoidal coil. A synchronization circuit ensures the generation of the electron beam at the instant when the axial magnetic field attains its peak value. The beam interacts with the SWS modes and generates microwaves due to Cherenkov interaction. Estimated power of 2 MW in TM₀₁ mode is observed.      

Role of MW-ECR hydrogen plasma on dopant deactivation and open-circuit voltage in crystalline silicon solar cells

Plasma hydrogenation is an efficient method to passivate intergrain and intragrain defects of polycrystalline silicon (pc-Si) solar cells. The hydrogenation experiments were carried out in hydrogen plasma generated in an electron cyclotron resonance system controlling different operating parameters such as microwave power (P _MW), process time (t _H) and hydrogenation temperature (T _H) for a fixed hydrogen flux of 30 sccm. The hydrogenation of n⁺pp⁺ pc-Si solar cells resulted in an improvement in the open-circuit voltage. The improvement was correlated with the dopant deactivation due to the formation of boron–hydrogen bonding. This was demonstrated from the changes in the doping level after hydrogenation of n⁺p diode structures made using single crystalline silicon as a reference material. It was found that deactivation of boron was more pronounced at high microwave plasma power, in good agreement with the high open-circuit voltage values obtained on pc-Si mesa cells. On the other hand, the effect of longer hydrogenation time and higher temperature resulted in a decrease of boron deactivation, while an increase in V _oc with a tendency of saturation at high T _H was observed. Reasons for such behavior were thoroughly explained.     

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

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.     

Optical emission spectra from microwave oxygen plasma produced by surfatron discharge

This paper is devoted to a systematic study of the production of molecular and atomic ions and excited atoms in a microwave oxygen plasma generated by surfatron in a quartz tube (i.d. 6 mm). The content of O ₂ ⁺ , O⁺ and O is measured, using the optical emission spectroscopy, in a wide range of oxygen pressures (from 3.5 Pa to 10² Pa) and microwaves powers (from about 30 W to 300 W) delivered into the plasma. It is shown that the content of individual species O2⁺, O⁺ and O strongly depends on the conditions, particularly operating pressurep and the microwave powerP, under which plasma is created. It means that the chemical reactivity of microwave plasma also strongly depends onp andP. It is of great importance for many practical applications, e.g. for a reproducible production of thin films with prescribed properties.The authors wish to express their thanks to the Grant Agency of Academy of Sciences of the Czech Republic for the support of this research. This work was done under Academy of Sciences Grant No. 11020.     

Imaging the Spatial Distribution of Microwave Intensity Using the Recombination Continuum Emitted by the Positive Column of the Cs–Xe DC Discharge

We study experimentally the possibility of using the recombination continuum (RC) emitted by a nonequilibrium plasma of the positive column (PC) of a Cs–Xe discharge for real-time imaging of the spatial distribution of microwave intensity. A uniform plane slab of dense plasma of the PC of a Cs–Xe discharge with aperture 10×8 cm² was used for the imaging. A continuous-wave (CW) magnetron was the microwave source with frequency 35.4 GHz and power up to 20 W. We measured the dependence of the RC intensity on the intensity of the microwaves incident on the plasma. Spatial distributions of the microwave intensity for a microwave beam and the H₀₁ mode of a circular waveguide were imaged. The results of these experiments show that spatial distributions of microwave intensity measured using RC agree well with the distributions obtained using other methods. A temporal resolution of 10 μs was demonstrated and an energy-flux sensitivity of about 4·10^-5 J/cm² was achieved in the microwave-imaging experiments.     

OH number densities and plasma jet behavior in atmospheric microwave plasma jets operating with different plasma gases (Ar,Ar/N<Subscript>2</Subscript>, and Ar/O<Subscript>2</Subscript>)

OH radical number density in multiple atmospheric pressure microwave plasma jets is measured using UV cavity ringdown spectroscopy of the OH (A–X) (0–0) band at 308 nm. The plasma cavity was excited by a 2.45 GHz microwave plasma source and plasma jets of 2–12 mm long were generated by using three different plasma gases, argon (Ar), Ar/N₂, and Ar/O₂. Comparative characterization of the plasma jets in terms of plasma shape, stability, gas temperature, emission intensities of OH, NO, and N₂, and absolute number density of the OH radical was carried out under different plasma gas flow rates and powers at various locations along the plasma jet axis. With three different operating gases, the presence of OH radicals in all of the plasma jets extended to the far downstream. As compared to the argon plasma jets, the plasma jets formed with Ar/N₂ and Ar/O₂ are more diffuse and less stable. Plasma gas temperatures along the jet axis were measured to be in the range of 470–800 K for all of the jets formed in the different gas mixtures. In each plasma jet, OH number density decreases along the jet axis from the highest OH density in the vicinity of the jet tip to the lowest in the far downstream. OH density ranges from 1.3 × 10¹² to 1.1 × 10¹⁶, 4.1 × 10¹³ to 3.9 × 10¹⁵, and 7.0 × 10¹² to 4.6 × 10¹⁶ molecule/cm³ in the Ar, Ar/N₂, and Ar/O₂ plasma jets, respectively. The OH density dependence on plasma power and gas flow rate in the three plasma jets is also investigated.     

Microwave power absorbed by and scattered from a multilayered zero-index anisotropic metamaterial-semiconductor cylinder

We present here dependencies of scattered and absorbed powers of incident perpendicularly and parallel polarized microwaves by a multilayered cylinder. We consider here the normal (angle ??=90^°) and oblique (angles ??=60^°,30^°,5^°) incidence of microwave on the cylinder. The one consists of a glass core that is coated by the six anisotropic metamaterial and lossy n-Si semiconductor alternative layers. Characteristics of a cylinder with the semiconductor external layer are presented. The dispersion dependency of n-Si losses was taken into account. The metamaterial is a uniaxial anisotropic medium with the electric and magnetic plasma resonances in the frequency range from 1 until 4?GHz. The anisotropic metamaterial can include the constitutive parameters equal to zero. The multilayered cylinder has the external radius equal to 2?mm. The glass core has a radius equal to 0.5?mm. The thickness of all layers is the same. We have compared the scattered and absorbed power dependencies on the microwave polarization, the angle of microwave incidence (the normal and oblique directions of the incidence to the z-axis), and the n-Si specific resistivity. We discovered specific dependencies of scattered and absorbed powers on the parameters.     

Microwave power coupling with electron cyclotron resonance plasma using Langmuir probe

Electron cyclotron resonance (ECR) plasma was produced at 2.45 GHz using 200–750 W microwave power. The plasma was produced from argon gas at a pressure of 2 × 10^???4 mbar. Three water-cooled solenoid coils were used to satisfy the ECR resonant conditions inside the plasma chamber. The basic parameters of plasma, such as electron density, electron temperature, floating potential, and plasma potential, were evaluated using the current–voltage curve using a Langmuir probe. The effect of microwave power coupling to the plasma was studied by varying the microwave power. It was observed that the optimum coupling to the plasma was obtained for ~ 600 W microwave power with an average electron density of ~ 6 × 10¹¹ cm^???3 and average electron temperature of ~ 9 eV.     

Experimental investigation of a compact relativistic magnetron with axial TE11 mode radiation

As one of the relativistic electron tubes having compact configuration and high efficient output, the relativistic magnetron with direct axial radiation is very attractive in pulsed power and high power microwave fields for industrial and military applications. In this paper, the experimental investigation of a relativistic magnetron with axial TE11 mode radiation is reported. Under a total length of - 0.3 m, volume of - 0.014 m3, working at an applied voltage of 508 kV and a magnetic field of - 0.31 T, the relativistic magnetron radiates a microwave of 540 MW with the TE11 mode at 2.35 GHz in the axial direction. The power conversion efficiency is 15.0%. After a lot of shots, the detected amplitudes of microwaves are nearly the same. The fluctuations of wave amplitudes are less than 0.3 dB.     

Nonlocal theory of stimulated Raman backscattering in a strongly magnetized plasma

A nonlocal theory of stimulated Raman backscattering (BSRS) parametric instability of a large amplitude electromagnetic (EM) mode in a strongly magnetized plasma e.g., one encountered in a plasma filled backward wave oscillator, is reported. The EM mode is unstable to parametric instability in a magnetized plasma and decays into a Trivelpiece-Gould (TG) mode and a sideband EM mode. The growth rate of instability (Γ) scales proportional to three-fourth power of plasma density. For a typical BWO the growth rate is ∼10⁸ s^-1     

Particle-In-Cell Simulations from a X Band Coaxial Relativistic Backward Wave Oscillator

A high-power x-band coaxial relativistic backward wave oscillator has been conceptually designed and analyzed. The TM_on dispersion relations in the coaxial corrugated cylindrical waveguide used in the device was calculated. MAGIC,an electronmagnetic particle-in-cell code, is being used to investigated the nonlinear beam-wave interaction and other design optimization issues. Preliminary simulation results shows that the coaxial BWO can generate 2. 2 GW peak power microwave at 10. 15GHz in the TM₀₂ mode driven by a 600-KV 20-KA electron beam, the peak power efficiency is about 18%.     

Excitation of ion cyclotron waves at the difference frequency of two microwave beams in a semiconductor

This paper presents an investigation of the resonant excitation of the electrostatic ion cyclotron wave at the difference frequency of two microwave beams propagating in a magnetoactive solid state plasma, viz. n InSb. The resonant excitation of the electrostatic ion cyclotron wave occurs when the difference frequency of the two microwave beams and the difference of their propagation vectors satisfy the dispersion relation corresponding to the electrostatic ion cyclotron wave. For typical plasma parameters of n InSb and microwave beams of power densities 1 MW cm^?2, the power density of the excited ion cyclotron wave is 0.40 kW cm^?2 when external magnetic field is $\text{1.46 kG (}\text{Ω}{}_{\mathrm{c}}\text{ω}\text{) = 0.1)}$. The power density of the excited ion cyclotron wave increases with the magnetic field. This study may provide new means for the characterisation and diagnostic of semiconductors.     

Coherent backscattering of electromagnetic waves in a magnetised plasma

Summary A microwave coherent backscattering experiment has been carried out on Mirabelle, a weakly ionised plasma device, with the objective of measuring the electron density fluctuation level. The experiment is a preliminary step in order to prepare the detection system for a microwave stimulated backscattering experiment. The incident electromagnetic wave is focused in front of a plane grid which excites ion acoustic or electron Bernstein waves inducing fluctuations in the plasma. The backscattering signal is collected by the launching circuit and detected by homodyne mixing. The typical ratio of the scattered power to the incident power is about 10⁻¹² and the relative density fluctuations are of the order of δn _e/n _e=10⁻³ against a background electron density ofn _e=1–5·10⁹ cm⁻³. The backscattering measurement is compared with Langmuir probe measurements. The spectral width of the backscattered signal has also been studied, by taking into account effects due to the incident wave focusing and plasma wave damping. The authors of this paper have agreed to not receive the proofs for correction     

Microwave irradiation on a-axis oriented Y123/Pr123 two-stacked Josephson junctions device

Two-stacked submicron Josephson junctions devices were fabricated in a-axis oriented YBaCu₃O₇ and PrBa₂Cu₃O₇ (Y123/Pr123) multi layered thin films using focused ion beam milling technique. The transition temperature and critical current density (J_c) of the device are about 83 K and 5 × 10⁵ A/cm² at 20 K, respectively. The device was irradiated with external microwave of 10 GHz and studied at 20 K. The microwave induced voltage steps are observed in I–V characteristics. The supercurrent branch become resistive above a certain microwave power and also the J_c was suppressed as we increased the microwave power. Magnetic field modulation of critical current shows periodicity of about 2000 gauss correspond to the Josephson junctions in the stack.     

Attenuation of Electromagnetic Waves by a Plasma Layer at Atmospheric Pressure

Plasma layers at atmospheric pressure, are good broad band absorbers of electromagnetic radiation. However, to get substantial attenuations, two parameters have to be optimized. These are the plasma number density, and the thickness of the plasma layer. It is found that in order to be an effective attenuator of microwave radiation, a plasma layer has to have a number density in the 10¹³ cm^–3 range, and a thickness equal or larger than the wavelength of the incident wave. However, as the frequency increases, the amount of attenuation tends to reach a limiting value directly proportional to the number density.     

电子回旋共振等离子体特性及其对生长氮化镓晶膜的影响

为了解并优化在电子回旋共振等离子体辅助化学汽相沉积GaN晶膜的工艺研究中的等离子体特性,利用朗缪尔探针及法拉第筒系统地测量了离子密度(N_i)、等离子体势(V_p)、电子温度(T_e)及离子流强(J_i)等多个等离子体参量随微波功率(P_w)及沉膜室气压(p)变化的关系.给出了在P_w＝850W,p＝0.22Pa时,上述等离子体参量的轴向及径向分布.GaN晶膜的生长速率、电学及晶体学性能 关键词：     

激光等离子体对反射波频移影响的实验研究

 研究了微波被激光等离子体反射时反射波的频率变化。实验中微波发生器产生的微波入射到激光等离子体并被等离子体反射,反射波的频率由频谱分析仪测量,发现反射波的频率与入射波的频率明显不同,分析了在激光等离子体膨胀和熄灭两种情况下激光功率密度和入射波频率对反射波频移影响的原因。结果表明：在等离子体膨胀和熄灭过程中,反射波频移的最大值随激光功率密度的增加而增加;随入射微波频率的增加而减小。     

同轴虚阴极谐振效应研究

中国工程物理研究院应用电子学研究所的一些实验表明由阳极反射板、阳极网和阴极发射的电子束形成的环状虚阴极围成的准谐振腔是决定同轴虚阴极输出微波功率和传输模式的关键所在. 在二极管电压350kV，电流23kA条件下，获得了500MW的微波输出功率，能量转换效率约6.2％，工作频率3.3GHz，输出微波主要由TM₀₁模式和TE₁₁模式组成. 对同轴虚阴极的谐振效应进行了分析. 关键词： 高功率微波 同轴虚阴极 谐振腔 模式竞争