Cyclotron effects in relativistic backward-wave oscillators operating at low magnetic fields

In order to overcome the problem of efficiency decrease in the relativistic backward wave oscillator (RBWO) operating at low magnetic field, an S-band klystron-like RBWO with a three-period slow-wave cavity-chain and a dual-cavity extractor is proposed and investigated in detail in this paper. In the device, several measures are taken to enhance the conversion efficiency at low magnetic field, which include increasing the beam bunching depth by a two-stage modulation structure comprised of a resonant reflector upstream and a three-period cavity-chain modulator downstream, strengthening the energy exchange process from the electron beam to the microwave by a dual-cavity extractor, and optimizing the envelope distribution of the electron beam in the interaction region by adjusting the drift length before the resonant reflector. Under the condition of the guiding magnetic field of 0.28 T, the device could generate a microwave with a power of 4.3 GW, a frequency of 3.79 GHz, and the conversion efficiency of 43.3% in the simulation.

Based on the working principle of the backward wave oscillator (BWO) with a low magnetic field, a frequency-agile relativistic backward wave oscillator (RBWO), whose frequency can be counterchanged between X band and C band by changing the strength of the guiding magnetic field, is designed. To actualize frequencys agile counterchange, two sections of slow wave structure (SWS), whose appeals on parameters of electron beams are accordant, are linked. When the energy of the electron is 670 keV while the beam current is 8 kA, the simulated result of 1.0 GW output microwave power with 6.28 GHz frequency is obtained on conduction of 0.5 T guiding magnetic field, while 0.75 GW output microwave power with 9.26 GHz frequency is obtained on conduction of 0.8 T guiding magnetic field.

In this paper, a high power relativistic backward wave oscillator (BWO) experiment is reported. A 230 kV, 2 kA, and 150 ns relativistic electron beam source is developed using a Marx generator. The beam is then injected into a hollow rippled wall metallic cylindrical tube that forms a slow wave structure (SWS). A BWO is a slow wave structure in the form of a rippled wall wave-guide and is used as a source of high power microwave generation when excited by a relativistic electron beam (REB). The beam is guided using an axial pulsed magnetic field having, the duration 1 ms and strength being 1.0 Tesla, respectively. This field is generated by 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 electron beam interacts with the SWS waveguide modes and generates microwaves due to Cherenkov interaction. Estimated power of ~2 MW in TM01 mode is observed.

The design and preliminary results for a C-band relativistic backward wave oscillator (RBWO), which is magnetically well insulated, are presented. Under an external magnetic field of 0.36 T, the RBWO generated high power microwave radiation with a power of 3.3 GW and a frequency of 4.37 GHz for a diode voltage of 870 kV and a beam current of 13.5 kA. The electric field on the surface of the cathode holder was below the emission threshold, and an inlaid graphite cathode was designed to suppress the shunting current in the diode area. The device, operating with a low magnetic field and diode insulation, is a promising candidate for use as a permanent magnet package in high power microwave systems.

The effects of low-frequency (10-500 Hz) magnetic fields on the electrical properties of channel-free bilayer membranes, and on the single-channel conductance and macroscopic gating characteristics of porin channels incorporated into membranes, have been studied for field strengths in the range 10-100 mu T. The field conditions that could in theory give rise to 'cyclotron resonance' effects were also studied. No evidence has been found to support the concept that cyclotron resonance and membrane ion channel effects are involved in the reported biological effects of ELF magnetic fields.

Low magnetic field — assisted synthesis of MCM-41

Summary form only given, as follows. Recent experiments with the plasma-loaded, 8.5 GHz, relativistic backward wave oscillator (BWO) showed an increase in the microwave interaction efficiency up to 40% and the possibility of operation at beam currents beyond the vacuum limit. Our goal is to analyze and optimize interaction between the electron beam and electromagnetic fields in plasma-loaded periodic slow wave structures filled with plasma. In this work, we measured electromagnetic dispersion characteristic of a plasma-loaded corrugated slow wave structure. A hydrogen flashover gun generated a plasma column which was guided by a magnetic field (2-15 kG) and filled a periodic slow wave structure. Since the structure is of finite length, resonances occur only for discrete values of the wavenumbers. Introduction of plasma into the slow wave structure was expected to cause upward frequency shifts of the resonances. The frequency upshifts associated with the TM/sub 01/ mode in the periodic slow wave structure were measured as a function of the background plasma density by a single port (S/sub 11/) method. In order to determine the background plasma density the same technique was used for a smooth wall cavity. We measured frequency shifts on the order of 0.1 GHz around a center frequency of 8.5 GHz for the plasma density of about 10/sup 11/ cm/sup -3/. The obtained resonance frequency upshifts in the plasma-loaded slow wave structure showed good agreement with theoretical calculations.

Summary form only given. A series of experiments were made to characterize the electron component of a transient plasma column immersed in magnetic field. These long plasma columns (radius /spl ap/1 cm and length /spl ap/100 cm) were used to fill the slow wave structure of a relativistic backward wave oscillator (BWO). The addition of plasma increased the efficiency of microwave generation, the operating frequency, and the maximum allowable beam current. The plasma was generated by a coaxial hydrogen flashover gun located just outside the solenoid on the center axis. The plasma electron radial and axial density profiles and the plasma electron temperature were studied over a range of magnetic fields (2-15 kG), gun positions (24-40 cm from the solenoid end), and gun voltages (10-14 kV). This knowledge will allow more precise control of the operating conditions of plasma-loaded BWOs. A single movable Langmuir probe and a cylindrical resonant cavity were used.

Density peaking of a plasma at a low magnetic field is investigated. In this experiment, a helicon plasma source with a 15-cm plasma column diameter is used. An rf power of 200 ∼ 800 W with a frequency of 6 ∼ 14 MHz is fed to a half-turn helical antenna located outside a Pyrex tube plasma enclosure. Experimental results show a loading resistance and an ion density peaking at several tens of gauss, and the peaking magnetic fields of these two parameters are reasonably the same. In addition, the dependency of the peaking magnetic field on the ion density seems to be matched with the dependence of the mode coupling magnetic field on the ion density. Mode coupling of a short-wavelength electrostatic wave called the TG mode and the helicon mode is formed at the corresponding magnetic field when the effect of electron inertia is included in the rf plasma with an axial magnetic field. The experimental result supports that density peaking at a low magnetic field is possibly caused by eigenmode formation around a mode coupling magnetic field. The loading resistance is usually found to be high for the eigenmodes, and a higher loading resistance yields an efficient rf power coupling to the plasma, thus, a higher plasma density. A plasma with a low magnetic field may have advantages, such as a relatively uniform plasma profile and a simple system for magnets and the power supply. This kind of density enhancement at a low magnetic field is, therefore, useful for applications requiring an efficient, uniform plasma.

Low temperature and magnetic field X-ray diffraction study for Nd 0.5Sr 0.5MnO 3

Microwave output power from a high-power helium-filled backward wave oscillator (BWO) operating at X band is measured as a function of the strength of guiding magnetic field B and helium gas pressure. Tenuous helium gas filling the slow wave structure is ionized by an injected electron beam with typical parameters of energy 100 keV and current 0.5 kA. The enhancement of output power from the present gas-filled BWO is less than those reported in some previous publications. Near B=0.8 T, we observe the minimum value of output that is judged to result from cyclotron absorption caused by resonant interaction of the fast electron cyclotron mode with the Cherenkov radiation. On both sides of the absorption, two peak outputs near B=0.5 and 1.0 T are measured. In the former case, the output is enhanced compared with vacuum case by the presence of helium gas with pressure of a few mtorr. In the latter case, on the other hand, the output monotonously decreases with the gas pressure. The output powers for the latter case are larger than those of the former case. Both cases are observed repeatedly for various experimental conditions. The physical reasons for the observed results are discussed.

The combination of hyperpolarization methods with low and inhomogeneous fields promises to expand the application of nuclear magnetic resonance (NMR) to fields that nowadays are not standard. Reducing the magnetic field strengths and homogeneity requirements opens the door to a wide variety of magnet designs that can allow for monitoring chemical reactions in the benchtop or even larger reactors. In this work, we propose a method to monitor 15 N chemical reactions at low and inhomogeneous fields based on parahydrogen‐induced polarization. The concept was demonstrated at a magnetic field of 66 mT and 81 ppm inhomogeneity. Specifically, we measured the conversion of 15N‐boronobenzyl‐2‐styrylpyridinium (15N‐BBSP) to 15N‐2‐styrylpyridin (15N‐SP). This enables the detection of H2O2 with high sensitivity and selectivity, exhibiting a significant chemical shift difference of 88.4 ppm, when 15N‐SP is obtained after a base‐assisted 1,6 elimination/rearrangement. The long T1 of 15N‐BBSP at low magnetic fields (194 s at 100 mT) exemplifies the potential of hyperpolarized 15 N for low field applications. We envision the implementation of our methods for studying chemical reactions in benchtop devices, drug screening, investigating biological systems in their native environments, preclinical research and contrast development for low‐field and portable magnetic resonance imaging (MRI).

A klystron-like relativistic backward wave oscillator (RBWO) with a local decompression magnetic field is presented. There is a local non-uniform magnetic field near the extractor. As the electron beam travels through the RBWO, it will transport upwards and approach the extractor wall, then move downwards, and enter a coaxial collector. Owing to a higher kinetic energy of the electron beam which can be converted into microwave energy efficiently and a stronger electric field that can interact with the electron beam during the beam decompression in the extractor, the efficiency of the klystron-like RBWO is significantly increased to about 65%, compared to 55% with the conventional magnetic field.

Fluxons in stacked Josephson junctions at low magnetic fields

High-power microwaves (20–50 MW, 8.8 GHz) have been produced in repetitive pulses at a rate of 10 Hz in a joint experiment by Harry Diamond Laboratories and Sandia National Laboratories. These microwaves were produced with Harry Diamond Laboratories’ backward-wave oscillator (BWO), driven by a 600 to 800-kV, 2-kA electron beam, which was guided by a magnetic field of variable strength from 0.75 to 1.05 T. The beam was produced with Sandia National Laboratories’ repetitive high-voltage pulser, RAVEN. Microwave diagnostic instrumentation included a directional coupler, heterodyne receiver, dispersive waveguide delay line, and calorimeter. In a train of 10 pulses, the shape, duration, and amplitude of the injection voltage and beam current remained unchanged for each individual pulse. The intensity of the final microwave pulse was 40% of the first pulse, and the final pulse was 80% as long as the first. The basic pulse shape remained unchanged. More intense microwave radiation should be obtained with a higher magnetic field. Computer simulations using the code magic predicted the characteristic dip in the magnetic field strength versus microwave power output curve. Code predictions of microwave power output with a warm beam agree very well with experimental results. Finally, cyclotron resonance is presented in a descriptive fashion to account for the power output dip. This is the first experiment with a repetitively pulsed high-power BWO driven by a relativistic electron beam performed outside the USSR.