Helical Electron Research Articles

Experimental study of the possibility to increase the pitch factor of an intense relativistic helical electron beam

We present the results of experiments on the formation of a high-quality relativistic helical electron beam (HEB) in a magnetron-injection gun. It is shown that suppression of parasitic excitation of microwaves in the input part of the transportation channel allows eliminating high-voltage breakdowns in the gun and achieving greater beam compression. A modulation of the electron beam current at the frequency of longitudinal electron oscillations between the cathode and the magnetic mirror in the trap, which is related to the instability of the helical electron beam, has been observed for the first time. The modulation depth can reach tens percent. Pickup of reflected electrons by a special diaphragm makes it possible to increase the achievable pitch factor, eliminate the beam-current modulation and, as a result, form although with a current loss on the diaphragm, an HEB with record-breaking values of the pitch factor, which exceed 2. For a moderate HEB compression, when the portion of reflected electrons is relatively low, their pickup by the diaphragm allows one to form a beam in which the total energy of the transverse motion of electrons conserves despite the loss of part of the current. After the optimization, a beam with an electron energy of 300 keV, a current of 100 A, a pitch factor of 1.5, and a velocity spread of 20% is obtained for a 15% loss of the current on the diaphragm.

Electron Gun for Powerful Short Pulse Gyrotron with Operating Magnetic Field 8 T

High power short pulse gyrotron with operating frequency 395 GHz operating on the second cyclotron harmonic is now under developing at FIR FU. The gyrotron is planned to use in future experiments for plasma diagnostics. For this purpose the output power about 100 kW and pulse duration 100 ns at least are needed. Preliminary estimations of parameters of some versions of the electron guns with accelerating potential U0 = 70-100 kV were performed. Possibilities of non-adiabatic as well as adiabatic guns were considered. It was shown that non-adiabatic system is not reliable for such rather low value of U0, the adiabatic magnetron injection gun (MIG) is more preferable for the gyrotron design. Analytical estimations of the suitable MIG dimensions and operating regime to form good quality electron beam were fulfilled. Numerical optimization of the gun shape and position was performed. It was shown that in spite of the extremely big ratio of the operating current (I0 = 18 A) to the Langmuir current of the gun, close to 0.4-0.5, the suggested MIG can form the helical electron beam (HEB) which is suitable for gyrotron operation properties.

Experiments on the formation of an intense helical electron beam under conditions of picking-up of the electrons reflected from the magnetic mirror

We propose a method of increasing the pitch factor and decreasing the spread of rotational velocities of helical electron beams (HEBs) formed in nonadiabatic magnetron-injection guns of gyroresonant devices. The method is based on the effect of a special diaphragm mounted at the starting point of the transport channel on the process of formation of the laminar electron beam. The diaphragm located at one of the trajectory minima has such a diameter that it cannot be bent around by electrons with minimum rotational velocities. Such electrons land on it, whereas the remaining electrons pass further, moving in an increasing magnetic field. Then the electrons with the maximum rotational velocities reflect from the magnetic mirror adiabatically and land on the other side of the same diaphragm. Thus, the electron beam in the cavity contains electrons with a smaller resulting spread of rotational velocities. In the region of the HEB formation, the accumulation of the space charge of reflected electrons is eliminated, and the shielding of the electric field at the cathode is reduced, which eventually leads to an increase in the HEB pitch factor. Using such a diaphragm in the regime of current limitation by the space charge, the HEB with a high pitch factor (about 1.4) and a velocity spread acceptable for gyrotron applications (lower than 30%) was formed.

Numerical simulation and experimental study of an electron-optical system of a megawatt gyrotron with step frequency tuning in the range 100–170 GHz

We present the results of numerical simulation and experimental studies of the systems of forming helical electron beams with different topologies for a 1-MW gyrotron with step frequency tuning in the range 100–170 GHz. We analyze variations in the beam parameters including the distribution of electrons over the oscillatory velocities, as functions of the beam current for various accelerating voltages and magnetic fields. The results of experimental studies of a prototype of the multifrequency gyrotron in the oscillation regime such that the designed optimized electron-optical system forms an intense helical electron beam with specified parameters in a wide interval of magnetic fields are shown.

Increasing Power and Efficiency of Gyrotrons

Until recently, the development of new gyrotrons was directed mainly at the increase of their operating frequency, power, and efficiency. The output power of modern continuous-wave (cw) gyrotrons has reached 1 MW, and there is a clear tendency to increase this power further to at least up to 1.5 to 2 MW. The efficiency of the best gyrotron tubes reaches 40% without recovering the residual energy of the spent electron beam [collector potential depression (CPD)] in the continuous regimes and 50% in the pulsed one and achieves 50% with one-step CPD in the cw regimes and near 70% in the pulsed regimes. We analyze limitations of the gyrotron output power and efficiency imposed by systems forming helical electron beams, the cavity interaction processes, the transmitting capability of the output window, and the losses of stray radiation in the built-in converter and power dissipation on the collector (including CPD). Some specific examples in applying the different limits to real cases of gyrotrons are discussed. Ways to enhance the power and efficiency of gyrotrons based on the results of this analysis are shown.

Hot electron plasmas trapped in helical magnetic surfaces

Experimental studies on nonneutrai (pure electron) plasmas of finite temperature, trapped in helical closed magnetic surfaces have been conducted. The helical electron plasmas are produced with thermal electrons launched from the outside of the last closed flux surface (LCFS). About 150 μs after the electron injection, the plasmas reach equilibrium state. Around the LCFS, a steep gradient of plasma space potential ϕs is formed. The corresponding radial electric field is about 2.5 kV/m. On the other hand, around the magnetic axis of helical magnetic surfaces, ϕs is almost constant, indicating that there are little electrons there. The volume-averaged electron density is on the order of 1013 m−3, smaller than the Brillouin density limit. The confinement time seems to be limited by a disruptive instability, and is so far about 1.5 ms.

Experimental study of a powerful magnetron-injection gun for relativistic gyrodevices

We show that it is possible to increase significantly (up to one-half of the Langmuir value) the current of the electron beam in a magnetron-injection gun operated in the nonadiabatic regime of formation of a relativistic helical electron beam if the velocity spread is small. The obtained beam has a current of 200 A, a velocity spread of about 10%, a pitch factor of about 1, and an electron energy of 340 keV.

Development of new generation software tools for simulation of electron beam formation in novel high power gyrotrons

Computer aided design (CAD) based on numerical experiments performed by using adequate physical models and efficient simulation codes is an indispensable tool for development, investigation, and optimization of gyrotrons used as radiation sources for electron cyclotron resonance heating (ECRH) of fusion plasmas. In this paper, we review briefly the state-of-the-art in the field of modelling and simulation of intense, relativistic, helical electron beams formed in the electron-optical systems (EOS) of powerful gyrotrons. We discuss both the limitations of the known computer codes and the requirements for increasing their capabilities for solution of various design problems that are being envisaged in the development of the next generation gyrotrons for ECRH. Moreover, we present the concept followed by us in an attempt to unite the advantages of the modern programming techniques with self-consistent, first-principles 3D physical models in the creation of a new highly efficient and versatile software package for simulation of powerful gyrotrons.

Automodulation onset in a gyro-backward-wave oscillator with external feedback

The onset of multimode oscillation regimes (automodulation) in a helical electron beam-backward electromagnetic wave system (gyro-backward wave oscillator, gyro-BWO) with external feedback has been studied. Thresholds for the self-excitation and the onset of multimode oscillations in the gyro-BWO significantly depend on the feedback parameters and are determined by the quality and the natural frequencies of a resonator formed by an electrodynamic system of the gyro-BWO and the delay-feedback chain.

On potentials of gyrotron efficiency enhancement: measurements and simulations on a 4-mm gyrotron

In a 74.2-GHz 100-kW pulse gyrotron, the characteristics of parasitic low-frequency oscillations and their influence on electron energy spread have been determined. A method for suppressing parasitic oscillations by varying the magnetic field distribution in the beam compression region is studied. The results of measurements demonstrate the influence of azimuthal inhomogeneity of electron emission on the excitation of parasitic oscillations and on the gyrotron output parameters. Obtained data allows one to determine the ways of gyrotron efficiency enhancement by improved quality of the helical electron beam

Study of the magnetic analyzer of relativistic helical electron beams

The paper presents the results of studying the analyzer of helical electron beams experimentally. The analyzer is based on separation of electron velocity fractions in the adiabatically growing magnetic field. Optimization of the analyzer design to minimize the amount of electrons reflected from the analyzer elements made it possible to practically eliminate the systematic error in measurements of the velocity spread and pitch-factor.

Electron-Optical System of a High-Power Gyrotron with Nonadiabatic Electron Gun

We describe a nonadiabatic electron gun operated in the mode of a currentconfined by spatial charge and intended for producing helical electron beams for high-power gyroresonantdevices.Results of numerical simulation of electron trajectories and electron-beam parameters arepresented. It is shown that for a high operating current, the pitch factor and the electronvelocity spread can be significantly better than in the conventional adiabatic systems.

Development of a Prototype of a 1-MW 105-156-GHz Multifrequency Gyrotron

We present the results of studying the possibility of numerical and experimental optimization of the helical electron beam (HEB) formation system and the processes of mode interaction in the electron beam – cavity system for a prototype of a 1-MW 105–156-GHz step-tunable gyrotron with various operating modes. The system parameters are optimized to achieve the maximum efficiency of the gyrotron with an acceptable ohmic load in the cavity. We also analyze the influence of mode competition and depression of the electron-beam potential as well as the possibility of energy recovery of the collector electron beam. The possibility of optimizing the mode converter system for various operating modes is demonstrated.

Transient Processes in the Distributed Active Medium “Helical Electron Beam–Counterpropagating Electromagnetic Wave”

UDC 621.385 In this paper, we study the transient processes in a nonautonomous distributed active medium formed by a gyro-counterpropagating-wave oscillator (gyro-CWO) synchronized by an external signal. We analyze how the time required to achieve the synchronous mode depends on the ratio between the external-signal phase and the self-oscillation phase. The characteristics of transient chaos in a gyro-CWO near the synchronization area are also examined.

Low-Frequency Parasitic Space-Charge Oscillations in the Helical Electron Beam of a Gyrotron

We developed a medium-power pulsed gyrotron equipped with diagnostic systems for studying low-frequency parasitic space-charge oscillations in the helical electron beam and the energy spectrum of electrons in the collector region. Methods for suppressing parasitic oscillations are proposed and tested. An explanation of the mechanism of suppression of these oscillations is given.

Formation and Diagnostics of Intense Relativistic Helical Electron Beams for Gyrotrons

We develop a magnetron-injection gun for a 1-cm wavelength gyrotron, which allows us to increase the operating current up to 50% of the Langmuir current of the gun by reducing the time required to bring electrons to the high-potential region. To measure the velocity distributions in helical electron beams for energies of several hundreds of keV, we propose, develop, and study a new electron-velocity distribution analyzer based on the phenomenon of electron reflection from an adiabatic magnetic mirror. The results of our study demonstrate that the gun is capable of forming high-quality intense beams suitable for generating microsecond microwave radiation with power higher than 10 MW in gyrotrons.

Electron Gun for Powerful Large Orbit Gyrotron

In the article, results of numerical simulation of the gun with the cusp of magnetic field are presented. Short pulse version of the gun with explosion emission is investigated. Some preliminary analytical estimation of the beam and gun parameters are performed. Then, numerical optimization of the electrodes shape as well as magnetic field distribution is carried out. For preliminary separation of electrons and formation of the rectilinear beam, anode diaphragm is installed. After then, additional selection of electrons for decreasing the ripple is performed. For this purpose, channel walls are used for interception of some part of the electron beam. Reverse of the magnetic field in the diode part of the gun is formed. So, the formation of the rectilinear beam is combined with the region, where electrons obtain initial gyration energy. To prevent the disperse action of the own beam space charge forces, the system with big gradient of magnetic field (about 0.5-1 kGs/mm) is needed. According to results of the simulation, helical electron beam can be performed even at total compression ratio about 1000 and current density more than 50 kA/cm2. The designed electron gun provides acceptable performance for the large orbit gyrotron, such as operating current close to 300 A, the pitch-factor value about 1.5-1.7, deviation of the guiding centers from the axis (the ripple) λ/10 and λ/6 for operation on 3-rd and 5-th cyclotron harmonic correspondingly (wavelength λ=0.5 and 0.3 mm) and velocity spread within the range 10-15%.

Unified model of electron beam shot noise and coherent spontaneous emission in the helical wiggler free electron laser

A numerical algorithm for simulating electron beam shot noise in free electron lasers (FELs) is presented. Shot noise is a source of spontaneous emission that may be amplified in the self-amplified spontaneous emission regime of operation. This regime is of great importance to XUV and x-ray FELs where the spontaneous emission is currently the only effective source available for amplification. The algorithm uses a quasiuniform phase-space distribution of appropriately charge weighted macroparticles. The statistical properties of the macroparticles are derived directly from the temporal Poisson statistical properties of the real electron distribution. Unlike previous algorithms, ours does not rely upon any averaging over a resonant radiation period time scale and so more correctly describes the underlying physics. The algorithm also allows shot noise to be modeled self-consistently in unaveraged FEL models which are able to describe subwavelength phenomena such as coherent spontaneous emission (CSE). The algorithm is used in the unaveraged 1D FEL numerical simulation code FemFel and demonstrates spontaneous emission due to shot noise and CSE in both rectangular and Gaussian electron pulse current profiles. The preliminary results show good qualitative and quantitative agreement with theory.

Signal amplification in a gyro-backward-wave tube

The process of signal amplification in an active medium of the helical electron beam-backward electromagnetic wave type (gyro-backward-wave tube) has been theoretically analyzed. The same process has been studied in gyro-backward-wave tube with an electrodynamic structure representing coupled waveguide systems. © 2003 MAIK "Nauka/Interperiodica".

On the ultrafast synchronization of oscillations in a distributed active medium formed by a helical electron beam and a counterpropagating electromagnetic wave

Nowadays, problems related to synchronization of self-oscillations in distributed self-oscillatory systems of an electronic nature have become of urgent interest from different points of view. Analysis of these problems is important from both the standpoint of fundamental studies of synchronization phenomena and the applied aspects of developing microwave generators with controlled characteristics (frequency, output power, phase) of the output radiation. In [1–3], nonautonomous phenomena in certain reference distributed models of microwave electronics and nonlinear oscillation theory and wave theory were analyzed. We imply an electron beam carrying a supercritical electric current or an electron beam with a backward (counterpropagating) wave in cyclotron-resonance conditions [3]. Presently, such a system (a laser based on the cyclotron resonance with a counterpropagating wave) is being intensely investigated as a promising easily adjustable powerful radiation source in the millimeter and submillimeter wavelength ranges [4].