Gyrotron Radiation Research Articles

He2+ source based on Penning-type discharge with electron cyclotron resonant heating by millimeter waves

Electron cyclotron resonance (ECR) ion sources are particularly useful for nuclear, atomic and high energy physics, as unique high current generators of multicharged ion beams. Plasmas of gas discharges in open magnetic traps heated by pulsed high power high-frequency microwaves of gyrotrons are promising in the field of research in the development of ECR sources for high charge state ion beams. It is necessary to decrease pressure and increase plasma density to reach higher average charge of ions in the plasma. The ECR discharge could be ignited at very low gas pressure, but it takes longer time for the discharge start-up. Hence, it is impossible to realize the ECR discharge with limited microwave heating pulse duration at gas pressure below a certain threshold value. We suggest a Penning-type discharge as a trigger discharge for fast development of pulsed-ECR plasma. The Penning-type discharge glows at a low pressure as needed. Gyrotron radiation (75 GHz, 200 kW, 1 ms) was used for plasma heating. Fully striped helium ions were demonstrated, average charge of ions in the plasma was ≈2. Temporal evolution of charge state distribution as a function of helium pressure and microwave power was investigated.

ECR Breakdown of Heavy Gases in Open Mirror Trap

Present work is devoted to experimental demonstration of possibility of short pulsed (<100 μs) multicharged ion beams creation. Two regimes of short pulsed beams generation are discussed: quasi-stationary and non-stationary in preglow regime.Experiments with ECR discharge stimulated with gyrotron radiation @ 37.5 GHz, 100 kW were performed to reach the minimum duration of the pulse.In quasi-stationary regime pulses with duration of 50 μs and more were obtained. “Preglow” effect was also observed and investigated in experiments. Received dependencies of the “Preglow” parameters are in good correspondence with results of numerical simulations. It was shown in experiments that generation of “Preglow” peak with duration about 20 μs is possible.

Experimental study of a continuous-wave high-stability second-harmonic gyrotron for spectroscopy of dynamically polarized nuclei

We present the results of the experiments on a continuous-wave gyrotron with an output frequency of about 260 GHz, which has been developed for dynamic polarization of nuclei and is operated at the second cyclotron-frequency harmonic. For an accelerating voltage of 16 kV and a beam current of 0.3 A, a maximum continuous-wave radiation power of about 200 W has been obtained. Measurements of the thermal load on the resonator cavity allow evaluating its actual ohmic Q-factor. Gyrotron radiation is also observed at other frequencies and modes. Their generation zones and the structure of the output radiation have been studied. Long-term stability of gyrotron parameters has been determined.

Generation of a high-current beam of multiply charged ions from a dense plasma produced by high-power millimeter-wave gyrotron radiation under ECR conditions

Experimental data for generating intense beams of multiply charged heavy gas ions are reported. The beams are extracted from the dense plasma of an ECR discharge with quasi-gasdynamic confinement provided by the powerful microwave radiation of a gyrotron in a magnetic trap. The axisymmetric magnetic trap makes it possible to extract uniform (in transverse coordinates) plasma fluxes from it, and the use of a multiple-aperture extraction system makes it possible to increase the beam current by more than an order of magnitude compared with the ECP sources currently available. Multiply charged nitrogen ion beams with a current of 150 mA and a normalized emittance of 0.9π mm mrad are obtained.

Effect of ECRH regime on characteristics of short-wave turbulence in plasma of the L-2M stellarator

This paper reports on studies of short-wave turbulence in the plasma of the L-2M stellarator under markedly different conditions: with doubling the ECR heating power (100 and 200 kW) and with restricting the plasma radius by a sector limiter. The role of such short-wave turbulence in anomalous transport can appear important for conditions of a thermonuclear reactor. Experiments were carried out in a basic magnetic configuration of the L-2M stellarator during ECRH at the second harmonic of the electron gyrofrequency (75.3 GHz) at average electron densities of (1.5–1.7) × 1013 cm−3. The energy confinement time was ∼3.5 ms at P0 = 100 kW and was reduced to ∼2 ms at P0 = 200 kW. When the limiter was introduced inside the plasma to a depth of 2 cm from the last closed flux surface, τE decreased by a factor of 1.3–1.4. Plasma density fluctuations were measured from the scattering of gyrotron radiation at the second harmonic of operating frequency (∼150 GHz). A quasioptical receiving system allowed measurements of scattered radiation from plasma regions r/a ⩽ 0.6 at scattering angles π/4 ⩽ Θ ⩽ π/2 (24 cm−1 ⩽ k⊥ ⩽ 44 cm−1). The short-wave turbulence was studied for two radial positions of the scattering region: r/a = 0.3–0.4 and r/a = 0.5–0.6. Short-wave turbulence exhibits features of strong plasma turbulence. It is experimentally established that a change in the energy confinement time in the L-2M stellarator correlates with the level of short-wave turbulence.

Glow plasma trigger for electron cyclotron resonance ion sources

Electron cyclotron resonance ion sources (ECRISs) are particularly useful for nuclear, atomic, and high energy physics, as unique high current generators of multicharged ion beams. Plasmas of gas discharges in an open magnetic trap heated by pulsed (100 micros and longer) high power (100 kW and higher) high-frequency (greater than 37.5 GHz) microwaves of gyrotrons is promising in the field of research in the development of electron cyclotron resonance sources for high charge state ion beams. Reaching high ion charge states requires a decrease in gas pressure in the magnetic trap, but this method leads to increases in time, in which the microwave discharge develops. The gas breakdown and microwave discharge duration becomes greater than or equal to the microwave pulse duration when the pressure is decreased. This makes reaching the critical plasma density initiate an electron cyclotron resonance (ECR) discharge during pulse of microwave gyrotron radiation with gas pressure lower than a certain threshold. In order to reduce losses of microwave power, it is necessary to shorten the time of development of the ECR discharge. For fast triggering of ECR discharge under low pressure in an ECRIS, we initially propose to fill the magnetic trap with the plasmas of auxiliary pulsed discharges in crossed ExB fields. The glow plasma trigger of ECR based on a Penning or magnetron discharge has made it possible not only to fill the trap with plasma with density of 10(12) cm(-3), required for a rapid increase in plasma density and finally for ECR discharge ignition, but also to initially heat the plasma electrons to T(e) approximately = 20 eV.

Axisymmetric multistage cavity resonators

We study theoretically multistage cavity resonators as well as the gyrotrons manufactured on their basis. Coupled cavities with mode conversion and echelette cavities are particular cases of such devices. The possibilities of reducing the diffraction Q-factor of the operating mode of a cavity, with the preservation of its selective properties and significant increase in the gyrotron radiation power, and provision of high efficiency and acceptable heat ohmic load of the cavity at the first harmonic of the gyrofrequency are considered. Achievable parameters of the gyrotrons at the second and third harmonics with typical electron guns and cryomagnetic systems are analyzed.

Collective backscattering of gyrotron radiation by small-scale plasma density fluctuations in large helical device

A version of the collective backscattering diagnostic using gyrotron radiation for small-scale turbulence is described. The diagnostic is used to measure small-scale (k(s) approximately 34 cm(-1)) plasma density fluctuations in large helical device experiments on the electron cyclotron heating of plasma with the use of 200 kW 82.7 GHz heating gyrotron. A good signal to noise ratio during plasma production phase was obtained, while contamination of stray light increased during plasma build-up phase. The effect of the stray radiation was investigated. The available quasioptical system of the heating system was utilized for this purpose.

Thermometry of microwave discharge in powder mixtures by the thermal radiation spectrum

The temperature is determined in pulsed microwave discharges in powder mixtures at atmospheric pressure. The effect of microwave radiation of gyrotron (wavelength of 4 mm, power up to 200 kW, pulse duration of 1 to 10 ms) on a finely divided medium causes the emergence of plasma in a gas medium between solid particles; the thresholds (with respect to power) of generation of plasma in powder mixtures are hundreds of times lower than those in gas. The method of determining the discharge temperature consists in recording the radiation in a wide spectral range (200 to 850 nm) and comparing the obtained spectrum with the Planck spectrum in the Wien region. In the case of similarity of two spectra, the discharge temperature is determined as a parameter of the spectrum under observation. In so doing, no data on the emissivity of discharge are required for determining the temperature. It is demonstrated that microwave-discharge plasma in powders is characterized by a temperature of 2000–3000 K.

High current multicharged metal ion source using high power gyrotron heating of vacuum arc plasma

A high current, multi charged, metal ion source using electron heating of vacuum arc plasma by high power gyrotron radiation has been developed. The plasma is confined in a simple mirror trap with peak magnetic field in the plug up to 2.5 T, mirror ratio of 3-5, and length variable from 15 to 20 cm. Plasma formed by a cathodic vacuum arc is injected into the trap either (i) axially using a compact vacuum arc plasma gun located on axis outside the mirror trap region or (ii) radially using four plasma guns surrounding the trap at midplane. Microwave heating of the mirror-confined, vacuum arc plasma is accomplished by gyrotron microwave radiation of frequency 75 GHz, power up to 200 kW, and pulse duration up to 150 micros, leading to additional stripping of metal ions by electron impact. Pulsed beams of platinum ions with charge state up to 10+, a mean charge state over 6+, and total (all charge states) beam current of a few hundred milliamperes have been formed.

Gyrotron Radiation Affected by a Controlled Modulated Reflector: High Power Experiment

An experiment performed with the gyrotrons of the TJ-II ECRH system has been performed to influence the gyrotron output radiation with a relatively weak wave returned from an external mechanically modulated reflector. An original method is proposed for directly measuring the wavelength of electromagnetic radiation in the free space. The method has been demonstrated in measurements of the wavelength of a high-power gyrotron radiation in the millimeter wavelength range used for plasma ECR heating in the TJ-II stellarator.

Gasdynamic ECR source of multicharged ions based on a cusp magnetic trap

One of the modern trends in the development of electron cyclotron resonance (ECR) sources of multicharged ions is enhancement of the power and frequency of microwave pumping. Therefore, gyrotrons—powerful sources of radiation in the millimetre wavelength range—are now used for the creation and heating of plasma. These generators of microwave radiation are capable of producing and confining plasma of very high density (1013 cm−3 and higher), thus providing conditions for a substantial increase in the extracted ion beam current. Most ECR sources use for plasma confinement mirror magnetic traps with ‘min B’ configuration, which suppress MHD plasma instabilities. However, it is extremely difficult to construct such systems for pumping frequencies higher than 30 GHz because strong magnetic fields of complicated configuration are needed. All this makes the search for simpler axisymmetric MHD-stable systems for plasma confinement very topical.A cusp trap produced by two coils with opposite currents is the simplest MHD-stable magnetic trap. Magnetic lines at any point of plasma in such a trap have a curvature that suppresses MHD perturbation in the plasma. In the present work the possibility of creation of an effective ECR source of multicharged ions based on a cusp magnetic trap was investigated numerically and in experiments. The pioneer result is the realization of the quasi-gasdynamic regime of plasma confinement in the trap of the multicharged ion ECR source of multicharged ions with cusp configuration of the magnetic field. Experiments were made with a small cusp trap which was designed for plasma creation by 37.5 GHz gyrotron radiation under ECR conditions. The total current and ion charge state distribution in the extracted ion beam were studied. The results of experiments with the simplest kind of cusp trap have demonstrated good correspondence with theoretical calculations, and therefore the adequacy of the developed approach and the possibility to build more effective source on this basis. Two ways of possible evolution of ECR ion sources based on a cusp magnetic trap are proposed.

Generation of multiply charged refractory metals in an electron-cyclotron resonant discharge in a direct magnetic trap

The possibility of additional ionization of refractory metal ions in the vacuum arc plasma injected to a magnetic trap due to additional heating of electrons by microwave radiation under the conditions of electron-cyclotron resonance is demonstrated. High-power short-wave radiation of gyrotrons used in experiment makes it possible to work with a higher (on the order of 1013 cm−3) density of the plasma and to ensure the confinement parameter at a level of 3 × 108 cm−3 s at an electron temperature sufficient for multiple ionization.

Multicharged Ion Generation in Plasma Created by Millimeter Waves and Confined in a Cusp Magnetic Trap

Modern way of ECR multicharged ion sources development is connected with increasing of microwave frequency up to tens of GHz. Millimeter wave gyrotrons are used now at a few laboratories. Traditional mirror magnetic traps with min B configuration for suppressing of MHD instabilities became too expensive because very strong magnetic field is needed. So prospection of axisymmetric plasma trap with MHD stabilization is a quite relevant subject of modern plasma physics. The simplest trap of such kind is CUSP one. The zero level of magnetic field in the center of the trap doesn’t allow to achieve a good confinement in classical regime. In a gasdynamic regime of plasma confinement was studied. The same regime may be realized in the CUSP geometry trap if plasma density is high enough. A small CUSP trap was designed for plasma creation by 37,5 GHz gyrotron radiation under ECR conditions. Temporal evolutions of plasma parameter in experiments were quite smooth and stable. Experimental ion charge state distribution corresponds with calculated one for optimal plasma conditions.

Local enhanced evaporation of carbon pellets in a Wendelstein 7-AS stellarator

Experiments with second-harmonic electron cyclotron resonance plasma heating performed on a Wendelstein 7-AS stellarator revealed a new regime of the carbon pellet evaporation, whereby sharp peaks with the characteristic spatial width of a few millimeters appeared on the evaporation rate profile. The peaks were observed in a peripheral region of the plasma column for an electron density in the central region below and the heating power above the threshold values. The appearance of these peaks is correlated with the observation of a superthermal emission in the low-frequency part of the cyclotron radiation spectrum. The observed effect is related to the excitation of superthermal electrons by the cyclotron radiation of gyrotrons. The energy and density of the superthermal electron population are estimated at 30–50 keV and 1011–109 cm−3, respectively.

Second harmonic of gyrotron radiation: New potentialities of plasma diagnostics

It is shown that the relative intensity of the second harmonic of gyrotron radiation on the axis of a microwave beam after quasi-optical filtering in a four-mirror quasi-optical transmission line is about −50 dB of the total radiation intensity. The second-harmonic radiation is used in collective-scattering diagnostics of turbulent density fluctuations in the plasma column of the L-2M stellarator. At an electron temperature of 0.8–1.0 keV and average plasma density of 2.0–2.5×1013 cm−3 (a plasma energy of about 0.6 kJ), which was achieved after the boronization of the vacuum chamber, spatiotemporal structures in plasma density fluctuations were observed in the central region of the plasma column. The correlation time between the structures was found to be on the order of 1 ms. It is shown that the spectrum of the signal from the second-harmonic scattering extends to higher frequencies compared to that from the fundamental-harmonic scattering.

Studies of fluctuations in the high-temperature plasma of modern stellarators by the microwave scattering technique

Microwave scattering diagnostics are described that allow direct measurements of the turbulent processes in the high-temperature plasma of magnetic confinement systems. The first physical results are presented from fluctuation measurements carried out in 2000–2001 in three stellarators: L-2M (Institute of General Physics, Moscow), LHD (National Institute of Fusion Science, Toki), and TJ-II (CIEMAT, Madrid). Plasma density fluctuations in the axial (heating) regions of the L-2M and LHD stellarators were measured from microwave scattering at the fundamental harmonic of the heating gyrotron radiation. In the TJ-II stellarator, a separate 2-mm microwave source was used to produce a probing beam; the measurements were performed at the middle of the plasma radius. Characteristic features of fluctuations, common for all three devices, are revealed by the methods of statistical and spectral analysis. These features are the wide frequency Fourier and wavelet spectra, autocorrelation functions with slowly decreasing tails, and non-Gaussian probability distributions of the magnitudes and the increments in the magnitude of fluctuations. Observations showed the high level of coherence between turbulent fluctuations in the central region and at the edge of the L-2M plasma. The drift-dissipative instability and the instability driven by trapped electrons are examined as possible sources of turbulence in a high-temperature plasma.

Design of correcting mirrors for a gyrotron used at Large Helical Device

We report the design of a mirror system used at the Large Helical Device (LHD) to convert the output radiation of the CPI 84-GHz gyrotron into the HE 11 mode of a corrugated waveguide transmission line. The radiation pattern of the gyrotron is measured using an IR camera at LHD. The measured data is analyzed at Massachusetts Institute of Technology (MIT) to retrieve the phase distribution of the radiation. The moments of the measured amplitude arrays are calculated to improve the reliability of the IR image data. The retrieved radiation structure is treated to synthesize the correcting mirrors.

Design of a system converting an output radiation of frequency tunable gyrotron into a gaussian beam

A novel design of a versatile quasi-optical system for conversion of gyrotron radiation into collimated gaussian beams is presented and discussed. The proposed system consists of a quasi-optical antenna, two ellipsoidal mirrors and a spatial filter which truncates the sidelobe radiation. The system is appropriate as a transmission line for frequency tunable gyrotrons operating at TE0n and TE1n modes. As an illustration of our approach, we present results which demonstrate the applicability of the developed system for conversion of the radiation generated by the Gyrotron FU IVA. The examples include conversion of four TE1n mode outputs (TE12, 170 GHz; TE13, 271 GHz; TE14, 372 GHz; TE15, 472 GHz) into gaussian-like beams and three TE0n modes (TE02, 223 GHz; TE03, 323GHz; TE04, 423 GHz) into bigaussian-like beams.

Formation of Multicharged Ions in Plasma of ECR Discharge Sustained by Powerful Millimeter Wave Radiation in a Mirror Trap1

Essential increase of operation efficiency of electron cyclotron resonance (ECR) multi-charged ion (MCI) sources (that is the increase of the average charge number and current of ion beams) is connected with possibility to achieve more dense plasma and to achieve so called quasi-gasdynamic regime of the plasma confinement in mirror traps using RF radiation with higher frequency. Results of experimental investigation of ion charge state distribution in a pulsed ECR source of MCI with pumping by millimeter wave radiation of powerful gyrotron are presented in the paper. Millimeter wave radiation with maximum power W=130 kW, frequency f=37.5 GHz and pulse duration up to 1.5 ms is used in the experiments. The charge state distribution maximum corresponded to argon charge (+13)-(+15). Total ion saturation current density of the probe achieved 2 A/cm2.