Open Magnetic Trap Research Articles

Improved axial confinement in the open trap by the combination of helical and short mirrors

The paper presents experimental results from the SMOLA device, which was built in the Budker Institute of Nuclear Physics for the verification of the helical mirror confinement idea. This concept involves active control of axial losses from the confinement zone in an open magnetic trap through the use of multiple mirrors that move in the plasma frame of reference. The discussed experiments focused on determining the cumulative effect of a helical mirror system in combination with a short segment of a stronger magnetic field. Combination of these two methods of axial flow suppression results in higher efficiency compared with each method individually. Different combinations of the mirrors were tested. The most effective flow suppression was observed if the short mirror was placed between the confinement region and the helical mirror. In this configuration, an effective mirror ratio of $R_{{\rm eff}} = 32.6\pm 7.8$ was achieved, along with a more than three-fold increase in plasma density within the confinement region. The possibility of a cumulative effect of different types of magnetic mirrors offers a way to improve the confinement performance of the reactor-grade mirror confinement devices.

Experimental issues of energy balance in open magnetic trap

The paper presents an overview of experimental results of an investigation of different energy loss channels in the gas dynamic trap (GDT), which is a magnetic mirror plasma confinement device in the Budker Institute of Nuclear Physics. Energy losses along magnetic field lines are considered as well as losses onto radial limiters, which restrict the plasma column radius and provide its magnetohydrodynamic stability via the ‘vortex confinement’ mechanism. The losses along the field lines were measured using a set of pyroelectric bolometers on the plasma absorber and the losses onto the limiters were determined with thermistors from their temperature rise. Additionally, the losses due to charge exchange of fast plasma ions on the residual neutral gas in the GDT were measured using a longitudinal array of pyroelectric bolometers mounted on the wall of the central cell. An attempt was made to draw up the energy balance in the GDT in order to identify the predominant loss channels and reduce those losses in the future.

Optimization of the collective Thomson scattering diagnostic for the GDT open magnetic trap

The collective Thomson scattering of high-power millimeter-wave radiation on plasma density fluctuations (CTS) is widely used for diagnosing the velocity distribution function of energetic ions in fusion plasma. In this paper, we discuss a non-standard scheme of CTS measurements, which exploits strong refraction of probing and scattered microwave beams when reflecting from the cut-off layer in dense plasma. The scheme may be realized with the existing CTS diagnostic system at the Gas-Dynamic Trap (GDT) facility, a large open magnetic trap operating at Budker Institute (Novosibirsk, Russia). This requires a minor upgrade of the available hardware and essentially increases the range of plasma densities allowed for CTS measurements, as well as its sensitivity and spatial resolution. A detailed study of CTS efficiency for different geometries and plasma conditions at GDT is performed by means of an advanced numerical model that allows for an accurate description of non-Gaussian beam scattering in inhomogeneous plasma. To perform this task, we develop a quasi-optical theory of scattering, which itself may be of general interest.

On matching a microwave radiation with the plasma of an electron cyclotron resonance discharge

This article proposes a scheme for matching microwave radiation with a magnetized plasma of an electron cyclotron resonance (ECR) discharge in an open magnetic trap, using the effect of depolarization—the transformation of the ordinary wave into the extraordinary one upon reflection from the metal walls of a plasma chamber. An analysis of the efficiency of such a transformation is carried out, and it is shown that for grazing (less than 15° with metal surface) angles of incidence; it is possible to achieve an efficiency exceeding 90%.

Mathematical model of matter transfer in a helical magnetic field using boundary conditions at infinity

The paper presents a mathematical model of plasma transfer in an open magnetic trap using the condition of zero plasma concentration at infinity. New experimental data obtained at the SMOLA trap at the Budker Institute of Nuclear Physics SB RAS were used. Plasma confinement in the plant is carried out by transmitting a pulse from a magnetic field with helical symmetry to a rotating plasma. The mathematical model is based on a stationary plasma transfer equation in an axially symmetric formulation. The stationary equation of the transfer of matter contains second spatial derivatives. The optimal template for the approximation of the mixed derivative based on the test problem is selected. The numerical implementation of the model by the establishment method and the method of successive over-relaxation is compared.

Fast-Ion Diagnostics with Collective Thomson Scattering of Microwave Radiation for the GDT Open Magnetic Trap

Fast-Ion Diagnostics with Collective Thomson Scattering of Microwave Radiation for the GDT Open Magnetic Trap

First results of collective Thomson scattering diagnostic of fast ions at the GDT open magnetic trap

Fast-ion diagnostic based on measuring of the collective Thomson scattering of high-power gyrotron radiation has been developed for the gasdynamic trap facility in Budker Institute during the last 3 years. This diagnostic is aimed at reconstruction of ion distributions over transverse and longitudinal velocities driven in the plasma core by strong neutral beam injection. Here, we present the first experimental results which may be interpreted as the expected scattering signal from fast ions. The detected spectra with the effective noise temperature of about 100 eV detuned in a particular frequency range of 100–300 MHz from the probe gyrotron frequency are compatible with the early theoretical predictions.

Low Frequency Oscillations of Plasma Injected into an Open Magnetic Trap from an Independent Ultrahigh Frequency Source

Methods of plasma heating by electromagnetic waves, interaction of electro-magnetic waves with magnetoactive plasma, plasma turbulence and transport processes are investigated in the trap of the linear plasma device OMT2. An ultrahigh frequency (UHF) contactless method is used to fill the open magnetic trap by plasma. We propose a new method of open magnetic trap filling with plasma whereby plasma is injected in the trap along the magnetic field from an independent stationary UHF source. The source is located outside the trap and plasma formation in it takes place in the strongly nonuniform magnetic field, in the electron cyclotron resonance (ECR) regime by means of the UHF power. In this paper we study the efficiency of open magnetic trap filling by plasma and plasma low frequency (LF) oscillations in the trap.

Multimode Regime of Generation of Microwave Discharge Plasma in an Open Magnetic Trap

Multimode Regime of Generation of Microwave Discharge Plasma in an Open Magnetic Trap

Fast-Ion Diagnostics With Collective Thomson Scattering of Microwave Radiation for the GDT Open Magnetic Trap

Fast-Ion Diagnostics With Collective Thomson Scattering of Microwave Radiation for the GDT Open Magnetic Trap

Mathematical modelling of beam dynamics in diamagnetic confinement regime of open trap

The paper is devoted to the mathematical modelling of a beam injected into an open magnetic trap in the diamagnetic mode. The two-dimensional axially symmetric hybrid numerical model with the kinetic approximation for the ion component of the plasma and the MHD approximation for magnetized electrons is used. In the numerical experiments, the possibility of formation of the cavity in the magnetic field was shown. A detailed study of the evolution of the ion beam continuously injected into the trap is done, the dependencies on time, moment of injection and beam temperature are presented. The obtained results may be used for the data analysis of the laboratory experiments in open magnetic systems.

Charge exchange radiation diagnostic with gas jet target for measurement of plasma flow velocity in the linear magnetic trap

The study of longitudinal fluxes of energy and particles is one of the most important fundamental problems in the physics of open magnetic traps. The solution to this problem is the key to the successful implementation of fusion power in linear magnetic systems. The plasma electrostatic ambipolar potential largely determines the physics of longitudinal transport of particles and energy. In this work, we used the spectroscopic method CXRS (Charge eXchange Recombination Spectroscopy) to measure the plasma ion velocity distribution via the analysis of light emitted in this atomic process. The experiments were carried out in the gas dynamic trap (GDT), which is a linear plasma confinement system with an axially symmetric magnetic field configuration. We measured plasma potential and ion temperature of two plasma components: the main (hydrogen and deuterium) and the helium impurity. The paper presents the emission spectra of the lines H-α (656.28 nm) and He-I (667.8 nm). This method was applied for the first time to measure the spatial profile of the ambipolar potential in an open magnetic trap.

Development of fast-ion collective Thomson scattering diagnostics for the GDT experiment

In this paper, we describe hardware developed for the collective Thomson scattering (CTS) diagnostic for the large open magnetic trap GDT (Budker Institute, Russia) used in fusion researches. The diagnostic utilizes 400 kW/54.5 GHz gyrotron as a source of probe radiation and is aimed at the reconstruction of fast ion distributions over transverse and longitudinal velocities driven by high-power neutral beam injection. Formation and manipulation of the probe beam and collection of the scattered microwave radiation are done with large-aperture quasi-optical mirror systems. The receiving complex consists of two identical channels for simultaneous detection of scattering in two complement geometries. Each channel includes a highly sensitive radiometer/spectrum analyzer with the band of 54.470 ± 0.550 GHz, the noise temperature of 0.35 eV, the sensitivity less than 0.025 eV, the frequency/time resolution of 5–50 MHz/10 μs, and narrow-band notch filter providing at least -40 dB rejection in the band of 25 MHz with a central frequency tunable in 54.470 ± 0.050 GHz for protection of the input RF circuit from a backlight of powerful gyrotron radiation.

Recovery of the Two-Dimensional Ion Distribution Function in a Magnetic Mirror from Measurements of Collective Thomson Scattering Spectra

A method is proposed for tomography of the distribution function of energetic ions that are adiabatically trapped in an open magnetic trap, according to the diagnostic data by the method of collective Thomson scattering. This method is based on measurements of the scattering spectra from successive plasma cross sections corresponding to different values of the magnetic-field strength along a single line of force. It is shown that the problem of restoring the ion distribution function in the velocity space from the measurement data in this situation is reduced to an integral equation of the first kind that allows an analytical solution. Several ways to construct exact and approximate solutions of the resulting integral equation are considered.

Long-pulse plasma source for SMOLA helical mirror

A plasma gun for forming a plasma stream in the open magnetic mirror trap with additional helicoidal field SMOLA is described. The plasma gun is an axisymmetric system with a planar circular hot cathode based on lanthanum hexaboride and a hollow copper anode. The two planar coils are located around the plasma source and create a magnetic field of up to 200 mT. The magnetic field forms the magnetron configuration of the discharge and provides a radial electric insulation. The source typically operates with a discharge current of up to 350 A in hydrogen. Plasma parameters in the SMOLA device are Ti ~ 5 eV, Te ~ 5–40 eV and ni ~ (0.1–1) × 1019 m−3. Helium plasma can also be created. The plasma properties depend on the whole group of initial technical parameters: the cathode temperature, the feeding gas flow, the anode-cathode supply voltage and the magnitude of the cathode magnetic insulation.

Performance improvement of particle-in-cell method for numerical modelling of open magnetic system

The work is connected with numerical simulation of plasma dynamics in open magnetic trap in diamagnetic regime. The hybrid particle-in-cell model we develop allows to perform numerical experiments for the high ratio of Larmor radius of the ions and the one of the electrons due to the combining of the kinetic description for the ions and MHD description for the electron plasma component. The disadvantage of the model is the stability condition and the corresponding requirements for the time step. In practice a doubling of the grid nodes in each direction leads to the decrease of the time step x6 times. For the characteristic times of the plasma processes 102 reciprocal ion cyclotron frequencies the computations required few days for grids 100x500. The processing of the particle data takes more than 85% of the total computation time, thus its effective realization yields significant gain in performing. In our algorithm we combine the dynamic load balance and vectorized computations of densities and current densities. We present the results of numerical experiments on its basis including the highly non-uniform particle distribution in the domain and the increasing of the particle number due to the beam injection.

Numerical study of diamagnetic regime in open magnetic trap

The article presents a two-dimensional axially symmetric numerical model of a diamagnetic plasma confinement regime in a linear magnetic system as applied to laboratory experiments at the BINP SB RAS, Novosibirsk. The model is based on the kinetic approximation for the ion components of the background plasma and the injected beam. The MHD approximation is used for the electron component, taking into account the mechanisms of energy dissipation due to conductivity and thermal conductivity. The particle-in-cell method is used to solve the Vlasov equation. The dependences of the magnetic field profile and plasma density on the current of the ion beam and its energy are found.

Граничные условия в источнике электронного пучка на поверхности анодной плазмы со сверхзвуковым потоком ионов

A solution for the one-dimensional Poisson problem for a layer on the surface of a plasma ion emitter is obtained. The case of plasma with a directed ion velocity greater than the ionic sound one is considered. Moreover, the plasma surface is irradiated with an electron beam. As applied to the parameters of deuterium plasma flowing out of a gas dynamic open magnetic trap GDT, a physical model of a layer with boundary conditions on the plasma surface is formulated. It allows a two-dimensional numerical simulation of the source of an electron beam injected into a trap.

Measurements of axial energy loss from magnetic mirror trap

The work represents a next step in studies of longitudinal energy losses from linear magnetic plasma confinement devices. The paper describes the experimental technique that has been developed and used to measure the absolute values of longitudinal particle and energy density fluxes from an open magnetic mirror trap. Using this technique, it was possible to measure and account for the secondary electron emission coefficient from plasma-facing end plate, and perform a comparison with the existing theory of energy confinement in open traps. It is shown that the average energy transported by an ion-electron pair leaving the trap is close to theoretically predicted minimum value of 6–8 electron temperatures. As confirmed by the measured dependencies, this conclusion is valid in a broad range of parameters and the threshold value of the magnetic expansion ratio does not exceed its theoretically predicted minimum of .

Collective Thomson scattering diagnostic for the GDT open magnetic trap

In this paper, we propose a collective Thomson scattering (CTS) diagnostic for fast ion measurements for the GDT (gas-dynamic trap) facility at the Budker Institute. The diagnostic utilizes the 54.5 GHz gyrotron usually used for electron cyclotron resonance heating as a source of probe radiation and is aimed at reconstruction of distributions over transverse and longitudinal velocities of NBI-driven ions in the plasma core. Here we present a feasibility study of this concept showing a possibility to receive a strong CTS signal of several hundred eVs for a wide range of GDT parameters. The main limitations come from the refraction of the probe and scattered radiation propagating in inhomogeneous plasma: to provide well-resolved CTS measurements the on-axis plasma density should be kept less than 1.5 · 1013 cm−3 while usual GDT discharges correspond to the density of (0.9 − 1.3)· 1013 cm−3.