H. Wakabayashi

Experimental verification of nonconstant potential and density on magnetic surfaces of helical nonneutral plasmas

For the first time, nonconstant space potential ϕs and electron density ne on magnetic surfaces of helical nonneutral plasmas are observed experimentally. The variation of ϕs grows with increasing electron injection energy, implying that thermal effects are important when considering the force balance along magnetic field lines. These observations confirm the existence of plasma equilibrium having nonconstant ϕs and ne on magnetic surfaces of helical nonneutral plasmas.

Observation of collisionless inward propagation of electrons into helical vacuum magnetic surfaces via stochastic magnetic fields

Electrons are injected into a stochastic magnetic region (SMR) of a stellarator vacuum configuration. Remarkably, when the SMR is present, some field-following electrons in the SMR move inwardly across the last closed flux surface. This inward propagation occurs in a collisionless process, but it is never observed for cases where the SMR is lost, nor is the electron density small in the SMR. These suggest the existence of cross-field transport that is associated with free-streaming of electrons along the stochastically wandering field lines in the SMR.

Filament size of floating-emissive probe for low density plasmas with large space potential

Space potential φs of non-neutral plasmas with a low density of ne∼1012 m−3 are measured by two floating-emissive probes. Nothing is different between them except the area S of filaments. Despite the fact that the thermionic current is sufficiently emitted, floating potential φf outputted from the smaller filament is much larger than the realistic φs at some measurement points, which is contrary to the widely known relation of φf⩽φs in probe measurements. The result is attributed to the insufficient probe current Ip collected in low-ne plasmas with a large φs. This is because, in such a plasma, Ip does not always satisfy the necessary condition of Ip>φs/RHI, where RHI is a high impedance resistor, although the value of Ip required for the floating emissive method is very small. In order to correctly determine the φs of the plasmas, S must be larger than φs/ene〈ve〉RHI, where e is the electron charge and 〈ve〉 is the mean speed of electrons collected to the probe.

Experiments on injecting electrons into helical magnetic field configuration

An experimental study on injecting an electron beam into a helical vacuum magnetic field via stochastic (chaotic) magnetic field region has been conducted on the compact helical system device. Remarkably, despite being launched from the outside of the last closed flux surface (LCFS), the injected electrons move across the LCFS and, moreover, penetrate deeply into the magnetic surfaces. This penetration of electrons occurs in a collisionless process that has never been observed in past studies on toroidal nonneutral plasmas confined in axisymmetric geometry. Data clearly suggest that the stochastic magnetic region, which confines a considerable number of electrons, plays a key role in the observed collisionless penetration.

Experimental Investigation of Helical Non‐neutral Plasmas

For the first time, an experimental study on helical nonneutral plasmas is performed on the Compact Helical System (CHS) device. (1) Remarkably, despite being launched from the outside of closed magnetic surface, the injected electrons travel across the magnetic field and penetrate deeply inside the magnetic surfaces. This penetration of electrons is caused by a collisionless mechanism that has never been observed in past toroidal nonneutral plasmas confined in axisymmetric geometry. (2) The penetrated electrons form a helical non‐neutral plasma inside the magnetic surface. The electron density is about 1011–13 m−3 much smaller than the Brillouin density limit. (3) The stable phase of the helical non‐neutral plasma continues only for 1 – 4 ms and then starts to disrupt. About 50 kHz of disruptive instability is observed. This frequency and the other parameters related to the onset time of the instability suggest an ion‐related instability as the possible mechanism.

Recent Results of Helical Nonneutral Plasmas on Compact Helical System (CHS)

First of all, non‐constant space potential φs and electron density ne on magnetic surfaces of helical nonneutral plasmas are verified experimentally. The difference in φs enlarges significantly at the outer region inside the closed magnetic surfaces, and the corresponding equipotential surfaces are inferred to shift upward vertically with respect to magnetic surfaces. Meanwhile, larger value of ne is clearly observed in the downward region (z < 0) of magnetic surfaces, which seems to be consistent with the φs measurement. These results are the first evidence which strongly suggests the equilibrium proposed for nonneutral plasmas confined in closed magnetic surfaces. Secondly, in order to investigate the mechanism of the multiple disruption of helical nonneutral plasmas observed in experiments, space and time evolutions of electron flux are measured carefully inside the magnetic surfaces, when the plasma disruption occurs. Surprisingly, a set of data show that the observed disruption is at first happened a...

Penetration of electrons into helical magnetic field configuration

Summary form only given, as follows. Experimental studies on injecting an electron beam inside a helical magnetic field via stochastic (chaotic) field region have been conducted on the Compact Helical System (CHS) device. Electrons are launched from a LaB/sub 6/ cathode located in the stochastic region. Remarkably, despite launching from the outside of the separatrix, the electrons travel across the separatrix and furthermore, penetrate deeply inside the closed field region. The length of the penetration is about 20 cm, while the Larmour radius of electrons is about 0.2 mm in experiments. In consequence of that, a strong radial electric field up to 10 kV/m is established in the boundary layer of the helical field configuration. Such penetration of electrons strongly depends on the emission current of electrons. In fact, measured profiles of the space potential inside the separatrix show a transition-like jump when the emission current from the gun exceeds a critical value. Simultaneously, the electron flux measured inside the separatrix increases significantly within 50 ps. This is shorter than the electron-neutral collision time. Thus, the observed penetration is categorized as a collision-free phenomenon. Actually, the signal of the measured electron flux shows fluctuations with 40-60 kHz frequency. No dependence on the initial direction of launched electrons is observed in experiments. Electrons penetrate even when being launched quasi-parallel to the magnetic field. Also, no penetration of electrons appears when the gun is distanced about 5 cm outside the separatrix. This corresponds to the scale length of the stochastic layer around the separatrix. Thus, these results suggest the existence of some collective motion of electrons induced around the separatrix with increasing the electron flux. As a result, the collective motion might cause the penetration of electrons inside the separatrix.

Penetration of Electrons inside Helical Field Configuration via Stochastic Magnetic Layer

An experimental study on motions of electrons in a stochastic magnetic field has been performed on the Compact Helical System (CHS) device [1]. The purpose of this research is to experimentally investigate how strongly magnetized electrons move in the stochastic region where the field line itself is chaotic. As an experiment, electrons with 1.2 keV directed energy are launched from the stochastic magnetic layer. Remarkably, the electrons penetrate the stochastic layer and moreover, reach near the central core of the helical configuration. No theory on the loss cone orbit can be applied to the experimental data. The result may reflect some chaotic or collective effect which allows the electrons to move completely in a non‐ordered way in the stochastic magnetic region.

Long‐Time Confinement of Toroidal Electron Plasmas in Proto‐RT

Long‐term confinement (the upper limit is set by the diffusion due to neutral collisions) of toroidal electron plasmas is achieved in a dipole magnetic field configuration of an internal conductor device by the optimization of internal potential profiles. The application of a negatively biased electrode makes possible the elimination of a potential hall inside the plasma and results in the stabilization of the diocotron instability. In the experimental parameters of the magnetic field B ∼ 100 G and back pressure P ∼ 10−4 Pa (∼ 10−6 Torr), the obtained maximum decay time of the trapped charge (number density ne ∼ 1012 m−3) is 200 msec, which is comparable to the classical neutral diffusion time. It is demonstrated that toroidal magnetic surface configurations have excellent confinement properties for non‐neutral plasmas, and might be useful as applications for novel traps of charged particles such as anti‐matters or other multi‐fluid non‐neutral plasmas.

Parameter Dependence of Inward Diffusion on Injected Electrons in Helical Non‐Neutral Plasmas

Experimental studies on an electron injection into a helical magnetic field and characteristics of non‐neutral plasmas have been performed. It is found that the space potential φs has a weak dependence on the injection angle except for a narrow ‘window’ region in which φs significantly drops. A calculation shows that because of the electric field Eg of the electron gun (e‐gun), the emitted electrons are launched quasi‐parallel to the helical magnetic field B, regardless of α. This seems to agree with the observation. The ‘window’ seen in the data may be attributed to an current‐driven instability which might result in the insufficient electron penetration or the degradation of electron confinement in the magnetic surface.