Atsushi Kojima

A gold film detector for radial profile measurement of plasma density by using a gold neutral beam probe

A gold film detector was developed in order to measure the plasma density profile by use of the gold neutral beam probe. The detector is a kind of neutral beam detector composed of a rectangular entrance collimator, two sets of grid plates, a gold thin film, a quartz backing, and a microcarbon coated copper plate. The secondary electrons emitted from the gold film were utilized for the neutral beam detection. The yield curves of the secondary electrons were measured as a function of the incident angle and electron energy in both cases of the gold neutral probing beam and ultraviolet ray injection. Time evolution of the plasma line density was measured by adjusting the incident angle and adding a beam chopping method. The result was in good agreement with the line density measured by a microwave interferometer. It was found that this film detector was quite useful in the viewpoint of simultaneous measurements of the electrostatic potential and plasma density profiles.

Improvement of uniformity of the negative ion beams by tent-shaped magnetic field in the JT-60 negative ion source

Non-uniformity of the negative ion beams in the JT-60 negative ion source with the world-largest ion extraction area was improved by modifying the magnetic filter in the source from the plasma grid (PG) filter to a tent-shaped filter. The magnetic design via electron trajectory calculation showed that the tent-shaped filter was expected to suppress the localization of the primary electrons emitted from the filaments and created uniform plasma with positive ions and atoms of the parent particles for the negative ions. By modifying the magnetic filter to the tent-shaped filter, the uniformity defined as the deviation from the averaged beam intensity was reduced from 14% of the PG filter to ∼10% without a reduction of the negative ion production.

Measurement of the magnetic field fluctuations by use of a gold neutral beam probe in the tandem mirror GAMMA 10

The beam deflection coefficients to the magnetic field fluctuation using a gold neutral beam probe was examined by numerical simulations. The beam deflection parallel to the magnetic field line was mainly caused by the azimuthal component of the magnetic field fluctuation in a case of the beam passing through the plasma center; on the other hand, the beam passing through the plasma edge was deflected by the radial component of the magnetic field fluctuation because the radial and azimuthal velocity components of the beam depend on the injection angle into the plasma. Then, the coefficients could be described as being the function of the injection angle which indicated the radial position of the ionization point and the measured point. As a result of numerical simulation, the coefficients were estimated to be 0.1 and 1.3 for the radial and the azimuthal components of the magnetic field fluctuation in the case of the beam passing through the plasma center. We also estimated the potential effects on the beam...

Direct measurements of the electrostatically and magnetically bounced ions in the tandem mirror

Existence of the bounced ion by the plug potential (PP) bounce ion is essential in order to improve the ion confinement in the tandem mirror. The trajectories of the PP bounce ion were calculated on the assumption that the equipotential surface was slightly deviated from the equilibrium state, and it was found that the radial electric field affected the radial transport of the bounce ion. A charge exchange neutral particle analyzer was located at the inner mirror throat region as a bounce ion measurement device. The energy distribution function of the bounce ion was measured using the device, and compared with the energy distribution function of the end-loss ion.

Measurement of heat load density profile on acceleration grid in MeV-class negative ion accelerator

To understand the physics of the negative ion extraction/acceleration, the heat load density profile on the acceleration grid has been firstly measured in the ITER prototype accelerator where the negative ions are accelerated to 1 MeV with five acceleration stages. In order to clarify the profile, the peripheries around the apertures on the acceleration grid were separated into thermally insulated 34 blocks with thermocouples. The spatial resolution is as low as 3 mm and small enough to measure the tail of the beam profile with a beam diameter of ∼16 mm. It was found that there were two peaks of heat load density around the aperture. These two peaks were also clarified to be caused by the intercepted negative ions and secondary electrons from detailed investigation by changing the beam optics and gas density profile. This is the first experimental result, which is useful to understand the trajectories of these particles.

Observation of Radial Particle Transport Induced by the Fluctuation Measured with a Gold Neutral Beam Probe

The radial particle flux induced by the fluctuation is measured by a Gold Neutral Beam Probe. Then the transient transport phenomenon induced by the fluctuation is investigated in the tandem mirror GAMMA 10. When the drift wave is excited at the central cell, the density near the center is reduced and the divergence of the flux becomes similar to the time derivative of the electron density. It shows that the density reduction is caused by the flux induced by the drift wave. After the density reduction, the drift wave is saturated and comes to the steady state because the density reduction accompanies the reduction of density gradient. Therefore, the transport phenomenon accompanying the growth and saturation of the drift wave is observed experimentally. In the steady state, the phase difference obeys the boltzman relation including the electron non-adiabatic term.

Detection of bounce ions by use of charge exchange bounce ion analyzer

Existence of the plug potential (PP) bounce ion is quite essential for effective improvement of axial confinement in the tandem mirror, which is bounced by the confining potential hill. We paid attention to the neutral particles changed from the bounce ions through the charge exchange process and measured simultaneously both energy and emergence angle of the neutral particles by use of a charge exchange neutral particle analyzer for measuring bounce ions located near the inner mirror throat (IMT) of the plug barrier cell. We detected successfully the bounce ions during confining potential formation and assigned the bounce ions to the PP bounce ion and the OMT bounce ion which is bounced near the outer mirror throat (OMT) of the plug/barrier cell. The trajectories of the PP bounce ion were calculated, and it was found that the confinement of the PP bounce ion was sensitive to the radial profile of the confining potential in relation to the radial transport.

Effect of the Radial Potential Profile on the Transport of the Bounced Ions by the Plug Potential and Radial Potential Control in the Tandem Mirror

We estimate the influence of the discrepancy of the cross sectional shapes between the magnetic flux tube and the equi-potential surface at the mirror throats of the anchor cell on the radial drift of the plug potential bounce ion. The radial potential profiles are assumed to be Gaussian. It is found that the discrepancy enhances the radial drift of the bounce ion and the spread radial potential profile moderates the enhancement. The radial potential profile of the core plasma is adjusted by controlling the electrostatic potentials of the coaxially separated end plate. It is found that the spread type of radial potential profile is effective for the retardation of the radial transport of the bounce ions.

Status of ion sources at the national institutes for quantum and radiological science and technology (QST)

The National Institutes for Quantum and Radiological Science and Technology (QST) manages various types of ion sources for research and development in the fields of life sciences, medical and industrial applications, and fusion energy science. The QST is currently developing on electron cyclotron resonance ion sources, negative ion sources (ion sources for fusion and for tandem accelerators), ion sources for radioactive beams, laser ion sources, and miscellaneous ion sources. Its intra- and inter-institutional collaborations make QST a promising platform for future ion source technologies.The National Institutes for Quantum and Radiological Science and Technology (QST) manages various types of ion sources for research and development in the fields of life sciences, medical and industrial applications, and fusion energy science. The QST is currently developing on electron cyclotron resonance ion sources, negative ion sources (ion sources for fusion and for tandem accelerators), ion sources for radioactive beams, laser ion sources, and miscellaneous ion sources. Its intra- and inter-institutional collaborations make QST a promising platform for future ion source technologies.

Beam Sweeping for Long-Pulse Operation of an Ion Source in Neutral Beam Injectors

Abstract Long-pulse operation has been initially and successfully demonstrated during a 100-s stable beam extraction in the neutral beam test stand (NBTS) system of the Korea Atomic Energy Research Institute (KAERI) for the positive ion source (IS) of the JT-60SA neutral beam injector. The NBTS system was constructed at KAERI to develop 300-s deuterium beam extractions of 100 kV/50 A as an auxiliary heating system of the Korea Superconducting Tokamak Advanced Research (KSTAR). The IS of the JT-60SA neutral beam injector is composed of a plasma generator and a set of tetrode accelerators. The beamline components include an optical multichannel analyzer duct, a neutralizer, a bending magnet (BM), a calorimeter, and a vacuum pump system. The beam power deposition of the IS and the beamline components along the NBTS have been measured by water flow calorimetry (WFC), and a total of 99.7% of the extracted beam power (Vacc∙Iacc) was counted for a hydrogen beam of 82 kV/25 A (2.05 MW) during 100-s beam extraction. To reduce the localized heat load on the calorimeter plate, a method of small-angle deflection for the ion beam particles was applied using a small alternate current of 8 A, 0.5 Hz for the BM coil.