Search

In the GAMMA 10/PDX tandem mirror, plasma with strong ion-temperature anisotropy is produced by using the ion cyclotron range of frequency waves. This anisotropy of ion temperature causes several Alfven-Ion-Cyclotron (AIC) waves to spontaneously excite in the frequency range just below the ion cyclotron frequency. In addition, difference-frequency (DF) waves are excited in the radial inner region of the plasma by wave–wave coupling among the AIC waves. The radial density profiles were measured at multi-axial positions using a frequency-modulation reflectometer with an axial array of microwave antennas, and an axial variation of the density was found to be significant. In addition, a relative phase difference of the DF wave between axially separated two points was first obtained by finely choosing the probing frequency of the reflectometers with a maximum coherence used as a measure, indicating that the DF wave is a propagating wave, while the pump AIC waves are standing waves in the axial region of measurement.

To investigate end-loss plasma control, several heating operations were carried out using ion-cyclotron range of frequency (ICRF) heating systems in GAMMA 10/PDX. In anchor region, one of the important heating positions to control the end loss particle and heat fluxes, excited ICRF waves are measured as the density fluctuations using an upgraded reflectometer system and the detailed radial profiles are observed. The efficiency of anchor heating was changed using the phase control between two antennas installed in the anchor and/or the central region. The interference of two waves during a phase-control ICRF heating has been directly measured inside the plasma. In addition, several phase dependencies were observed using absorption methods with photodiode in anchor region.

Effective ICRF heating creates a high ion-temperature plasma with strong ion-temperature anisotropy in the GAMMA 10/PDX central cell, where several Alfvén-ion-cyclotron (AIC) waves are spontaneously excited. It is clearly measured with a microwave reflectometer installed in the central cell that waves with the difference frequencies of the AIC waves are excited in a core region. In addition, it has been observed with an end-loss high-energy ion detector that high-energy ions of over 6 keV are axially transported along the magnetic field lines with the same frequencies as the difference frequencies of the AIC waves. In this study, in order to understand the related wave-particle interaction, another end-loss high-energy ion detector using a micro-channel plate has been developed. By changing the ion retarding grid voltage, which the new detector equips, it is found that an energy dependence exists in the loss mechanism; some frequencies that the axial transport includes disappear for high-energy ions of over 6 keV while they are significant for low-energy ions. In addition, by changing the radial position of the new detector, a radial localization of the axial transport to a core region, which is consistent with the profile of the difference frequencies waves, is indicated.

We used the additional Ion Cyclotron Range of Frequency (ICRF) heating in the anchor cell for the high density plasma production. When the ICRF waves are excited with antennas installed in the anchor cell, it is observed that the plasma potential increases significantly and forms non-axisymmetric potential distribution in the periphery of the central cell. Experiments with different frequencies were carried out to understand the physical mechanism of this potential change due to the ICRF waves. It is confirmed that the ion heating does not play an important role to the potential increase. The floating potentials on some tips set outside of the plasma in the anchor cell remarkably decrease during the additional heating and non-axisymmetric distribution of the floating potential in the central cell depends on the antenna location in the anchor cell. It is suggested that the radial transport of electrons near the antennas in the non-axisymmetric anchor cells with the ICRF antennas is a possible candidate of the potential increase in the central cell.

In the GAMMA 10/PDX tandem mirror, studies on divertor physics have been performed in the west end region by utilizing end-loss plasmas flowing from the confinement region. Since the plasma density in the end region is quite low (∼1016 m−3), an increase of the end-loss ion flux is required. The increase of the end-loss ion flux has been obtained by increasing the density in the confinement region on previous experiments. When an additional ICRF heating using the antennas in the anchor cells has been performed, a significant increase of the ion flux has been observed with the increase of the potential in the central cell although the change of line densities in the confinement region is little. The effect of the potential on the ion flux has been examined on the GAMMA 10/PDX using about 10,000 discharges. The ion flux increases almost linearly with the potential and the density increase. By comparing with a simple calculation, the increase of the ion flux is explained by the expansion of loss cone boundary of ions and the decrease of the transport time of the end-loss ions from the confinement region to the end region.

Beach heating using a slow Alfvén wave in ion cyclotron range of frequencies would be the first candidate for ion heating in a DEMO-relevant divertor testing linear plasma device if it is applicable to a high-density regime. To clarify its availability, the density dependence of a slow wave excitation is investigated using a full wave simulation with TASK/WF3D code in the GAMMA 10/PDX central cell configuration, where there is an extensive track record of a beach heating. A shielding effect is successfully demonstrated and well understood under a three-dimensional configuration in the limit of cold plasma approximation. As the density increases, excitable left-handed electric field, which contributes to ion cyclotron heating, degrades more and more from a core region, and resultantly the ion absorption region goes outwards with reducing its power. For core densities above 1020 m−3, the wave field exists only at a very edge, and ion heating becomes negligible unless the wave frequency is much increased with a correspondent magnetic field enhancement.

Slow Alfvén wave with a frequency just below an ion cyclotron frequency can be destabilized by the anisotropy of ion temperature and called as Alfvén-Ion-Cyclotron (AIC) wave. In the GAMMA 10/PDX central cell, several AIC waves simultaneously appear in an unstable frequency range. In addition, low-frequency waves with the difference frequencies (DF) between the AIC waves are observed, and their characteristics have to be clarified to reveal their unknown contributions to the ion transport suggested in GAMMA 10/PDX. In this study, simultaneous two-point measurements in the azimuthal and axial directions were conducted by using a two-channel microwave reflectometer with multi horn antennas. The azimuthal mode number (m) of the DF waves was confirmed at inner plasma region for the first time, and was zero, which satisfies azimuthal mode matching with the m = −1 AIC waves. In addition, it was shown that the DF waves were radially localized in an inner region, which is different from rather broad profiles of the AIC waves.

In the west end of GAMMA 10/PDX, the end-loss ion flux is utilized for divertor simulation experiments. It is important for controlling parameters of the end-loss ion flux to investigate the efficiency of ion cyclotron range of frequency (ICRF) heating in the west plug/barrier cell, which is next to the west end region. The ICRF heating experiments in the west plug/barrier cell have been performed. In this study, three-dimensional simulations of the ICRF wave propagation were carried out in the plug/barrier cell for the first time. Two types of antennas, a Double Half Turn (WB-DHT) and a Nagoya Type-III (WB-Type-III) antennas, are evaluated. As the results, it is clarified that the WB-Type-III antenna is more effective for the ion heating than the WB-DHT antenna when the frequency is near the ion cyclotron resonance frequency at the midplane of the plug/barrier cell. An additional experiment with the WB-Type-III antenna which is located near the midplane of the plug/barrier cell has been performed. The improvement of the power absorption has been observed. The WB-Type-III antenna located near the midplane is more effective for the ion heating. The experimental results are consistent with the simulation of the wave propagation analysis.