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To achieve high-thrust-density operation, we propose electrostatic–magnetic hybrid ion acceleration in which the empirical thrust density limit of the electrostatic acceleration is surpassed without violent plasma oscillation by combing the collisional momentum transfer mechanism, which is the ion acceleration mechanism of the electromagnetic acceleration. To achieve hybrid ion acceleration, we experimentally obtained two design criteria: one near anode propellant injection and another at the on-axis hollow cathode location. The thrust characteristics of three thrusters composed of a slowly diverging magnetic field between an on-axis hollow cathode and a coaxially set ring anode were examined. By injecting xenon propellant along the anode inner surface, the electron impact ionization process was enhanced, and generated ions are electrostatically accelerated through the radial-inward potential gradient perpendicular to the axial magnetic lines of force. The hybrid ion acceleration characteristics were obtained only if these two criteria were satisfied and the obtained thrust was consistent with the thrust formula derived for steady-state, quasi-neutral plasma flows. In addition to the criteria, strengthening the magnetic field and enhancing the propellant mass flux were effective for improving thrust density without deteriorating thrust efficiency. Among the experimental conditions in this study, the maximum thrust density was 70 N/m2 with an anode specific impulse of 1200 s, which cannot be achieved in a purely electrostatic thruster with thrust density 6.3 times than that of a typical Hall thruster. [ABSTRACT FROM AUTHOR]

In this study, a novel single-piece thin multi-layer tungsten resistive heater was successfully fabricated using additive manufacturing and tested as an electrothermal thruster. The heater has 12 resistive layers, with each layer having a thickness and height of 0.15 and 81 mm, respectively, and can provide high heating efficiency. A single-piece or monolithic heater was manufactured via additive manufacturing technique, which drastically improved its reliability and decreased its manufacturing cost. In the heating and thrust measurement tests that used nitrogen gas as a propellant, the heater reached a gas temperature of ∼2000 K at a 140-A heater current without experiencing any failure. The tungsten-heater resistance linearly increased with an increase in temperature due to the temperature dependence of tungsten's resistivity. The specific impulse and thrust increased with the heater temperature in accordance with the theoretical prediction. Even including a voltage drop due to a contact resistance, the achieved heater efficiency reached 63% at a 100-A heater current even without a thermal insulation around the thruster. The heater efficiency decreased with an increase in the heater temperature due to heat loss to the surroundings. The heat-loss analysis indicated that both thermal conduction and radiation heat losses were crucial for improving the heater performance at a high-temperature operation of over 2000 K.

Urodele amphibians are thought to have poorer immune responses than evolutionary more ancestral vertebrate classes, such as bony fish. We investigated skin graft rejection and transplantation immunity in Urodele amphibians, Japanese newts, and Asiatic salamanders, and compared these findings to those from transplants in several species of frogs. The skin grafts used in this study were either allogeneic or xenogeneic. The mean survival time of the first set of allografts at 20°C was approximately 60 days for chronic responses in Urodela and 20 days for acute responses in Anura. As the graft survival times of urodeles were significantly longer than those of anurans, even when urodeles were repeatedly grafted from identical donors, there appear to be substantial differences in transplantation immunity between Urodela and Anura. These slow responses in Urodela may not be accompanied by the expansion of cytotoxic T cells, as observed in fish and anuran species, which are known to have functional major histocompatibility complex (MHC)-class I systems. In our study, approximately five histo-incompatible immunogenic components were found to be involved in chronic responses in newts. Similar chronic responses were also observed in xenograft rejection in newts. In contrast, xenografts were rejected in frogs due to an accelerated acute response, possibly involving natural killer cells. Our findings that some anti-allogeneic components appear to be shared with xenogeneic components indicate that the diversification of the acquired immune system is poorly developed in Urodela.

Urodele amphibians are unique due to their greatly reduced immune responsiveness compared to bony fishes, which show acute immune responsiveness. In newts, the mean survival time of allogenic skin grafts in the transplantation immunity was 48.8 ± 8.3 days at 25°C, suggesting that it occurs in a chronic manner. The graft rejection process was categorized into three stages: a latent stage with frequent blood circulation, or the immune induction phase; a vascular stoppage stage with dominant infiltrating cells of T cells; and a rejection stage showing the change of the dominant cells to monocytes/macrophages, probably as effector cells, tetntatively referred to as the immune effector phase. The immune induction phase is susceptible to the cyclophosphamide (CY) mitosis inhibitor, but not to a temperature shift from 18 to 27°C, while the immune effector phase is susceptible to temperature shifts, but not CY-treatment, although the temperature shift failed to shorten the graft survival time to less than 25 days, which nearly equals that of the secondary set of grafts where the lack of complete blood circulation is remarkable and graft rejection is resistant to CY-treatment. In contrast, a very low temperature (5-10°C) completely prevented effector generation in newts; in frogs, however, it is reported that such low temperatures did not prevent the generation of effectors. Taken together, these data suggest that chronic responses in newts are due to effector cells other than cytotoxic T cells; possible effector cells are discussed.