On Optimization of Energy-Mass and Energy-Physical Characteristics of a High-Voltage Diode of Current Conversion Systems for Space Nuclear Power Propulsion Systems

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Дата публикации:
03 декабря 2021, 12:50
Секция 04. Космическая энергетика и космические электроракетные двигательные системы – актуальные проблемы создания и обеспечения качества, высокие технологии
The issues of optimization of electric power and mass-energy characteristics of high-voltage plasma thermionic diodes used in current conversion systems of space nuclear power propulsion systems are associated with the development and creation of powerful current conversion systems at operating temperatures of 600...1000 K, i.e. in areas where traditional semiconductor elements cannot be used. The relations for the optimal electrical characteristics of the high-voltage plasma thermionic diodes and its thermophysical parameters are obtained.
Ключевые слова:
high-voltage plasma thermionic diode, current conversion system, space nuclear power propulsion system, ion laer, electrical power, interelectrode gap, reverse breakdown voltage
Основной текст труда

In this paper, the optimization of the mass and energy efficiency of a high-voltage thermal emission diode with an operating temperature of a «cold» electrode of 600...1000 K, used in the NPPS CCS for matching the electrical parameters of the TRP and the ERDU, is evaluated.

The electric power characteristics of the HPTD are characterized by the operating (or reverse breakdown Ub) voltage Up and the discharge current density jp) in the conductive state, these parameters determine its specific electrical power (NHPTD = jp•Up). The optimum power corresponds to the minimum specific gravity and optimal operating temperature of the electrodes and vapor pressure in the interelectrode gap (IEG) HPTD, functioning as part of the NPPS CCS. The task of the study is to determine the optimal parameters of the HPTD (TA, TC, pCs) and its electrical power NHPTD.

A study of reverse ignitions in a variety of diodes [1] using spectral diagnostics has shown that this process is associated with stepwise ionization of excited atoms («Aston glow») in the ion layer. At the same time, the temperature of the excited atoms in the ion layer at the surface of the negative electrode (anode – in reverse current mode) reaches a certain critical value at which their energy balance is disrupted. As a result, a dependence is obtained linking the critical temperature of the excited atoms of the ion layer with the ignition voltage of the reverse arc discharge [2]:

    U_{npo\sigma }=\left(\left(T_{ak}^{*}-T_{a0}\right){\frac {m_{a}\chi _{\text{areac }}^{2}}{ek\varepsilon _{0}n_{a}}}\right)^{1/3},                                                                                               (1)

associated with the heating of excited atoms to the temperature T*ak, where Ta0 is the temperature of the atoms at the boundary of the ion layer — plasma column, ma, na is the mass of the atom and their concentration, respectively, e is the electron charge, k is the Boltzmann constant, ε0 is the dielectric constant, χareac is the «reactive» thermal conductivity of the vapor taking into account ionization and dissociation of atoms [3, 4]:

                                                              \chi _{\text{areac }}=E\lambda _{ia}D_{am}\left({\frac {\partial n_{e}}{\partial T_{a}}}\right)_{p={\text{ const }}},                                                                                  (2)

where Dam is the coefficient of ambipolar diffusion. Ta0 ≈ TA whereTA is the temperature of the anode of the HPTD.

Thus, by determining the critical vapor temperature in the ion layer (anode temperature), it is possible to obtain the breakdown voltage of the layer during the transition from a glow discharge to an independent arc discharge (reverse arc breakdown voltage HPTD).

Since the voltage of the reverse arc breakdown is a function of the vapor pressure in the IEG according to (1), in order to ensure the optimal specific electrical power of the HPTD, the maximum value of the density of the thermal emission current from the HPTD cathode in the conductive state should be realized.

To achieve this goal, one can use the S-shaped dependences of the Reiser to determine the point of maximum current density, or the results of processing experimental material on the work of the output in cesium and barium vapors [5, 6]. This is realized by optimizing the cathode temperature («moving» the temperature of cathode TC to the point of maximum thermal emission current density on the constant vapor pressure curve in the IEG — S-shaped curve). Thus, the maximum value of the cathode current density in the conductive state determines the optimal value of the cathode temperature of the HPTD.

As a result of the proposed approach, the HPTD with cesium filling is optimized for the cathode temperature and the specific electrical power (per unit of the electrode surface). The optimization parameter is the vapor pressure per month — pCs and the temperature of the anode HPTD — TA (set according to the operating conditions).

The disadvantage of a cesium-filled HPTD is the low density of the thermal emission current due to the small values of the cesium vapor pressure in the IEG — 10–3...10– 2 torr [1, 2], therefore, the specific electric power does not exceed 1...3 kW/cm2.

To increase the NHPTD, it is advisable to use binary filling of the IEG HPTD (cesium and barium), which allows the optimization to be divided into two independent parts: the cesium pressure pCs and the anode temperature TA determine the breakdown (and therefore operating) voltage of the HPTD, and the barium pressure pBa and the cathode temperature TC determine the current density in the conductive state. Note that the density of the thermal emission current in barium vapor is one or two orders of magnitude higher than that in cesium vapor [5, 6]. In this case, with constant values of pCs and TA, it is possible to increase the electrical power of the HPTD to 25...30 kW/cm2.

Experimental studies of the HPTD have revealed a feature that the anode temperature should not exceed 700...720 K [1, 2], otherwise the breakdown voltage of the HPTD is sharply reduced (less than 500...300 V). In this regard, in order to increase the operating temperature of the anode (and the HPTD itself), it is advisable to switch to barium filling of the IEG [7]. The anode temperature can be 900...1000 K at a barium pressure of 10– 4...10– 3 torr. Experimental studies [7] have shown that the specific electrical power of HPTD with barium filling can reach 30-50 kW/cm2 at the anode operating temperature up to 940 K (reverse arc breakdown voltage 2200...2500 V).

The cathode temperature is optimized, as is the case in the HPTD with cesium filling.

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