Focused Practice

I. Answer the following questions:

1. What has the move toward small-scale optical circuits revealed?

2. Why is the so-called guest-host system a popular choice in the production of many prototype systems?

3. What is the most promising alternative to the straight-forward guest-host system?

4. What is the idea behind such a development?

5. Will a long-ranged network reduce the mobility of the nonlinear molecules?

6. What has already been demonstrated?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Guest-host systems.

Unit 2 Grammar: The Simple Predicate Word List:

1. SPT	стационарный плазменный реактивный двигатель
2. satellite	спутник
3. propulsion devices	двигатели
4. Hall thruster plasma thruster	ракетный двигатель малой тяги Холла плазменный ракетный двигатель малой тяги
5. orbit transfer	переход на другую орбиту
6. reposition	изменение положения
7. station-keeping	позиционирование, удержание станции на орбите
8. contamination	загрязнение
9. plume	струя, шлейфовый факел
10. propellent	ракетное топливо
11. charge exchange plasma	плазма, образующаяся за счет обмена зарядами
12. concern, n	беспокойство, озабоченность, проблема
13. influence of experimental facilities	инструментальные ошибки
14. codes = software	компьютерные программы
15. verification	подтверждение
16. Particle-in-Cell (PIC) technique	методика PIC
17. simulation	моделирование
18. DSMC	моделирование по методу Монте-Карло
19. momentum transfer	передача кинетической энергии
20. overabundance	переизбыток

Particle Simulations of the spt

As with other electric propulsion devices, Hall thrusters offer a high specific impulse well-suited for satellite station-keeping, repositioning, and orbit transfer. The Stationary Plasma Thruster (SPT) variety has been flown for many years on Russian satellites and has been reliable. However, there is concern about contamination of the satellite surface due to the plasma in the plume. Xenon gas is currently the propellent of choice for such devices, because it is an inert gas with a relatively low ionization potential. The charge exchange plasma, created by collisions between ions and un-ionized propellent in which an electron is transferred, is of particular concern. The charge exchange ions have much lower velocities than the beam ions; therefore they are more influenced by the self-consistent electric fields and may interact with spacecraft surfaces. Computational modeling allows the dynamics of the plume and its interaction with its environment to be examined. The ability to simulate the plumes of these devices allows a wider variety of operating conditions to be tested and also eliminates the influence of the experimental facilities. However, computer codes need verification from experimental data.

To assess the ability to simulate these plumes accurately a computer code which combines the direct simulation Monte Carlo (DSMC) and the Particle-in-Cell (PIC) techniques is being developed to understand in detail the plasma behavior of the plumes of Hall thrusters. The PIC method determines the trajectories of charged particles as predicted by imposed and self-consistent electric fields. The DSMC method is used to deal with the collisional effects in the flow field. Both charge exchange and momentum transfer collisions are modeled. Ions, neutral atoms from the thruster, and background atoms are simulated. The code has previously been verified for an ion thruster and is currently being applied to Hall thrusters. The plumes of these two types of thrusters are similar. The main differences in modeling are in the geometry and the ratio of neutral atoms to ions.

Since electric propulsion devices such as ion thrusters, arc-jets, and Hall thrusters inherently involve charged propellent, the PIC technique is well-suited to simulate their plumes.

The behavior of the ions is of primary interest. A sufficient number of ion particles are needed in each cell to avoid statistical effects in the PIC method. Therefore, to increase the ion particle count without creating an overabundance of neutrals, a particle weighting scheme is used.