Abstract. Let P be the category whose objects are posets and whose morphisms are partial mappings a: P-> Q satisfying (i) V p,q e dom a [p < q =* a(p) < a(q)] and The full subcategory Py of P consisting of all finite posets is shown to be dually equivalent to the category of finite Brouwerian semilattices and homomorphisms. Under this duality a finite Brouwerian semilattice A corresponds with M(A), the poset of all meet-irreducible elements of A. The product (in P,) of n copies (n E N) of a one-element poset is constructed; in view of the duality this product is isomorphic to the poset of meet-irreducible elements of the free Brouwerian semilattice on n generators.If V is a variety of Brouwerian semilattices and if A is a Brouwerian semilattice, then A is V-critical if all proper subalgebras of A belong to V but not A. It is shown that a variety V of Brouwerian semilattices has a finite equational base if and only if there are up to isomorphism only finitely many V-critical Brouwerian semilattices. This is used to show that a variety generated by a finite Brouwerian semilattice as well as the join of two finitely based varieties is finitely based. A new example of a variety without a finite equational base is exhibited. 0. Introduction. The interest in Brouwerian semilattices has been motivated mainly from two seemingly different directions. First there is intuitionistic propositional logic: The Lindenbaum algebra of its fragment £/c, which consists of formulas containing only implication and conjunction as logical connectives, is a free Brouwerian semilattice on a countable number of generators. Thus a formula is a tautology relative to £/c if and only if it is valid in every Brouwerian semilattice. In addition the validity relation between formulas and Brouwerian semilattices sets up a Galois connection between extensions of £/c and subvarieties of the variety of Brouwerian semilattices. Consequently algebraic methods can be successfully employed to deal with problems of a logical nature. Examples of this approach are the papers [31] and [52]; it must be said, however, that historically it was mostly full intuitionistic propositional logic (with disjunction and possibly negation) that was studied and thus on the algebraic side more interest was created in Brouwerian lattices and Heyting algebras.On the other hand the main pioneer in the study of Brouwerian semilattices, Nemitz, considered them as algebraic objects in their own right. In [34] he initiated

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There is no ideal atomic propellant for ion thrusters. Xenon commonly used as propellant becomes resource-critical in light of electric propulsion commercialization. Combining these considerations leads to seeking alternatives to xenon as propellant. In this review, we summarize the current literature on molecular propellants. We define two classes of molecules, group I and II, comprising diatomic molecules and more complex molecules, respectively. We identify basic properties which a candidate molecule belonging to either group, I or II, should possess in order to be suitable as molecular propellant. We discuss the pits and traps in testing such candidate molecules inside a thruster on the basis of our experiences with iodine (a member of group I) and adamantane (a member of group II). The thruster system needs to be individually adopted for each propellant candidate in order to enable a thorough testing inside the thruster. The same holds for optimizing the thruster’s performance when fed with a new propellant because the microscopic processes occurring inside the plasma will differ from molecule to molecule. These circumstances make such testing time-consuming and costly. To accelerate systematic screening of the vast number of molecular species in terms of suitability as propellant, we propose a screening and evolution procedure based on combining chemical engineering and fundamental physical measurements.

Many on-going European Space Agency (ESA) science and earth observation missions are based on precision attitude control and formation flying. All these missions impose strong requirements on propulsion system which should provide low thrust, low noise, and high-precision thrust vectors in up to 16 directions. Also as most of these missions have a platform with limited solar cell arrays, the power consumption of the propulsion system should be as low as possible. The idea of using a small high-efficiency multistage plasma thruster (HEMPT) system for such missions is very attractive because of its relatively low complexity and low system mass. Thus, the ability of downscaling a HEMPT to the µN range is investigated experimentally. A measurement campaign studying systematically the influence of the geometrical dimensions of main thruster parameters on operation, beam profile, and ion acceleration is presented. Additionally the anode material was varied and showed relevance to ion acceleration distribution. The minimum achieved thrust was 50 µN at an anode voltage of 600 V, corresponding to a specific impulse of 230 s. Operation points with thrusts of 180 and 360 µN demonstrate a specific impulse of 610 and 860 s, respectively.Index Terms-Electric propulsion, high-efficiency multistage plasma thruster (HEMPT), plasma devices, plasma diagnostics. NOMENCLATURE Airbus DSAirbus Defence and Space. CHT Cylindrical hall thruster. DCF Diverging cusped field thruster. DiDchamberInner diameter of discharge chamber.