The interaction between an electron emissive wall, electrically biased in a plasma, is revisited through a simple fluid model. We search for realistic conditions of the existence of a non-monotonic plasma potential profile with a virtual cathode as it is observed in several experiments. We mainly focus our attention on thermionic emission related to the operation of emissive probes for plasma diagnostics, although most conclusions also apply to other electron emission processes. An extended Bohm criterion is derived involving the ratio between the two different electron densities at the potential minimum and at the background plasma. The model allows a phase-diagram analysis, which confirms the existence of the non-monotonic potential profiles with a virtual cathode. This analysis shows that the formation of the potential well critically depends on the emitted electron current and on the velocity at the sheath edge of cold ions flowing from the bulk plasma. As a consequence, a threshold value of the governing parameter is required, in accordance to the physical nature of the electron emission process. The latter is a threshold wall temperature in the case of thermionic electrons. Experimental evidence supports our numerical calculations of this threshold temperature. Besides this, the potential well becomes deeper with increasing electron emission, retaining a fraction of the released current which limits the extent of the bulk plasma perturbation. This noninvasive property would explain the reliable measurements of plasma potential by using the floating potential method of emissive probes operating in the so-called strong emission regime.

Background and Aims Therapies for chronic hepatitis B virus (HBV) infection are urgently needed because of viral integration, persistence of viral antigen expression, inadequate HBV‐specific immune responses, and treatment regimens that require lifelong adherence to suppress the virus. Immune mobilizing monoclonal T Cell receptors against virus (ImmTAV) molecules represent a therapeutic strategy combining an affinity‐enhanced T Cell receptor with an anti‐CD3 T Cell‐activating moiety. This bispecific fusion protein redirects T cells to specifically lyse infected cells expressing the target virus‐derived peptides presented by human leukocyte antigen (HLA). Approach and Results ImmTAV molecules specific for HLA‐A*02:01‐restricted epitopes from HBV envelope, polymerase, and core antigens were engineered. The ability of ImmTAV‐Env to activate and redirect polyclonal T cells toward cells containing integrated HBV and cells infected with HBV was assessed using cytokine secretion assays and imaging‐based killing assays. Elimination of infected cells was further quantified using a modified fluorescent hybridization of viral RNA assay. Here, we demonstrate that picomolar concentrations of ImmTAV‐Env can redirect T cells from healthy and HBV‐infected donors toward hepatocellular carcinoma (HCC) cells containing integrated HBV DNA resulting in cytokine release, which could be suppressed by the addition of a corticosteroid in vitro. Importantly, ImmTAV‐Env redirection of T cells induced cytolysis of antigen‐positive HCC cells and cells infected with HBV in vitro, causing a reduction of hepatitis B e antigen and specific loss of cells expressing viral RNA. Conclusions The ImmTAV platform has the potential to enable the elimination of infected cells by redirecting endogenous non‐HBV‐specific T cells, bypassing exhausted HBV‐specific T cells. This represents a promising therapeutic option in the treatment of chronic hepatitis B, with our lead candidate now entering trials.

The performances of a simple circuit for fast sweep measurements using collecting and emissive Langmuir probes are evaluated. The probes are biased by means of a time dependent ramp voltage signal with a variable pulse frequency and the current voltage curves are measured along the increasing flange of this sawtooth voltage. The response of this fast probe polarization circuit was verified under actual experimental conditions by measuring the properties of a stationary Maxwellian plasma produced by means of a glow discharge. The results are independent of the experimental conditions and essentially rely on the discharge properties for polarization pulse repetition rates below a threshold. This upper bound lies below the ion plasma frequency and is related with the faster time scale involved in the sawtooth signal probe bias voltage. The motion of ions would not follow the rapid change of the electric field around the probe associated to the short decreasing edge of the sawtooth polarization voltage and, therefore, the probe perturbs the local electric field. We conclude that these time scales should be considered for the interpretation of these measurements in addition to the electron and ion plasma frequencies in fast sweep Langmuir probe techniques. We conclude that these time scales should be considered for the interpretation of these measurements in addition to the electron and ion plasma frequencies in fast Langmuir probe techniques.

The design and operation modes of a small, low power ion plasma thruster and the properties of the emitted plasma plumes are discussed. The ion beam is extracted from a primary plasma produced by a stationary low pressure electric discharge where the ion production rate is essentially determined by the discharge current. The experiments evidence that the electron neutralization current controls the space charge levels of the outgoing ion current and also influences the spatial properties of emitted plasmas. The electron plasma density increases with the discharge and the electron neutralization currents, while decreases as the plasma plume expands. However, the corresponding electron temperatures decrease when the electron neutralization currents increments. The collisional origin of this electron cooling effect is excluded because of the large collisional mean free paths involved. Then, these electron energy losses during the neutralization of the ion beam would be caused by more subtle physical mechanisms than collisions. The experimental results are compared with previous numerical simulations and similar phenomena found in other experiments.

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A three species one-dimensional kinetic model is presented for a spatially homogeneous weakly ionized plasma (WIP) subjected to the action of a time varying electric field. Planar geometry is assumed, which means that the plasma dynamics evolves in the privileged direction of the field. The energy transmitted to the charges is be channelized to the neutrals thanks to collisions and impacting the plasma dynamics.Charge-charge interactions have been designed as a one-dimensional collision term equivalent to the Landau operator used for fully ionized plasmas. Charge-neutral collisions are modelled by a conservative drift-diffusion operator in the Dougherty's form. The resulting set of coupled drift-diffusion equations is solved with the stable and robust Propagator Integral Method (PIM). This method feasibility accounts for non-linear effects without appealing to linearisation or simplifications, providing conservative physically meaningful solutions. It is found that charge-neutral collisions exert a significant effect since a quite different plasma dynamics arises if compared to the collisionless limit. In addition, substantial differences in the system evolution are found for constant and temperature dependent collision frequencies cases.

The characteristics of supersonic ion beams from the alternative low power hybrid ion engine (ALPHIE) are discussed. This simple concept of a DC powered plasma accelerator that only needs one electron source for both neutral gas ionization and ion beam neutralization is also examined. The plasma production and space charge neutralization processes are thus coupled in this plasma thruster that has a total DC power consumption of below 450 W, and uses xenon or argon gas as a propellant. The operation parameters of the plasma engine are studied in the laboratory in connection with the ion energy distribution function obtained with a retarding-field energy analyzer. The ALPHIE plasma beam expansion produces a mesothermal plasma flow with two-peaked ion energy distribution functions composed of low and high speed ion groups. The characteristic drift velocities of the fast ion groups, in the range 36.6–43.5 Km/s, are controlled by the acceleration voltage. These supersonic speeds are higher than the typical ion sound velocities of the low energy ion group produced by the expansion of the plasma jet. The temperatures of the slow ion population lead to ion Debye lengths longer than the electron Debye lengths. Furthermore, the electron impact ionization can coexist with collisional ionization by fast ions downstream the grids. Finally, the performance characteristics and comparisons with other plasma accelerator schemes are also discussed.