Growth and Saturation of the Electron Drift Instability in a Crossed Field Plasma.

The linear growth and nonlinear energy transfer of the electron drift instability (EDI) are experimentally measured in the plume of a low-temperature, Hall effect discharge. A frequency-based bispectral analysis technique applied to fast ion density fluctuation measurements shows a growth rate function that is qualitatively similar to predictions from the linear instability dispersion relation, but an order of magnitude smaller. Calculation of the nonlinear transfer function indicates multiple three-wave interactions between high-frequency resonances of the instability in addition to an inverse energy cascade toward lower-frequency modes. These results are discussed in the context of recent theoretical, numerical, and experimental efforts on the EDI in Hall effect discharges and how the EDI may impact anomalous cross field transport.

Anomalous cross-field transport in a Hall thruster inferred from direct measurement of instability growth rates.

The contribution of the electron drift instability to anomalous electron transport is experimentally assessed in a Hall effect discharge. The transport is represented by an anomalous collision frequency, which is related through quasilinear theory to the energy and growth rate of the instability. The wave energy is measured directly with ion saturation probes, while estimates of the growth rate are employed based on both linearized theory and direct measurement. The latter measurement is performed with a bispectral analysis method. The wave-driven collision frequency is compared to measurements of the actual collision frequency inferred from a method based on laser-induced fluorescence. It is found that estimates for transport using linearized theory for the growth differ by over an order of magnitude from the actual anomalous collision frequency in the plasma. The wave-driven anomalous collision frequency with measured growth, however, is shown to agree with the electron collision frequency in magnitude and capture aspects of the trends in spatial variation. This result demonstrates experimentally that wave-driven effects ultimately can explain the observed cross-field transport in these devices. The implications of this finding are discussed in the context of the key lengthscales that drive the transport as well as the implications identifying reduced fidelity models that could be used to predict anomalous collision frequency.

The glass is half empty: Negative self-appraisal bias and attenuated neural response to positive self-judgment in adolescence.

Substantial changes in cognitive-affective self-referential processing occur during adolescence. We studied the behavioral and ERP correlates of self-evaluation in healthy male and female adolescents aged 12-17 (N = 109). Participants completed assessments of depression symptoms and puberty as well as a self-referential encoding task while 128-channel high-density EEG data were collected. Depression symptom severity was associated with increased endorsement of negative words and longer reaction times. In an extreme group analysis, a negative appraisal-bias subsample (n = 28) displayed decreased frontal P2 amplitudes to both positive and negative word stimuli, reflecting reduced early attentional processing and emotional salience. Compared to the positive appraisal-bias subsample (n = 27), the negative appraisal-bias subsample showed reduced LPP to positive words but not negative words, suggesting attenuated sustained processing of positive self-relevant stimuli. Findings are discussed in terms of neural processes associated with ERPs during negative versus positive self-appraisal bias, and developmental implications.

Adolescent positive self, negative self: associated but dissociable?

Adolescence is a period of significant identity development and particular vulnerability to depression associated with negative self-evaluation. We investigated if increased depressive symptom severity was also associated with positive self-evaluation. We also considered pubertal developmental differences in positive and negative self-evaluation, and if these could reflect dissociated facets of the self. This cross-sectional sample consisted of healthy male and female adolescents (N = 109) aged 12-17 from the United States. Participants completed a self-referential encoding task, which required them to indicate if a single-word adjective was self-descriptive. We administered the Children's Depression Inventory, the Pubertal Development Scale, and the Child Narcissism Scale. Negative-word endorsement was significantly predicted by pubertal maturation level and depressive symptoms, but not by narcissism. Positive-word endorsement was significantly predicted by narcissism and negatively predicted by depressive symptoms, but not by pubertal maturation. In this typically developing sample, positive self-judgment does not vary across the pubertal range and is positively associated with narcissistic traits, and negatively associated with depressive symptom severity. Negative self-judgements are positively correlated with puberty and are associated with depressive symptom severity only. Our findings suggest that negative and positive aspects of the self are partially dissociable.