Oct4 is a widely recognized pluripotency factor as it maintains Embryonic Stem (ES) cells in a pluripotent state, and, in vivo, prevents the inner cell mass (ICM) in murine embryos from differentiating into trophectoderm. However, its function in somatic tissue after this developmental stage is not well characterized. Using a tamoxifen-inducible Cre recombinase and floxed alleles of Oct4, we investigated the effect of depleting Oct4 in mouse embryos between the pre-streak and headfold stages, ∼E6.0–E8.0, when Oct4 is found in dynamic patterns throughout the embryonic compartment of the mouse egg cylinder. We found that depletion of Oct4 ∼E7.5 resulted in a severe phenotype, comprised of craniorachischisis, random heart tube orientation, failed turning, defective somitogenesis and posterior truncation. Unlike in ES cells, depletion of the pluripotency factors Sox2 and Oct4 after E7.0 does not phenocopy, suggesting that ∼E7.5 Oct4 is required within a network that is altered relative to the pluripotency network. Oct4 is not required in extraembryonic tissue for these processes, but is required to maintain cell viability in the embryo and normal proliferation within the primitive streak. Impaired expansion of the primitive streak occurs coincident with Oct4 depletion ∼E7.5 and precedes deficient convergent extension which contributes to several aspects of the phenotype.
We investigate electron transport for a thermal plasma using a magnetostatic plasma model. Three cases are investigated; (1) fixed ions, only magnetic fluctuations; (2) fixed ions, magnetic and electrostatic fluctuations; (3) mobile ions, magnetic, and electrostatic fluctuations. For (1), magnetic islands as well as many unclosed field lines occurred; for (2) and (3) electrostatic convective cells occurred along with magnetic islands giving a much more turbulent situation. Generally the transport due to electrostatic convective cells dominated.Anomalous plasma diffusion due to thermally excited convective cells has been studied in detail. 1 Even for thermal equilibrium, plasma diffusion across a strong magnetic field can be dominated by zero-frequency fluctuations (convective cells). It has recently been pointed out 2 that in addition to the electrostatic convective cells, zero-frequency magnetic fluctuations exist in a two-dimensional situation which produce random magnetic islands as well as many open field lines for a shearless zero-order magnetic field. Since the particles can follow the magnetic field they can diffuse across the system; it is important to compare this diffusion with collisional diffusion and convective cell diffusion. Furthermore, since the motion of charged particles due to the convective cells can destroy the current filaments responsible for the magnetic fluctuations, a strong coupling between the electrostatic and magnetostatic fluctuations is expected to occur. This coupling should determine the correlation time of the thermal magnetic fluctuations. It is also possible for the fluctuating magnetic fields to give rise to shorting of the charges associated with the convective cells and affect their lifetime. This effect is only important at high ft In order to study the above processes in detail, a set of two-and-one-half-dimensional simulations have been carried out using a magnetostatic particle code with a uniform external magnetic field in the z direction. 3 The simulation parameters were the following: a 64x64 grid, 4096 ions and electrons, Sl e /(jo Pe = 1 (electron gyrofrequency/plasma frequency), m i /m e =3Q, T e /T { = 2, X De /6=l (electron Debye length/grid spacing), V Te /c-j$ (electron thermal speed/speed 753 of light) and, 0=0.04 (plasma pressure/magnetic pressure); note that fi^m e /m i here. To study the particle diffusion and the coupling between the electrostatic and magnetostatic fluctuations in detail, simulations are carried out in three steps. First, all the electrostatic fluctuations are suppressed in the code and the ions are treated as a stationary background (case 1). This will correspond to the case considered in Ref. 2. In Fig. I, test-particle diffusion ((Ax) 2 ) with time is shown. We observe that ((Ax) 2 ) increasx250 < I 0 (b) 1 1 Cose 2 / y Case 3 ^J ~~i i 1 FIG. 1. Test-particle diffusion ((Ax) 2 ) vs time for (a) fixed ions, only magnetic fluctuations (case 1); (b) fixed ions, magnetic and electrostatic fluctuations (case 2); mobile ions, magnetic and e...
A high-power gyrotron traveling-wave amplifier operating in the low-loss TE 01 mode has been constructed at the University of California, Davis that will be driven by a 100-kV, 5-A electron beam with a pitch angle () of unity and velocity spread of 5%. The amplifier is predicted by large-signal simulations to generate 140 kW at 92 GHz with 28% efficiency, 50-dB saturated gain and 5% bandwidth. The stability of the amplifier from oscillation has been investigated with linear codes. The threshold current for the absolute instability of the TE 01 operating mode for the chosen operating parameters is predicted to be 10 A. To suppress the potential gyro-backward-wave oscillator interactions, the interaction circuit with a cutoff frequency of 91 GHz has been loaded with distributed loss so that the single-pass attenuation is 90 dB at 93 GHz. The coaxial input coupler has a predicted and measured coupling of 1 and 2 dB, respectively. Index Terms-Absolute instability, coaxial input coupler, distributed loss, gyro-backward-wave oscillator (BWO), gyro-traveling-wave tube (TWT) amplifier, gyrotron traveling-wave amplifier, magnetron injection gun (MIG). I. INTRODUCTION W IDE-BAND high average power amplifiers are required in the 92-96-GHz atmospheric window for advanced radar applications [1], [2]. The highest average power from a conventional linear beam slow wave device at 94 GHz is 1 kW and is produced by the CPI 8783 extended interaction amplifier. Further advances are made difficult by the problem of passing an intense electron beam through the extremely narrow circuits needed to support a slow wave at this frequency. Fast wave devices are capable of significantly higher power because their circuits can be significantly larger. Gyrotron devices, which employ a fast wave circuit, generate the highest average power Manuscript
A unified small-signal amplification theory is developed to compare growth mechanisms responsible for a number of relativistic radiation generators. The theory is formulated from the basis that the electron resonance frequency produced by the external fields of the devices depends on γ−q, where γ is the beam Lorentz factor and q is a constant (q=1 for cyclotron masers, q=1/2 for ion-channel lasers, and q=0 for free electron lasers). It is concluded that for wave amplification, the sign of the electron mismatch frequency is required to be the same as the sign of bunching parameter that is determined by the total bunching both axial and azimuthal; this depends on the q value. The two bunching mechanisms exist, not only in the single electron resonance regime, but also in the collective gain regime. Competition or reinforcement between the two bunching mechanisms is determined by the q value, the electron axial velocity, and the wave phase velocity.
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