Input Power Dependence Research Articles

Analysis of plasma initiation by neutral beams in the Large Helical Device

An analysis of plasma initiation by neutral beams in the presence of a confining vacuum magnetic field is presented. Plasma startup by NBI alone was demonstrated in LHD in 1999, and since then this has become an indispensable technique for producing plasmas over a wide range of magnetic field strengths. The method is simple: injecting high power neutral beams into a thin gas target, waiting for a few hundred milliseconds until a thin `seed' plasma appears and then adding a gas puff or injecting a pellet to build up the main plasma. The mechanism of the process is considered to be as follows. A small number of fast ions created by collisions with background neutrals are confined in the magnetic field. They ionize the background neutrals and heat the thin seed plasma produced earlier. Numerical calculations based on this physical model can reproduce well the observed time evolution and input power dependence of the plasma buildup. The essential point is that its electron temperature should be high enough to ionize the neutrals added by gas puffing or pellet injection. Because the main energy loss process from this partially ionized plasma is by charge exchange of ions, the ionization degree should become high enough that the electron temperature increases. Therefore, the initial target gas density is important for building up plasma within a limited time.

Pressure and input power dependence of Ar/N2H2 inductively coupled plasma systems

Pressure and input power dependence of inductively coupled plasma systems was investigated with the gas systems used for the chemical vapor deposition of metals and metal nitrides; H2, N2, Ar, and the H2/Ar and N2/H2 mixtures. The plasma density and the electron temperature were measured with a Langmuir single probe and the ionic species in the plasma were analyzed with a quadrupole mass spectrometer. The plasma density of Ar plasma measured 11 cm below the rf coil was in the 1017 m−3 range, which is about 1 order of magnitude higher than N2 plasma and 2 orders of magnitude higher than the H2 plasma. Capacitively coupled power transfer was observed for H2 and N2 plasmas when the plasma density was below about 2×1015 m−3. When the plasma density increased effectively with increasing pressure and input power while the electron temperature was independent of the input power. In the high pressure regime, the plasma density showed saturation in H2 and N2 plasmas and even decrease in N2 plasma. With an appropriate addition of Ar to the H2 plasma (H2/Ar plasma), the plasma density increased greatly and thus the production of atomic hydrogen was enhanced. In N2/H2 plasma, the plasma density changed little over a wide range of gas mixing ratio but the concentration of atomic nitrogen was decreased abruptly as the portion of H2 increased while the concentration of atomic hydrogen remained high even when the portion H2 was small.

Microwave intermodulation study of YBa 2Cu 3O 7−δ films in the presence of an external d.c. magnetic field

Two tone intermodulation measurements on microstrip resonators made of YBa 2Cu 3O 7−δ (YBCO) superconducting thin films are reported. The effect of an external d.c. magnetic field on third order products is investigated. The data show that the presence of the d.c. field affects the input power dependence of intermodulation signals consistently with changes of the surface impedance.

The Excitation Spectra of Two-Photon Induced Fluorescence in Xanthene Dyes

Two-photon excitation spectra of xanthene dyes in ethanol were measured by observing fluorescence emission after excitation by light with wavelengths from 780 to 1260 nm. The normalized spectra showed large two-photon induced fluorescence at higher energies and two moderate peaks near the one-photon absorption band. These features agreed well with the two-photon excitation spectra reported by other authors. The peaks near the one-photon absorption band may be due to a two-photon allowed state overlapping the lowest singlet state or to excitation to the lowest singlet state. We believe the latter is the case; while a two-photon excitation is formally symmetry forbidden, both interaction with the counterion in solution as well as vibronic coupling will remove the symmetry, allowing this transition. Comparison of the spectra of dyes with slightly different structures supports this explanation. The input power dependence of fluorescence irradiance was also measured and some deviation from the expected quadratic dependence was observed in Rhodamine 6G and Rhodamine B.

Input power dependence of growth rate and quality of diamond films deposited in a d.c. arcjet system

Abstract Using a d.c. arcjet chemical vapor deposition (CVD) system, the dependence of growth rate and quality of diamond films on input power to the plasma torch was investigated. It was found that in a remote methane feed mode, the growth rate of diamond films increases as the input power increases, confirming the benefit of using high power torches. But unexpectedly, the content of non-diamond carbon or defect density in thin diamond films was found to increase as a function of input power, and a thick diamond film deposited at a low input power was of high quality. These results were analyzed considering the kinetics of gas activation taking place in the CVD process. It is believed that high input power along with a high density of atomic hydrogen will effectively activate hydrocarbon radicals, which will bring about both a high growth rate of diamond films and a large probability of defect incorporation into the growing materials.

Effect of mean lattice coordination on the backward-wave phonon echoes in chalcogenide glasses

We have studied backward-wave phonon echoes at 9.3 GHz in several chalcogenide glasses. A difference in the input-power dependence of the echopower between the different chalcogenide glasses has been observed and is discussed with respect to the mean coordination of the samples.

Mixing/Resonance of Electronic States and Optical Nonlinearity in a GaAs/AlGaAs Asymmetric Triple Quantum Well Structure

The input power dependence on the transmission of a light through a GaAs/AlGaAs asymmetric triple quantum well (ATQW) shows strong nonlinearity at 77 K, the behavior of which depends on the wavelength of the input light. By studying the relation of nonlinearity to the photoluminescence spectra, the resulting phenomenon is attributed to the Stark shift and change of the optical oscillator strength which is due to the triple resonance of the electronic states. The triple resonance is induced by the temporal separation of photo-excited electrons and holes.

Relationship between the Magnetic Properties of YIG Ferrites and the IMD (Intermodulation Distortion) of an Isolator

YIG ferrite is widely used for microwave applications such as isolators and circulators. This paper studies the origin of intermodulation distortion (IMD) in microwave isolators in terms of the influence of the ferromagnetic resonance linewidth (ΔH), which is a material property of YIG ferrites. ΔH is affected by the non-uniformity of the spin direction under a dc field, which results from crystal anisotropy and the demagnetizing field around pores. The IMD was decreased by minimizing ΔH, which in turn was achieved by minimizing these two contributions separately. It also appeared that the input power dependence of IMD was increased by using low-anisotropy YIG ferrites.

High power durability of diamond surface acoustic wave filter

The power durability of the ZnO/diamond surface acoustic wave (SAW) filter was investigated and compared with the LiTaO/sub 3/ SAW filter. The ZnO/diamond SAW filter maintained linearity of the input-output relation up to the input power of 36 dBm at 2.90 GHz, whereas the LiTaO/sub 3/ SAW filter, which operated at 822 MHz, was nonlinear above the input power of 23 dBm and degraded significantly at 27.7 dBm. The input power dependence of the time to failure was also investigated for both SAW filters. It was found that the ZnO/diamond SAW filter was durable for 8 dB higher input power than that of the LiTaO/sub 3/ SAW filter even at 3.5 times higher frequency. These results indicate the extremely high power durability of the ZnO/diamond SAW filter.

Optical emission spectra of microwave oxygen plasmas and fabrication of SiO2 films

The optical emission spectra of microwave oxygen plasmas under various gas pressures have been investigated. At the gas pressure of 0.01 Torr the plasma-generated particles are mainly excited atomic oxygen and ionic molecular oxygen species. As the gas pressure is increased to a value higher than 0.2 Torr, the plasma contains excited atomic oxygen with emission peaks at 777 and 844 nm and a new emission peak at 759 nm. When the gas pressure reaches 2.9 Torr, only a single emission peak at 759 nm appears. To the best of our knowledge, the optical emission spectra peak at 759 nm has not been reported before. This peak is thought to be due to the presence of excited molecular oxygen species. At low gas pressures, the concentrations of atomic oxygen and ionic molecular oxygen increase linearly with increasing input power to the plasma, while there is no such input power dependence for excited molecular oxygen. SiO2 films have also been fabricated by means of oxidation of silicon wafers in microwave oxygen plasmas at two gas pressures, 0.01 and 2.2 Torr. It is found that the SiO2 films grown at 0.01 and 2.2 Torr have properties similar to those of thermally grown oxide, and that the films grown at 0.01 Torr have a lower value of interface trap density (≊9×10−10 eV−1 cm−2) and a smaller leakage current. Atomic oxygen species are found to be the main species to contribute to the oxidation of silicon in microwave oxygen plasmas.

Ohmic H-mode studies in TUMAN-3

The spontaneous transition from Ohmically heated limiter discharges into the regime with improved confinement termed as "Ohmic H-mode" has been investigated in "TUMAN-3". The typical signatures of H-mode in tokamaks with powerful auxiliary heating have been observed: sharp drop of Dalpha radiation with simultaneous increase in the electron density and stored energy, suppression of the density fluctuations and establishing the steep gradient near the periphery. The crucial role of the radial electric field in the L-H transition was found in the experiments with boundary biasing. The possibility of initiating the H-mode using single pellet injection was demonstrated. In Ohmic H-mode strong dependencies of tau E on plasma current and on input power and weak dependence on density were found. Thermal energy confinement time enhanced by a factor of 10 compared to predictions of Neo-Alcator scaling. Longest energy confinement time (30 ms) was obtained in the small tokamak TUMAN-3. Absolute values of the energy confinement time are in agreement with scaling proposed for description of the ELM-free H-modes in devices with powerful auxiliary heating ("DIII-D/JET H-mode" scaling).

A comparison study of electroluminescence from Au/native oxide/p-Si and Au/porous Si diodes

Visible electroluminescence (EL) has been observed from a Au/native oxide/p-Si diode and has been studied in comparison with the EL from a Au/porous Si diode. Both diodes share similarities in current-voltage characteristics, EL spectra, and electrical input power dependence of integrated EL intensity. However, Au/porous Si exhibits quite strong photoluminescence (PL) while Au/native oxide/p-Si does not show any measurable PL. For the Au/native oxide/p-Si diode, when the native oxide layer was removed in 1% HF solution, no detectable EL has been observed. These experimental results indicate that the native oxide layer plays a key role in light emission from the Au/native oxide/p-Si diode, and show that the luminescence mechanism for Au/native oxide/p-Si diodes is similar to that for the Au/porous Si diode. This comparative study may aid in elucidation of the EL mechanism of porous silicon.

Backward-wave phonon echoes from tunneling systems in chalcogenide glasses

We have studied backward-wave phonon echoes at 9.3 GHz in several chalcogenide glasses. The input power dependence of the echo power can be modeled in terms of the non-linear mixing process of two laser fields in a gas consisting of two-level systems. A difference in this dependence between the arsenic and germanium chalcogenide glasses is observed.

Light power dependence of electro-optical transmission in InGaAs/AlGaAs multiple quantum wells

Dual wavelength electro-optical bistability in resistor-biased and symmetric self electro-optic effect devices and their input power dependence are investigated using InGaAs/AlGaAs multiple quantum wells. A new principle to drive the devices using two widely separated wavelengths for signal and control light is proposed and demonstrated to obtain optical transmission bistability between two stable states of the signal light at λ1 by varying the control beam power at λ2. Detailed experimental results of signal power dependencies of the optical responses are presented and rigorously explained based on the voltage switching mechanism of the signal diode by the second control light using a simple load line analysis. Advantages of this new device configuration are discussed taking full merit of the transparency of the GaAs substrate to the signal wavelength.

Nonlinear resolution improvement and second-harmonic generation of a pressurized superfluid 4He acoustic microscope

We observed the input power dependence of the second-harmonic generation of a 370-MHz focused acoustic beam in pressurized superfluid 4He at about 300 mK. We also measured the resolving power of a reflection mode acoustic microscope using pressurized superfluid 4He as a coupling liquid under 11 atm (anomalous dispersion region) and 21 atm (normal dispersion region). At the nonlinear power region of each pressure, the resolution was improved beyond the diffraction limit. Results of these two experiments confirmed that the higher harmonics are generated and they are converted to the fundamental as the wave propagates through the focal point. The higher harmonics play an important role in recently observed exotic nonlinear phenomenon in pressurized superfluid 4He.

Microwave superconducting filters

Four-pole, parallel-coupled microstrip line filters were fabricated using epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films deposited by off-axis sputtering on LaAlO/sub 3/ substrates. Filters were made with and without superconducting ground planes. Because it is not yet possible to grow in-situ epitaxial films on both sides of a substrate, two separate substrates were used, one for the filter and one for the ground-plane. The devices were packaged so that the microstrip-to-ground-plane distance was the thickness of one of the substrates (0.43 mm). Passband characteristics, input power, and temperature dependence were measured, as well as the thermal cycling stability.

Magneto-conductance of an ultra-small δ-doped GaAs channel grown by metalorganic chemical vapor deposition

Using electron-beam-induced-resist, we have fabricated δ-doped GaAs channel with structural width and length of 2700 Å and 2400 Å, respectively, which corresponds to 300-Å-wide and 4800-Å-long ultra-small channel, when depletion layer is taken into consideration. The positive magneto-conductance and universal conductance fluctuation are observed for the channel at 2.3 K. Phase-coherence-length estimated from quantum interference of electron is 1000 Å for input power of 2.7x10 −14 W per electron. This value is five times as long as our process resolution of 200 Å. Input power dependence of universal conductance fluctuation is also investigated.

High moment soft amorphous CoFeZrRe thin-film materials

CoFeZrRe thin films have been deposited and studied systematically for suitability as a material for thin-film recording heads. The addition of Fe and Re results in a higher degree of crystallinity, thus increasing the coercivity. The addition of Fe increases the magnetostriction of the films while the addition of Re decreases the magnetostriction. The large magnetostriction of films with a large Fe content results in high coercivity. Rotating field annealing is very effective in lowering the coercivity of the magnetostrictive films because of the relaxation of internal stresses. By properly choosing the composition of Fe and Re, we have deposited very soft magnetic thin films with a saturation magnetization of 14 kG, coercivity of 0.3 Oe, and near-zero magnetostriction. The input power dependence of the magnetostriction of this series of films is observed, although x-ray fluorescence analysis does not show any apparent compositional variations. Finally, the anisotropy field of the these films can be decreased by rotating field annealing from 25 to 5 Oe.

The rf input power dependence of chromium excited state level populations in the inductively coupled plasma

Excited state level populations have been measured for Cr(I) and Cr(II) in the inductively coupled plasma. The measurement was carried out using a 4096 pixel linear photodiode array spectrometer to detect emission intensities from 16 atomic and 16 ionic lines. The effect of changes in rf input power on the level populations was studied over the power range 0.75-1.75 kW. The results are consistent with a p-LTE model of excitation and ionization.

Theory of the input power-dependence of the electron heating in n-inversion layers

The average energy loss P of hot electrons due to the interaction with acoustic bulk phonons is calculated and used to determine the electron heating temperature Δ as a function of the input power eμ E 2. It is found that P creases proportional to Δ 2 and is independent of the carrier concentration. Consequently the ratio Δ/√ eμ E 2 turns out to be a constant (0.75 × 10 −2 K/(eV/s) 1 2 for n-Si and 2.04 × 10 −2 K/(eV/s) 1 2 for n-GaAs) in agreement with the experimental data deduced from FIR-emission experiments at T = 4.2 K.