Experimental investigation of evaporative cooling mixture of bosonic ^(87)Rb and fermionic ^(40)K atoms with microwave and radio frequency radiation

We investigate sympathetic cooling fermions 40K by evaporatively cooling bosonic 87Rb atoms in a magnetic trap with microwave and radio frequency induced evaporations in detail. The mixture of bosonic and fermionic atoms is prepared in their polarized spin states IF = 9/2, mF = 9/2） for 40K and IF = 2, mF=2〉 for 87Rb, which is trapped in Quadrupole-Ioffe-Configuration trap. Comparing microwave with radio frequency evaporatively cooling bosonic STRb atoms with sympathetically cooling Fermi gas 40K, we find that the presence of rubidium atoms in the [2, 1} Zeeman states, which are generated in the evaporative process, gives rise to a significant loss of 40K due to inelastic collisions. Thus, the rubidium atoms populated in the [2, 1} Zeeman states should be removed in order to effectively perform sympathetically cooling 40K with the evaporatively cooled STRb atoms.

Comparison of Cellulose Decomposition by Microwave Plasma and Radio Frequency Plasma

Biomass conversion by plasma has the advantage of mainly producing gaseous products, H2, CO and CO2. Though the thermal plasma has been used for this conversion, the plasma temperature is too high to be unfit for the conversion biomass. The temperature of cold plasma, however, is lower under 3000 K. It expects to be adequate for biomass conversion. Cold plasma can be obtained with irradiation microwave (2.45 GHz) or radio frequency (13.5 MHz) under reduce gas pressure. Therefore, in present study, the effective decomposition of cellulose by microwave plasma (MWP) and radio frequency plasma (RFP) is examined. The conversion of cellulose by MWP (XMWP) is higher than that by RFP (XRFP), irrespective of the reaction time. XMWP and XRFP reach 92.8 wt% at 10 min and 68.1 wt% at 30 min. The maximum yield of gaseous products (Ygas) by MWP is 85.1 wt% at 10 min, higher by 23.2 wt% than Ygas by RFP at 30 min. The amount of H2 and CO obtained by MWP is 18.0 mmol/g and 23.5 mmol/g, it is larger than that obtained by RFP. Comparing the relationship between conversion and yield, Ygas of MWP is slightly higher than that of RFP under X of 60 wt%, and both Ygas is almost same over 60 wt%. The amount of H2 and CO obtained by MWP is larger by 9.3 mmol/g and 9.6 mmol/g than that obtained by RFP. C, H and O element in cellulose is mainly distributed to H2 and CO by MWP. RFP mainly distributes H and O element to the other gases without H2 and CO. In addition, a large amount of C element is remains in the residue. Those results is found that MWP was more suitable for cellulose gasification than RFP, since MWP can highly convert C, H and O element to H2 and CO by higher energy of microwave frequency in comparison with radio frequency.

Power transfer efficiency in an air-breathing radio frequency ion thruster

Due to a series of challenges such as low-orbit maintenance of satellites, the air-breathing electric propulsion has got widespread attention. Commonly, the radio frequency ion thruster is favored by low-orbit missions due to its high specific impulse and efficiency. In this paper, the power transfer efficiency of the radio frequency ion thruster with different gas compositions is studied experimentally, which is obtained by measuring the radio frequency power and current of the antenna coil with and without discharge operation. The results show that increasing the turns of antenna coils can effectively improve the radio frequency power transfer efficiency, which is due to the improvement of Q factor. In pure N_2 discharge,with the increase of radio frequency power, the radio frequency power transfer efficiency first rises rapidly and then exhibits a less steep increasing trend. The radio frequency power transfer efficiency increases with the increase of gas pressure at relatively high power, while declines rapidly at relatively low power. In N_(2)/O_(2) discharge, increasing the N_(2) content at high power can improve the radio frequency power transfer efficiency, but the opposite was observed at low power. In order to give a better understanding of these trends, an analytic solution in limit cases is utilized, and a Langmuir probe was employed to measure the electron density. It is found that the evolution of radio frequency power transfer efficiency can be well explained by the variation of plasma resistance, which is related to the electron density and the effective electron collision frequency.

Experimental results of a magnetic field modification to the radio frequency driver of a negative ion source

A magnetic field produced by a current flowing through the plasma grid(PG) is one of the solutions to reduce the collisional loss of negative ions in a negative ion source, which reduces the electron temperature in front of the PG. However, the magnetic field diffused into the driver has some influence on the plasma outflowing. In order to investigate the effect of changing this magnetic field on the outflowing of plasma from the driver, a circular ring(absorber) of high permeability iron has been introduced at the driver exit, which can reduce the magnetic field around it and improve plasma outflowing. With the application of the absorber, the electron density is increased by about 35%, and the extraction current measured from the extraction grid is increased from 1.02 A to 1.29 A. The results of the extraction experiment with cesium injection show that both the extraction grid(EG) current and H-current are increased when the absorber is introduced.

Hollow cathode effect in radio frequency hollow electrode discharge in argon

Radio frequency capacitively coupled plasma source(RF-CCP)with a hollow electrode can increase the electron density through the hollow cathode effect(HCE),which offers a method to modify the spatial profiles of the plasma density.In this work,the variations of the HCE in one RF period are investigated by using a two-dimensional particle-in-cell/Monte-Carlo collision(PIC/MCC)model.The results show that the sheath electric field,the sheath potential drop,the sheath thickness,the radial plasma bulk width,the electron energy distribution function(EEDF),and the average electron energy in the cavity vary in one RF period.During the hollow electrode sheath's expansion phase,the secondary electron heating and sheath oscillation heating in the cavity are gradually enhanced,and the frequency of the electron pendular motion in the cavity gradually increases,hence the HCE is gradually enhanced.However,during the hollow electrode sheath's collapse phase,the secondary electron heating is gradually attenuated.In addition,when interacting with the gradually collapsed hollow electrode sheaths,high-energy plasma bulk electrons in the cavity will lose some energy.Furthermore,the frequency of the electron pendular motion in the cavity gradually decreases.Therefore,during the hollow electrode sheath's collapse phase,the HCE is gradually attenuated.

Microwave damage susceptibility trend of a bipolar transistor as a function of frequency

We conduct a theoretical study of the damage susceptibility trend of a typical bipolar transistor induced by a high-power microwave （HPM） as a function of frequency. The dependences of the burnout time and the damage power on the signal frequency are obtained. Studies of the internal damage process and the mechanism of the device are carried out from the variation analysis of the distribution of the electric field, current density, and temperature. The investigation shows that the burnout time linearly depends on the signal frequency. The current density and the electric field at the damage position decrease with increasing frequency. Meanwhile, the temperature elevation occurs in the area between the p-n junction and the n n＋ interface due to the increase of the electric field. Adopting the data analysis software, the relationship between the damage power and frequency is obtained. Moreover, the thickness of the substrate has a significant effect on the burnout time.

Tunable and broadband microwave frequency combs based on a semiconductor laser with incoherent optical feedback

Based on a semiconductor laser （SL） with incoherent optical feedback, a novel all-optical scheme for generating tunable and broadband microwave frequency combs （MFCs） is proposed and investigated numerically. The results show that, under suitable operation parameters, the SL with incoherent optical feedback can be driven to operate at a regular pulsing state, and the generated MFCs have bandwidths broader than 40 GHz within a 10 dB amplitude variation. For a fixed bias current, the line spacing （or repetition frequency） of the MFCs can be easily tuned by varying the feedback delay time and the feedback strength, and the tuning range of the line spacing increases with the increase in the bias current. The linewidth of the MFCs is sensitive to the variation of the feedback delay time and the feedback strength, and a linewidth of tens of KHz can be achieved through finely adjusting the feedback delay time and the feedback strength. In addition, mappings of amplitude variation, repetition frequency, and linewidth of MFCs in the parameter space of the feedback delay time and the feedback strength are presented.

Microwave frequency transfer over a 112-km urban fiber link based on electronic phase compensation

We demonstrate the transmission of a microwave frequency signal at 10 GHz over a 112-km urban fiber link based on a novel simple-architecture electronic phase compensation system.The key element of the system is the low noise frequency divider by 4 to differentiate the frequency of the forward signal from that of the backward one,thus suppressing the effect of Brillouin backscattering and parasitic reflection along the link.In terms of overlapping Allan deviation,the frequency transfer instability of 4.2×10-15 at 1-s integration time and 1.6×10-18 at one-day integration time was achieved.In addition,its sensitivity to the polarization mode dispersion in fiber is analyzed by comparing the results with and without laser polarization scrambling.Generally,with simplicity and robustness,the system can offer great potentials in constructing cascaded frequency transfer system and facilitate the building of fiber-based microwave transfer network.

Optimal Microwave Radiation Field Parameters for Mercury Ion Microwave Frequency Standards

We propose a method to determine the optimal power of the microwave resonance transition that simultaneously improves the signal-to-noise ratio and reduces line width based on saturation broadening theory and experiment. Saturation broadening spectra of the ground state hyperfine transition of trapped 199Hg＋ ions are measured and analyzed. The value of the optimal microwave power is obtained by using the proposed method and is verified. Rabi oscillations decay spectra of trapped 199Hg＋ ions are observed and the optimal microwave irradiation time for the maximum transition signal intensity is determined. This work will help to improve the short-term frequency stability of the mercury ion microwave frequency standard.

Photonic generation of RF and microwave signal with relative frequency instability of 10^(-15)

We demonstrate the ultra-stable frequency sources aiming to improve the short-time instability of primary frequency standards.These sources are realized by using photonic generation approach,and composed of ultra-stable lasers,optical-frequency-combs,optical signal detecting parts,and synthesizers.Preliminary evaluation shows that the sources produce fixed-frequency at 9.54(/9.63)GHz,10 MHz,and tunable-frequency around 9.192 GHz with relative frequency instability of 10^(-15) for short terms.

Effect of microwave frequency on plasma formation in air breakdown at atmospheric pressure

Microwave breakdown at atmospheric pressure causes the formation of a discrete plasma structure. The onedimensional fluid model coupling Maxwell equations with plasma fluid equations is used to study the effect of the microwave frequency on the formation of air plasma. Simulation results show that, the filamentary plasma array propagating toward the microwave source is formed at different microwave frequencies. As the microwave frequency decreases, the ratio of the distance between two adjacent plasma filaments to the corresponding wavelength remains almost unchanged（on the order of 1/4）, while the plasma front propagates more slowly due to the increase in the formation time of the new plasma filament.

Design and test of the microwave cavity in an optically-pumped Rubidium beam frequency standard

We are developing a compact rubidium atomic beam frequency standard with optical pumping and detection. The cavity for microwave interrogation is an important part of the clock. The cavity in our design is a Ramsey-type, E-bend one, which is the same as the conventional method in most cesium beam clocks. Requirements for the design are proposed based on the frequency shift associated with the cavity. The basic structure of the cavity is given by theoretical analysis and detailed dimensions are determined by means of electromagnetic field simulation with the help of commercial software.The cavity is manufactured and fabricated successfully. The preliminary test result of the cavity is given, which is in good agreement with the simulation. The resonant frequency is 6.835 GHz, equal to the clock transition frequency of87 Rb, and the loaded quality factor is 500. These values are adjustable with posts outside the cavity. Estimations on the Ramsey line width and several frequency shifts are made.

Broadband microwave frequency doubler based on left-handed nonlinear transmission lines

A bandwidth microwave second harmonic generator is successfully designed using composite right/left-handed non- linear transmission lines （CRLH NLTLs） in a GaAs monolithic microwave integrated circuit （MMIC） technology. The structure parameters of CRLH NLTLs, e.g. host transmission line, rectangular spiral inductor, and nonlinear capacitor, have a great impact on the second harmonic performance enhancement in terms of second harmonic frequency, output power, and conversion efficiency. It has been experimentally demonstrated that the second harmonic frequency is deter- mined by the anomalous dispersion of CRLH NLTLs and can be significantly improved by effectively adjusting these structure parameters. A good agreement between the measured and simulated second harmonic performances of Ka-band CRLH NLTLs frequency multipliers is successfully achieved, which further validates the design approach of frequency multipliers on CRLH NLTLs and indicates the potentials of CRLH NLTLs in terms of the generation of microwave and millimeter-wave signal source.

PHASE AND FREQUENCY LOCKING OF MICROWAVE AND MILLIMETER WAVE POWER COMBINING

The phase and frequency locking of microwave, millimeter wave power combining were analysed and summarized in an all-round way. The master/slave phase locking of cavity oscillators, the peer phase locking of mutually coupled oscillators, and the peer phase locking of ring-connected multiple oscillators were investigated. The results of numerical calculations, and the relations of phase to phase locking model and oscillator parameters were given. And the cavity and space power combining aspects for microwave and millimeter wave were presented.

Controllable microwave frequency comb generation in a tunable superconducting coplanar-waveguide resonator

Frequency combs are useful in a wide range of applications,such as optical metrology and high-precision spectroscopy.We experimentally study a controllable frequency comb generated in a tunable superconducting coplanarwaveguide resonator in the microwave regime.A two-tone drive is applied on one of the resonance modes of the resonator and comb generation is observed around the resonance frequency of the resonator.Both central frequency and teeth density of the comb are precisely controllable,and the teeth spacing can be adjusted from Hz to MHz.Moreover,we show that a few hundreds of sidebands can be generated using a sufficiently strong drive power and the weakest drive power needed to generate the comb can be reduced to approach the quantum limit.These experimental results can be qualitatively explained via theoretical analysis.

A low-noise X-band microwave source with digital automatic frequency control for electron paramagnetic resonance spectroscopy

We report a new design of microwave source for X-band electron paramagnetic resonance spectrometer.The microwave source is equipped with a digital automatic frequency control circuit.The parameters of the digital automatic frequency control circuit can be flexibly configured for different experimental conditions,such as the input powers or the quality factors of the resonator.The configurability makes the microwave source universally compatible and greatly extends its application.To demonstrate the ability of adapting to various experimental conditions,the microwave source is tested by varying the input powers and the quality factors of the resonator.A satisfactory phase noise as low as-135 d Bc/Hz at 100-k Hz offset from the center frequency is achieved,due to the use of a phase-locked dielectric resonator oscillator and a direct digital synthesizer.Continuous-wave electron paramagnetic resonance experiments are conducted to examine the performance of the microwave source.The outstanding performance shows a prospect of wide applications of the microwave source in numerous fields of science.

Dick Effect in a Microwave Frequency Standard Based on Laser-Cooled 113Cd+ Ions

The Dick effect is one of the main limits to the frequency stability of a passive frequency standard, especially for the fountain clock and ion dock operated in pulsed mode which require unavoidable dead time during interrogation. Here we measure the phase noise of the interrogation oscillator applied in the microwave frequency standard based on laser-cooled 113^Cd＋ ions, and analyze the Allan deviation limited by the Dick effect. The results indicate that the Dick effect is one of the key issues for the cadmium ion dock to reach expected frequency stability. This problem can be resolved by interrogating the local oscillator continuously with two ion traps.

Dependence of Switching Current Distribution of a Current-Biased Josephson Junction on Microwave Frequency

We investigate the distribution of the switching current of a current-biased Josephson junction （CBJJ） and its dependence on the microwave frequency using two theoretical methods, one of which is the quantum trajectory method and the other is the master equation method. Both the methods show that the distribution of the switching current of CBJJ will exhibit double peaks in a certain range of microwave frequency if proper microwave power is given, and the gap between the two peaks will increase with the microwave frequency. The obtained results can be used to identify the energy difference of the ground and first excited states in a Josephson junction for any bias current.

Design of synthetic aperture radar low-intercept radio frequency stealth

Not confined to a certain point,such as waveform,this paper systematically studies the low-intercept radio frequency(RF)stealth design of synthetic aperture radar(SAR)from the system level.The study is carried out from two levels.In the first level,the maximum low-intercept range equation of the conventional SAR system is deduced firstly,and then the maximum low-intercept range equation of the multiple-input multiple-output SAR system is deduced.In the second level,the waveform design and imaging method of the low-intercept RF SAR system are given and verified by simulation.Finally,the main technical characteristics of the lowintercept RF stealth SAR system are given to guide the design of low-intercept RF stealth SAR system.

Scattered and rapid intrahepatic recurrences after radio frequency ablation for hepatocellular carcinoma

AIM： To evaluate a series of patients with hepatocellular carcinoma （HCC） treated with several different protocols and devices. METHODS： We treated 138 patients [chronic hepatitis/ liver cirrhosis （Child-Pugh A/B/C）, 3/135 （107/25/3）] with two different devices and protocols： cool-tip needle [initial ablation at 60 W （standard method） （n = 37） or at 40 W （modified method） （n = 28）] or; ablation with a LeVeen needle using a standard single-step, full expansion （single-step） method （n = 39） or a multi-step, incremental expansion （multi-step） method. RESULTS： Eleven patients experienced rapid and scattered recurrences 1 to 7 mo after the ablation. Nine patients were treated by the cool-tip original protocol （60 W） （9/37 = 24%） and the other two by the LeVeen single-step method （2/39 = 5%）. The location of the recurrence was surrounding and limited to the site of ablation segment in three cases, and spread over one Iobule or both Iobules in the other eight cases. There was no recurrence in the patients treated with the modified cool-tip modified method （40 W） or the LeVeen multi-step method. CONCLUSION： There is a risk of rapid and scattered recurrence after RFA, especially when the standard cool- tip procedure is used. Because such recurrence would worsen the prognosis, we recommend that modified protocols for the cool-tip and LeVeen needle methods should be used in clinical practice.