a compelling need to utilize advanced technologies in the agriculture sector to increase agricultural productivity and reduce food losses to guarantee food security. [2] In this regard, "smart agriculture" or "precision agriculture" has been attracting increasing attention due to its capability for using less to grow more compared to traditional agricultural practices. In addition, it improves the quality of the work environment and social aspects of farming, ranching, and other relevant professions. [4] Smart agriculture comprises a set of technologies that combines sensors, information systems, enhanced machinery, and informed management to optimize production by accounting for variabilities and uncertainties within sustainable agricultural systems. [3][4][5] Among the set of technologies, advanced sensing systems that monitor soil health and conditions and crop developments are of paramount importance because they collect and evaluate critical data for decision making and management, especially when crop growth conditions vary considerably over space and time. Spatial variation may result from soil properties, diseases, weeds, pests, and previous land management. In particular, some soil properties (e.g., moisture, pH, nutrients) and plant diseases may form long-term spatial patterns. Temporal variability arises from weather patterns and management practices. In summary, the soil properties relevant to crop growth include a range of soil conditions including soil gas, moisture, temperature, nutrients, pH, and pollutants in the soil (Figure 1). [6,7] Monitoring the soil conditions will provide key information not only to improve resource utilization to maximize farming outputs and minimize environmental side effects but also to build site-specific databases of relationships between soil conditions and plant growth for intelligent and sustainable agriculture systems. Traditionally, soil properties are measured by soil sampling and offsite laboratory analysis or by on-site measurement to provide an extensive knowledge of soil information. [8] Seasonally varying crop growth conditions, such as water stress, lack of nutrients, diseases, weeds, and insects, are evaluated by visual inspection and laboratory analysis of plant tissues. The relatively periodically coarse sampling/measurement rate of these conventional strategies may not be sufficient to reveal variation at the appropriate spatial and temporal resolution. Novel technologies for collecting soil information with sufficient spatiotemporal resolutions are in demand to build efficient smart or precision agriculture systems. With the Soil sensors and plant wearables play a critical role in smart and precision agriculture via monitoring real-time physical and chemical signals in the soil, such as temperature, moisture, pH, and pollutants and providing key information to optimize crop growth circumstances, fight against biotic and abiotic stresses, and enhance crop yields. Herein, the recent advances of the important soil sensors in agricultural applications, in...

This paper presents for the generation of a small size high current density pseudospark (PS) electron beam for a high frequency (0.2 THz) Backward Wave Oscillator (BWO) through a Doppler up-shift of the plasma frequency. An electron beam ∼1 mm diameter carrying a current of up to 10 A and current density of 108 A m−2, with a sweeping voltage of 42 to 25 kV and pulse duration of 25 ns, was generated from the PS discharge. This beam propagated through the rippled-wall slow wave structure of a BWO beam-wave interaction region in a plasma environment without the need for a guiding magnetic field. Plasma wave assisted beam-wave interaction resulted in broadband output over a frequency range of 186–202 GHz with a maximum power of 20 W.

The growing impact of airborne pollutants and explosive gases on human health and occupational safety has escalated the demand of sensors to monitor hazardous gases. This paper presents a new miniaturized planar electrochemical gas sensor for rapid measurement of multiple gaseous hazards. The gas sensor features a porous polytetrafluoroethylene substrate that enables fast gas diffusion and room temperature ionic liquid as the electrolyte. Metal sputtering was utilized for platinum electrodes fabrication to enhance adhesion between the electrodes and the substrate. Together with carefully selected electrochemical methods, the miniaturized gas sensor is capable of measuring multiple gases including oxygen, methane, ozone and sulfur dioxide that are important to human health and safety. Compared to its manually-assembled Clark-cell predecessor, this sensor provides better sensitivity, linearity and repeatability, as validated for oxygen monitoring. With solid performance, fast response and miniaturized size, this sensor is promising for deployment in wearable devices for real-time point-of-exposure gas pollutant monitoring.

Experimental studies of the production and propagation of an electron beam from a multigap pseudospark discharge are presented. From a three-gap pseudospark, a beam up to 680 A was measured at the anode at an applied dc voltage of 23 kV. This beam can propagate downstream as far as 20 cm in a gaseous environment with no guiding magnetic field, which confirms that the transport of the electron beam was based on the neutralization of the space charge of the electron beam due to the ionization of the gas molecules by the beam itself. The beam is of very small size of 1-3 mm in diameter and is ideal to drive high frequency radiation. Higher energy electron beam pulses were generated using a 14-gap pseudospark discharge powered by a cable pulser capable of producing 120 ns duration and 170 kV voltage pulses. The beam measured had a current of up to 110 A. Interactions between the produced beam and a Ka-band Cherenkov maser and a W-band backward wave oscillator slow wave structure were simulated and designed. Millimeter wave pulses were detected from the Cherenkov maser and backward wave oscillator beam-wave interaction experiments

This paper presents the first experimental results of an extended interaction oscillator (EIO) based on a pseudospark-sourced electron beam, which produced a peak output power over 38 W at W-band. The advantages of the newly developed device are: 1) transport of the electron beam by the positive-ion focusing channel without the need of an external magnetic field and 2) high interaction impedance and high gain per unit length of the EIO circuit. The experimental results agree well with the 3-D particle-in-cell simulations.

A new method to generate ultrahigh-power microwave pulses compatible with mildly relativistic electron sources is proposed. This method involves a novel microwave compressor in the form of a metal helically corrugated waveguide, which can enhance the power of frequency-modulated nanosecond pulses up to the multigigawatt level. The results of the proof-of-principle experiments at kilowatt power levels are in good agreement with theory. DOI: 10.1103/PhysRevLett.92.118301 PACS numbers: 84.40.Ik, 41.20.Jb, 41.60.Cr, 84.40.Fe Multigigawatt pulsed rf power, which has recently become available due to the development of relativistic electronics, opens up many new opportunities both for fundamental studies and future applications [1]. For example, it is important to recall that the power P 0 m 2 c 5 =e 2 8:7 GW (m, e are electron mass and charge, respectively, and c is speed of light) focused on an area of the order of the wavelength squared imparts a relativistic oscillatory velocity to electrons. Power at the multi-GW level was obtained at relatively long wavelengths of 3-10 cm when using electron beams with particle energies higher than 1 MeV and currents of tens of kA that were available at a small number of unique high-current accelerators. In this Letter, an alternative method of producing multi-GW pulses is proposed. It uses the amplification of significantly lower power radiation, which is generated with a certain frequency modulation during the pulse. Compression of this pulse is achieved after propagation in a waveguide with the proper frequency dispersion. If a compression factor K p 10-100 is possible, then the input pulse power required will be of the order of hundreds of MW. Such pulses are produced routinely in Cherenkov relativistic traveling wave amplifiers (TWTs) or backward-wave oscillators (BWOs), driven with modest energy 0.5-1.0 MeV electron beams [2,3]. However, compression of power from these devices requires additionally a sufficiently broadband frequency modulation within the pulse. A novel rf compressor in the form of a helically corrugated cylindrical waveguide can have the necessary dispersion while simultaneously it can sustain high strength rf fields as well as possessing a low level of reflections. Similar helically corrugated waveguides were intensively studied for gyro-TWTs [4].Consider compression of a quasimonochromatic pulse with frequency, monotonously varying in time from ! 1 to ! 2 . If the wave group velocity in the dispersive medium is an increasing function of frequency, v gr ! 2 > v gr ! 1 , then the tail of the pulse will overtake its leading edge, resulting in pulse shortening and a corresponding growth in amplitude if the losses are sufficiently low. To produce ultrahigh powers, the compressor characteristics should be optimized in order to achieve a maximum power compression factor at reasonable energy losses. Among the various methods of compression (see, e.g., [5]), the use of a metal waveguide is attractive because of its capability of handling high power, as well as...

Detailed experimental results from the first free-electron maser experiment to use a pseudospark-based electron beam are presented in this paper. These include the design and realization of a pseudospark-based electron beam source and Cherenkov maser experiment. A pulsed, 70-80 kV, 10 A electron beam was obtained from the hollow cathode discharge phase of an 8-gap pseudospark (PS) discharge. The beam was used to produce coherent microwave radiation via a Cherenkov interaction between the electron beam and the TM01 mode of a 60-cm long alumina-lined waveguide. A gain of 29+/-3 dB was measured and an output power of 2+/-0.2 kW in the frequency range 25.5-28.6 GHz. Results from numerical simulations of the Cherenkov amplification are also presented and found to be consistent with the experimental results

Propagation and post-acceleration of a pseudospark-sourced electron beam from a three-gap pseudospark discharge chamber were studied in recent experiments. The pseudospark produced an electron beam of two phases, an initial 22 kV, 50 A hollow cathode phase beam of brightness 10(9-10) Am-2 rad(-2) followed by a 200 V, 200 A conductive phase (CP) beam of brightness 10(11-12) Am-2 rad(-2). The aim of these experiments was to post accelerate the lower-voltage, higher-current CP beam using an acceleration unit driven by a 40 kV, 125 ns voltage pulse produced by a cable Blumlein. The experiments were realized by attaching an acceleration unit to the downstream side of the anode of the discharge chamber. Both the pseudospark discharge and the cable Blumlein were triggered to ensure time correlation between initiation of the pseudospark discharge and post-acceleration of the beam