A simple power supply and control unit for pulsed operation of a microchannel plate imaging detector

A simple power supply and control electronic system to operate microchannel plate (MCP) detectors for imaging transient events such as X-ray emission from laser-produced plasmas is described. The power supply consists of two parts, viz. a high voltage pulse generator of 1 kV amplitude with a variable pulse duration of 100 μs to 1 ms for the MCP input, and a regulated 5 kV d.c. for biasing the phosphor screen. The control unit synchronizes the high voltage pulse with the event and operates a safety switch to guard the detector against any accidental rise in the background pressure. A signal-to-noise ratio ≥ 75 is obtained, limited by the noise contribution of the Charge Coupled Device (CCD)-frame grabber combination attached to the MCP. The system is compact, has provision for both positive and negative pulses, is extendable to higher voltages for operating two-stage MCP, and is immune to electromagnetic interference.     

Development of picosecond time resolution optical and X-ray streak cameras

We describe the development of an optical and an X-ray streak camera with picosecond time resolution. The entire peripheral electronics and testing systems have been developed indigenously. Both the streak cameras provide ∼ 15 mm/1 ns streak rate with a sweep voltage of ∼ 1 kV amplitude and rise time of 1 ns. The time and spatial resolution of the optical streak camera have been found to be ∼ 17 ps and 100 μm respectively. The sweep pulse generator developed for this purpose provides a step pulse of rise time ≦1 ns and amplitude ∼ 2 kV. The laser diode used for testing the optical streak camera provides multiple pulsation when the pump current is increased beyond a critical threshold.     

Subnanosecond pulsed X-ray source based on nanosecond discharge in air at atmospheric pressure

We have studied the characteristics of an X-ray source based on a gas diode filled with air at atmospheric pressure. Driven by a SLEP-150 pulser with a maximum voltage amplitude of ∼140 kV, a pulse full width at half maximum (FWHM) of ∼1 ns, and a leading front width of ∼0.3 ns, a soft X-ray source produces subnanosecond pulses with an FWHM not exceeding 600 ps and an exposure dose of ∼3 mR per pulse. It is shown that the main contribution to the measured exposure dose is due to X-ray quanta with an effective energy of ∼7.5 keV.     

Discharge dynamics and characteristics of atmospheric glow discharge excited by sub-microsecond high voltage pulses

A study of sub-microsecond high voltage pulse excited glow discharges in atmospheric helium is carried out here. The discharge is generated between the powered copper electrode and ground electrode covered with a ceramic sheet. The electrical and optical characteristics of two discharge events in each voltage pulse in terms of discharge current amplitude, current pulse duration, time instant at current peak and accumulated charge in discharge are investigated on voltage pulse duration and pulse voltage magnitude. The time-resolved imaging with 5 ns exposure time is used to demonstration the spatio-temporal evolution of discharge. It is found that the first discharge depends markedly on pulse voltage magnitude and space charge accumulation on ceramic sheet surface plays an important role on ignition of second discharge.     

Count rate and dynamic range of an X-ray imaging system with MCP detector

X-ray imaging system with edge-on MCP detector has recently been evaluated for potential application in medical imaging. Although this system has been able to provide good quality images, its clinical implementation is limited by its low count rate. The system count rate was limited by a delay line position encoding electronics, which processes the events serially. The count rate of the MCP was also limited because of high gain required for delay line electronics. In this work, the count rate and dynamic range of the system was evaluated. The count rate limitation due to readout electronics and MCP was measured separately. Pulse counting and charge integrating modes of the MCP detector was used. Photon beams for these measurements were generated using an X-ray tube at 50 and 90 kVp peak potentials with 41 and 75 keV average photon energies, respectively. The electronics dead time was measured to be 20% at 200 kHz total system count rate. In addition, 15–30% of the photon signals were not detected by delay line electronics due to exponential pulse height distribution of the MCP signals. The exponential pulse height distribution of the MCP was due to the edge on illumination mode of the MCP and the high energy X-ray photon beam. Image artifacts due to dead time and signal loss were evaluated. Multichannel ASIC readout electronics was considered as a potential solution to the count rate and dynamic range problem. It was shown that the edge-on MCP detector can operate in pulse counting mode at low gains of 10⁴–10⁵ e/photon required for ASIC electronics. It was also shown that at this low gain, the MCP count rate can be increased up to 1.33×10⁶ count/s/pixel, for 100 μm detector pixels, which is appropriate for clinical X-ray imaging.     

Generation of intense soft X-rays from capillary discharge plasmas

X-ray lasing through high voltage, high current discharges in gas filled capillaries has been demonstrated in several laboratories. This method gives highest number of X-ray photons per pulse. The fast varying current and the jxB magnetic force compress the plasma towards the axis forming a hot, dense, line plasma, wherein under appropriate discharge conditions lasing occurs. At Laser Plasma Division, RRCAT, a program on high voltage capillary discharge had been started. The system consists of a 400 kV Marx bank, water line capacitor, spark gap and capillary chamber. The initial results of the emission of intense short soft X-ray pulses (5–10ns) from the capillary discharge are reported.     

Wide-aperture plasma jet source based on low-voltage spark discharge with ferroelectric electrode

Low-voltage vacuum spark discharge initiated at a storage capacitor voltage of 75–600 V using a metal grid cathode situated on the front surface of a polarized ferroelectric (FE) electrode has been experimentally studied. The discharge was initiated when a control voltage pulse with an amplitude of 1 kV and a duration of 100 ns at only negative polarity was applied to the rear FE surface (irrespective of the direction of its polarization vector). Optical measurements showed that the emitting surface area on the cathode increases approximately in proportion to the discharge voltage. According to the collector measurements, the ion plasma flux has slow and fast components, the velocities of which remain almost constant when the discharge current amplitude varies in a wide interval.     

Design and performance studies of an MCP based pinhole camera for laser-produced plasma diagnostics

An X-ray pinhole camera system has been developed and tested to obtain spatially resolved time-integrated X-ray images of laser-produced plasmas (Nd: YAG laser of 75mJ energy in 35 picoseconds) using a single stage micro channel plate (MCP) detector coupled to a CCD camera-frame grabber system instead of X-ray film. Thick slab targets of aluminium, copper and gold were used to produce plasmas with different X-ray yields at laser intensities ranging from 8 × 10¹¹ to 8 × 10¹² W/cm². MCP gain was increased with the help of biasing voltages to record low intensity X-ray images, which have been analysed using an image processing software “PROMISE” developed in-house. The experimental results are presented.     

Development of a 300-kV Marx generator and its application to drive a relativistic electron beam

We have indigenously developed a twenty-stage vertical structure type Marx generator. At a matched load of 90–100 Ω, for 25 kV DC charging, an output voltage pulse of 230 kV, and duration 150 ns is obtained. This voltage pulse is applied to a relativistic electron beam (REB) planar diode. For a cathodeanode gap of 7.5 mm, an REB having beam voltage 160kV and duration 150ns is obtained. Brass as well as aluminum explosive electron emission-type cathodes have been used     

Measurement of scintillator parameters using pulsed X-ray excitation

A setup for measurement of the major scintillator parameters, including decay time constants, light yield, and emission spectrum, has been developed. It consists of three main components: a small-size source of short X-ray pulses, a cryostat, and a registration system. The ultimate parameters characterizing the source are as follows: pulse duration, ～0.5 ns; repetition rate, 100 kHz; X-ray tube voltage, 30 kV; and current amplitude, 0.5 A. The registration system allows the measurements to be carried out in the 200–800 nm spectral range over the time intervals from 0.5 ns to 50 μs with resolution not worse than 0.1 ns.     

Binary semiconductor In2Te3 for the application of phase-change memory device

Nonvolatile phase-change memory devices with 500 nm contact hole based on In₂Te₃ were successfully fabricated by using focused ion beam, pulsed laser deposition, and dc magnetic sputtering techniques. In₂Te₃ films were characterized by using differential thermal analysis, X-ray diffraction, and UV–vis diffuse absorption spectroscopy, respectively. The devices can be switched between high and low resistance states repeatedly with the programmed voltage pulses. The reset operation (crystalline to amorphous) was done by the voltage pulse with a magnitude of 3.5 V and a duration of 30 ns, and the set operation (amorphous to crystalline) was done by the voltage pulse with a magnitude of 1.4 V and a duration of 100 ns. A dynamic resistance switching ratio (OFF/ON ratio) of 3.2 × 10³ has been obtained.     

Dense plasma formation on the surface of a ferroelectric cathode

An enhanced electron emission mode of the ferroelectric plasma cathode operation is reported. The enhanced emission is achieved due to the generation of dense plasma (10¹⁹-10²⁰ m⁻³). This plasma is formed by a flashover which is initiated by charged particles. These particles are attracted to the ferroelectric surface by a driving electric field and are released during its decay. Generation of an electron beam with current amplitude ?2.5 kA is demonstrated in a diode under an accelerating voltage of 150-300 kV and pulse duration of 300 ns.     

Discharge characteristic of nanosecond-pulse DBD in atmospheric air using magnetic compression pulsed power generator

Dielectric barrier discharge driven by repetitive nanosecond pulses can offer highly efficient non-thermal plasma at atmospheric pressure and is widely used for plasma applications. In this paper, the discharge is generated using a compact pulsed power generator based on one-stage magnetic compression. The output pulse can be up to 30 kV with a rise time of about 40 ns and a full width at half maximum of 70 ns. The electrical characteristics of the discharge parameters are studied by the measurement of voltage and current waveforms. The effects of applied voltage amplitude, voltage polarity, pulse repetition frequency, and barrier dielectric on discharge characteristics are investigated, respectively. The experimental results show that the discharge current, discharge power and electron density increase with the increase of the applied voltage, and the pulse repetition frequency has a slight effect on the electrical parameters. Moreover, the discharge current is influenced by the dielectric barrier, but it is not varied with the polarity of applied pulses.     

Electron emission from La-Doped lead zirconate stannate titanate antiferroelectric ceramic under fast electric field pulses

High current density pulsed-electron emission is observed from a lead zirconate stannate titanate lanthanum-doped (PLZST) antiferroelectric ceramic disc on application of positive or negative triggering voltage pulses. Electron-emission pulse with a peak current density 1,400 A/cm² and a full-width at half-maximum (FWHM) duration of 560 ns was recorded in the presence of a 3.5 kV dc extraction voltage. It is higher than the various earlier results obtained using lead zirconate titanate ferroelectric ceramic. Self emission of electrons with a current density of 1.3 A/cm² and the FWHM duration of about 100 ns were also observed. Strong electrons emission was the co-effect of surface plasma and noncompensated charges at the surface of the antiferroelectric. Field-induced local phase transition in the vicinity close to triple junction results in primary electron emission from these areas. These primary emission electrons ignited surface plasma and then led to the strong emission.     

Short-pulse range-gated optical imaging in turbid water

Using a very short laser pulse and camera gate in a range-gated optical imaging system, we have demonstrated an improvement in imaging in turbid water. A 0.5-ns FWHM laser pulse at 532 nm and a camera gate time ranging down to 120 ps were used to record images of 0.635-cm-bar-resolution targets at 6.5 attenuation lengths through 4.572 m of turbid water. With a four-channel version of this camera, coupled to a CCD camera, we can observe a quasi-three-dimensional image in real time.     

Ultrafast current switching using the tunneling-assisted impact ionization front in a silicon semiconductor closing switch

Ultrafast current switching in semiconductors, based on the mechanism of tunneling-assisted impact ionization front, has been experimentally implemented and theoretically studied. A voltage pulse with an amplitude of 220 kV and a front duration of 1 ns was applied to a semiconductor device containing 20 serially connected silicon diode structures. After switching, 150-to 160-kV pulses with a power of 500 MW, a pulse duration of 1.4 ns, and a front duration of 200–250 ps were obtained in a 50-Ω transmission line. The maximum current and voltage buildup rates amounted to 10 kA/ns and 500 kV/ns, respectively, at a switched current density of 13 kA/cm². The results of numerical simulation are presented, which show that the current switching is initiated at a threshold field strength of about 1 MV/cm in the vicinity of the p-n junction, where the tunneling-assisted impact ionization begins.     

Recombination X-ray radiation from plasma liners

The possibility of creating a high-power X-ray source based on the recombination radiation mechanism has been studied and analyzed. The recombination X-ray radiation from neon, aluminum, and argon was obtained in experiments with plasma liners performed on the GIT-12 setup using a current pulse amplitude of ∼2.5 MA and a leading pulse front width of ∼300 ns. The obtained results show that the recombination mechanism of X-ray generation offers good prospects for the development of a high-power X-ray source with photon energies in the 7–20-keV range.     

Ir TES Electron-Phonon Thermal Conductance and Single Photon Detection

We have operated an Ir TES as detector for single photon at 450 nm in a temperature range of 100–120 mK. The decoupling of the electron gas from the phonons in the film, caused by the fifth power dependence of the temperature, is measured from the pulse decay time. The detection of single photon generated by a laser diode with a pulse length of 500 ns in a 25×25 μm² detector area at a base temperature of 90 mK is shown.      

IGBT-based kilovoltage pulsers for ultrasound measurement applications

Two high-voltage pulser designs are presented that offer advantages in some ultrasound measurement applications, such as driving thick ultrasonic source transducers used for broadband measurements of attenuation or hydrophone frequency response and directivity. The pulsers use integrated gate bipolar transistors (IGBTs) as the switching devices, and in one design an output voltage pulse is produced that has a peak amplitude nearly twice that of the supply voltage. The pulsers are inexpensive and relatively easy to construct. The power supply need only provide the average current to charge the capacitors, as opposed to the much higher peak pulse current. With a 1200 V supply and a pulse repetition frequency of 200 Hz, the nondoubling and doubling pulsers provided peak voltages of greater than 1100 V and 2200 V, respectively, into loads ranging from 50 /spl Omega/ to 500 /spl Omega/. For a 50 /spl Omega/ load, slewing rates of 38 V/ns and 23 V/ns were measured for the nondoubling and doubling pulsers, respectively. For a 500 /spl Omega/ load these values were 56 V/ns and 36 V/ns.     

Dark noise in microchannel plate X-ray detectors

We present a detailed study of dark noise sources in microchannel plate (MCP) X-ray detectors. Previously postulated noise mechanisms are critically reviewed. Noise measurements carried out in the light of the review are then reported. The sea-level background count rate in two-stage MCP detectors is shown to have two principal components. The first — variable with position across the MCP and decaying with time under vacuum in a manner dependent on the plate history — is attributed to outgassing of the channel plate structure. The second — isotropic, independent of detector bias voltages, time and temperature — is shown to be consistent, in magnitude and in terms of its output charge spectrum, with the results of beta decay from the radioactive potassium content of the multiplier lead glass. Based on this identification of noise sources, prospects for the production of ultralow-noise MCP detectors for imaging X-ray astronomy are discussed. Measurements of noise reduction by coincidence methods using a partitioned anode are described.