kV Range Research Articles

Paschen Problems in Large Coil Systems

When a breakdown of the thermally insulating vacuum occurs in an energized high power coil system (e.g. a leakage of the He-cooling system or to the surrounding air) the coils warm up and have to be discharged fast to avoid destruction. Due to the coil inductance and the necessary discharge time, a high voltage is produced in the coil and the feeder system. Therefore an unspecified pressure coincides with the occurrence of a high voltage at the coil and feeder system. It is well known that under these conditions arcing can occur at a magnet when it is not perfectly insulated even for voltages 1 kV (Paschen-effect). The gas dependent breakdown voltage minimum occurs typically in the range of some 100 V at a pressure value that depends from geometry and temperature. For lower and higher pressures the breakdown voltage is increasing. For a high power magnet system an insulation is necessary that can withstand high voltage in the 10 kV range although any possible pressure is applied to the system. Usually a pressure independent solid insulation is used to solve this requirement. In order to test the reliability of this electrical insulation under all pressure conditions, a so-called Paschen test has to be performed. This Paschen test is a high voltage test with pressures varying between good vacuum and ambient pressure. In this paper examples for insulation failures on large coils found with Paschen testing are given. In a series of experiments with samples that have been prepared with intended insulation faults the necessity of Paschen testing is demonstrated to identify insulation problems that are not visible under good vacuum or ambient pressure. Therefore, a Paschen-test is also a useful high voltage diagnostic tool to localize insulation faults in solid insulated magnets.

Development of a Scintillating optical fiber dosimeter with silicon photomultipliers

A radiation dosimeter for low dose rate based on a scintillating optical fiber coupled to a high gain photon counting capable silicon photomultiplier (SiPM) for light readout satisfies most of the requirements for in-vivo and real-time dosimetry. It uses a small size scintillator, it is flexible and reasonably water-equivalent. Promising results were obtained when operating the device in current mode from low to high dose rates, detecting radiation from an X-ray tube operating in the 15–40 kV range and for anode currents as low as a few μA. As single-photon detectors, the major drawback of SiPMs is their high dark count rate (noise), which represents a problem for low dose rate measurements in single photon counting mode. This drawback can be reduced by cooling the SiPMs or by using a much more efficient proposed solution which uses two SiPMs operating in coincidence mode reading out the same optical fiber and making it possible to reject false events triggered by dark noise. We have implemented a simple and low-cost system with dedicated front-end electronics operating in pulse mode for coincidence detection. Studies of the dosimeter’s performance operating in current mode, as a function of the X-ray tube current and voltage, show good sensitivity even for low radiation dose. Operating in pulse mode with the coincidence system, under low activity gamma irradiation, it was possible to reduce the dark noise to residual values.

SU‐E‐T‐113: Measurement of the Cutout Output Factor on Ortho‐Voltage Unit Using EBT‐2 Model GafchromicTM Film

Purpose: In this study, we evaluate feasibility of using EBT‐2 model GafchromicTM film to measure patient specific cutout output factors on an ortho‐voltage treatment machine. Methods: A Gulmay Orthovoltage D3225 unit was used to produce clinical beams in kV range from 50 kVp to 220 kVp. Previously established air kerma based radiochromic film dosimetry protocol for low energy x‐ray beams was used. A closed‐end applicator with 10 × 10 cm2 field size and SSD 50 cm was employed. Cutouts were made from a lead sheet of 1.8 mm in thickness. A 6‐cm thick slab of Solid WaterTM was used as a backscatter material for output factor measurement. In‐house made MatLab routines were used to analyze scanned images of exposed and unexposed films and a PTW 23342 parallel‐plate ion chamber (PP IC) was used to measure the cutout output factors for comparison. Results: Air‐Kerma based calibration curve was created in the 1–300 cGy range. Film dosimetic accuracy was verified for known doses to water of 1.00 and 2.00 Gy, respectively, with maximum observed difference of 0.4%. Two‐dimensional dose maps and dose profiles were obtained for various cutout sizes ranging from 0.8 cm diameter circle to 10 × 10 cm2 square. Comparisons of output factors between film and pp ion chamber measurements show that there are no differences for large cutouts but the chamber measurements are up to 11 % higher for small cutout size. Conclusions: We demonstrated that the air‐Kerma based radiochromic film dosimetry protocol using EBT‐2 model GafChromicTM film for low energy x‐ray beam is adequate for patient specific cutout output measurements on orthovoltage units, especially for small size cutout measurements, where IC measurements give higher output factor than films due to ICˈs over responses to electron contaminations from lead sheets.

SU‐E‐T‐233: Commissioning of An Implantable Dosimeter for External Beam Radiation Therapy

Purpose: To commission a Dose Verification System (DVS) with permanently implanted in vivo dosimeters. To acquire and evaluate sufficient data for establishing standard of quality in a clinical setting.Methods: DVS is permanent implantable wireless radiation sensor based on MOSFET for absolute dose measurements. To evaluate characteristics of dosimeter, following tests were performed: temperature dependence, reproducibility, field size dependence, post‐irradiation signal drift, dependence on average dose‐rate, linearity test, angular dependence, dose‐rate dependence, irradiation depth dependence and a multiple reading effect. Dosimeter is not currently calibrated for use in kV range. Nonetheless, effects of conebeam CT(CBCT) imaging to dosimeter were investigated. Phantom studies were performed in both air and water using an Elekta Synergy‐S Beam‐Modulator linear accelerator. Results: DVS dosimeters have been calibrated to be used at 37C. Reproducibility tests were performed at room temperature. Readings were −3.2% low (SD=2.5%, or 5.06cGy for 200cGy delivered). Same tests in water at human body temperature showed 1.5% absolute difference, with a similar deviation (SD=2.85%). It was observed that measurement signal decayed significantly with time post‐irradiation, from +3% after 0.5min to −3.5% after 10min. Dose linearity was within 2% for absorbed total dose of 75Gy. Signal rapidly decayed after total dose of 82Gy. Angular dependence was −0.3% (SD=2.31%). Readings decreased with increase in depth, about 0.3%/cm. No filed size and dose‐rate difference were observed. Average dose reading due to CBCT imaging using the pelvis protocol was 7.89cGy (SD=1.46cGy) Conclusions: Extensive acceptance and commissioning measurements have shown considerable post‐radiation signal drift, temperature dependence, reproducibility variation, and rotational isotropy. This process confirmed manufacturerˈs stated performance and provided baseline QA necessary to implement DVS clinically. System is currently implemented with ±7% tolerance. Dosimeter can be used to quantify dose at depth, as well as to evaluate adherence between dose from treatment plan and daily delivery.

Cell-Specific Radiosensitization by Gold Nanoparticles at Megavoltage Radiation Energies

Gold nanoparticles (GNPs) have been shown to cause sensitization with kilovoltage (kV) radiation. Differences in the absorption coefficient between gold and soft tissue, as a function of photon energy, predict that maximum enhancement should occur in the kilovoltage (kV) range, with almost no enhancement at megavoltage (MV) energies. Recent studies have shown that GNPs are not biologically inert, causing oxidative stress and even cell death, suggesting a possible biological mechanism for sensitization. The purpose of this study was to assess GNP radiosensitization at clinically relevant MV X-ray energies. Cellular uptake, intracellular localization, and cytotoxicity of GNPs were assessed in normal L132, prostate cancer DU145, and breast cancer MDA-MB-231 cells. Radiosensitization was measured by clonogenic survival at kV and MV photon energies and MV electron energies. Intracellular DNA double-strand break (DSB) induction and DNA repair were determined and GNP chemosensitization was assessed using the radiomimetic agent bleomycin. GNP uptake occurred in all cell lines and was greatest in MDA-MB-231 cells with nanoparticles accumulating in cytoplasmic lysosomes. In MDA-MB-231 cells, radiation sensitizer enhancement ratios (SERs) of 1.41, 1.29, and 1.16 were achieved using 160 kVp, 6 MV, and 15 MV X-ray energies, respectively. No significant effect was observed in L132 or DU145 cells at kV or MV energies (SER 0.97-1.08). GNP exposure did not increase radiation-induced DSB formation or inhibit DNA repair; however, GNP chemosensitization was observed in MDA-MB-231 cells treated with bleomycin (SER 1.38). We have demonstrated radiosensitization in MDA-MB-231 cells at MV X-ray energies. The sensitization was cell-specific with comparable effects at kV and MV energies, no increase in DSB formation, and GNP chemopotentiation with bleomycin, suggesting a possible biological mechanism of radiosensitization.

Planar LTCC Transformers for High-Voltage Flyback Converters

This paper discusses the design and use of low-temperature (850°C to 950°C) cofired ceramic (LTCC) planar magnetic flyback transformers for applications that require conversion of a low-voltage to high-voltage (> 100-V) with significant volumetric constraints. Measured performance and modeling results for multiple designs show that the LTCC flyback transformer design and construction imposes serious limitations on the achievable coupling, and significantly impacts the transformer performance and output voltage. This paper discusses the impact of various design factors that can provide improved performance by increasing transformer coupling and output voltage. The experiments performed on prototype units demonstrate LTCC transformer designs capable of greater than 2-kV output. Finally, the paper investigates the effect of the LTCC microstructure on transformer insulation. Although this paper focuses on generating voltages in the kV range, the experimental characterization and discussion presented in this paper applies to designs requiring lower voltage.

Outcome of shock wave lithotripsy as monotherapy for large solitary renal stones (>2 cm in size) without stenting

Purpose:To evaluate the outcome of shock wave lithotripsy (SWL) as monotherapy for solitary renal stones larger than 2 cm without ureteral stenting.Materials and Methods:Our retrospective study included patients with solitary renal radio opaque stones larger than 2 cm treated with SWL using electromagnetic Dornier Compact S lithotripter device (Wessling, Germany) for a period of 3 years (September 2002-2005). Stone clearance was assessed at 1 week, 1 month, and 3 months with plain X-rays of kidney, ureter, and bladder region, ultrasonography, and tomograms. Stone-free status, morbidity of the procedure, and fate of clinically insignificant residual fragments (CIRF) were studied. A stone-free state was defined as no radiologic evidence of stone. Success was defined as complete clearance + CIRF.Results:Fifty-five patients, aged 11–65 years (mean 49.8) underwent SWL. Of them, only two were children. Male-to-female ratio was 3:1. The stone size was 21–28 mm (average 24 mm). The mean number of shocks were 3732 (range 724–12,100) and average energy level was 14 kV (range 11–16 kV). The mean follow-up was 18 months (range 3–22 months). Over all, stone-free status was achieved in 50% and success in 81% at 3 months. Stone clearance was not affected by stone location. Stones <25 mm had better stone-free rate (54.16% vs. 28.5%) and lesser CIRF (14.6% vs. 28.5%) when compared to larger stones (>26 mm) (P = –0.10). Of 54 patients, 39 developed steinstrasse with mean length of 3.2 cm (range 1.4–6.2 cm) and only four required intervention. Effectiveness quotient (EQ) for SWL monotherapy for solitary renal stones more than 2 cm was 25.3%. The EQ for stones <25 mm and those more than 25 mm were 28.4% and 10% (P = –0.12), respectively. There was a lesser trend of difference between stones with size <25 and more than 25 mm. During the last review, the overall stone-free rate was 67.2%.Conclusions:SWL monotherapy was safe but significantly less effective for solitary renal stones larger than 2 cm. It can only be suggested to those who refuse surgical intervention. Pretreatment DJ stenting is not mandatory when closer follow-up is ensured.

A compact submicrosecond, high current generator

Pulsed current generator was developed for experiments with current carrying pulsed plasma. Main parts of the generator are capacitor bank, low inductive current driving lines, and central load part. Generator consists of four identical sections, connected in parallel to one load. Capacitor bank is assembled from 24 capacitor blocks (100 kV, 80 nF), connected in parallel. It stores 9.6 kJ at 100 kV charging voltage. Each capacitor block incorporates a multigap spark switch, which is able to commute by six parallel channels. Switches operate in dry air at atmospheric pressure. The generator was tested with an inductive load and a liner load. At 17.5 nH inductive load and 100 kV of charging voltage it provides 650 kA of current amplitude with 390 ns rise time with 0.6 ohms damping resistors in discharge circuit of each capacitor block. The net generator inductance without a load was optimized to be as low as 15 nH, which results in extremely low impedance of the generator (approximately 0.08 ohms). It ensures effective energy coupling with a low impedance load such as Z pinch. The generator operates reliably without any adjustments in 70-100 kV range of charging voltage. Jitter in delay between output pulse and triggering pulse is less than 5 ns at 70-100 kV charging voltage. Operation and handling are very simple, because no oil or purified gases are required for the generator. The generator has dimensions 5.24x1.2x0.18 m(3) and total weight about 1400 kg, thus manifesting itself as simple, robust, and cost effective apparatus.

On the destruction of volatile organic compounds using a dielectric pellet bed reactor

Volatile organic compounds (VOC) are known to be dangerous for human health. If they are part of a precursor gas mixture undesired effects in e.g. industrial processes may occur. Therefore, the destruction of unwanted VOCs in a gas stream is an important aim. For that purpose a five stage planar dielectric barrier discharge reactor working at atmospheric pressure has been developed and tested. Each stage consists of two parallel stainless steel meshes serving as electrodes. The space between them is filled with dielectric beads. The ac voltage applied to the electrodes is in the kV and kHz range and several gas flow rates are possible. The gases of interest have been measured using a Fourier Transform Infrared Spectrometer (FTIR) combined with a 32.5m long path cell. First result showed, that the VOC ethylene (C2H4) can be removed.

SU‐GG‐J‐130: Monte Carlo Simulation of Elekta XVI Cone Beam System and Dose Distribution in CTDI Phantom

Purpose: (1) To validate a Monte Carlo model for kV cone beam CT simulations. (2) To determine whether a bow‐shaped radial dose distribution with equal weighting can be applicable for cone beam CT. Method and Materials: A BEAMnrcMP Monte Carlo package has been employed and modified to simulate the Elekta XVI system operated at 120 kVp with collimator M20 and bowtie filter F1. Percentage Depth Dose (PDD) has been measured with PTW Farmer and Roos chambers in a CNMC water tank as well as a GAMMEX Solid Water phantom at 100cm SSD with 10cm backscatter. Dose profile at depth of 1cm has been measured with a Wellhofer IC‐10 chamber in Solid Water. Sample dose calculation is performed on a standard CDTI phantom aligned at Linac isocenter. Results: The simulated PDD curve agrees very well with both Farmer and Roos chamber measurements in water, but differ slightly with the Farmer chamber measurement in solid water at large depths and build‐up region. This is due to the higher attenuation of solid water in kV range and the loss of scatter equilibrium at phantom surface. Excellent agreement has been shown between the measured dose profile and the Monte Carlo simulations along both in‐plane and cross‐plane directions. The CTDI calculations have indicated that equal weighting on both the center and the periphery can result in errors of −1.1% while unequal standard weighting (1/3 for center and 2/3 for periphery) can result in errors of up to 8.1%. Conclusion: Our Monte Carlo model has been validated by various measurements to simulate the kV cone beams from the Elekta XVI system. A bow‐shape radial profile in CDTI phantom results in equal weightings for volumetric average dose calculations using only center and peripheral CTDI measurements. Investigation on patient dose deposition due to kV CBCT is in progress.

The use of a quadrupole electrostatic trap to transfer the dimensions of the volt in the 10 V–10 kV range

The problems involved in using a quadrupole trap for charged particles in order to increase the accuracy of the measurement of dc voltage in the 10 V–10 kV range are considered.

Mega-ampere submicrosecond generator GIT-32

The GIT-32 current generator was developed for experiments with current carrying pulsed plasma. The main parts of the generator are capacitor bank, multichannel multigap spark switches, low inductive current driving lines, and central load part. The generator consists of four identical sections, connected in parallel to one load. The capacitor bank is assembled from 32 IEK-100-0.17 (0.17 microF, 40 nH, 100 kV) capacitors, connected in parallel. It stores approximately 18 kJ at 80 kV charging voltage. Each two capacitors are commuted to a load by a multigap spark switch with eight parallel channels. Switches operate in ambient air at atmospheric pressure. The GIT-32 generator was tested with 10, 15, and 20 nH inductive loads. At 10 nH load and 80 kV of charging voltage it provides 1 MA of current amplitude and 490 ns rise time with 0.8 Omega damping resistors in discharge circuit of each capacitor and 1.34 MA530 ns without resistors. The net generator inductance without a load was optimized to be as low as 12 nH, which results in extremely low self-impedance of the generator ( approximately 0.05 Omega). It ensures effective energy coupling with low impedance loads like Z pinch. The generator operates reliably without any adjustments in 40-80 kV range of charging voltage. Maximum jitter (relative to a triggering pulse) at 40 kV charging voltage is about 7 ns and lower at higher charging voltages. Operation and handling are very simple, because no oil and no purified gases are required for the generator. The GIT-32 generator has dimensions of 3200 x 3200 x 400 mm(3) and total weight of about 2500 kg, thus manifesting itself as a simple, robust, and cost effective apparatus.

Multiple high-voltage pulse stressing of conventional thick-film resistors

In this paper, results from a study of multiple high-voltage pulse (MHVP) stressing effects on resistance and low-frequency noise of thick-film resistors based on resistor compositions with sheet resistances of 1, 10 and 100 kΩ/sq are presented. For experimental purposes, a series of thick-film test resistors with identical geometries were realized and exhibited to two types of tests: multiple series of 10 pulses with increasing amplitudes from the 0.5 to 4.0 kV range and multiple series of 10 pulses with constant 3 kV and 4.5 kV amplitudes. Obtained experimental results were analyzed and correlation between resistance and low-frequency noise changes with resistor degradation due to MHVP stressing was observed. Comparing the resistance and noise index changes it is shown that low-frequency noise parameters are more sensitive to this kind of resistor stressing than resistance. During high-voltage pulse stressing, certain number of resistors failed and progressive resistor degradation that led to catastrophic failure is also presented in this paper.

Extracorporeal shock wave lithotripsy in children: Evaluation of the results considering the need for auxiliary procedures

Objective To retrospectively evaluate the efficacy of extracorporeal shock wave lithotripsy (ESWL) in children with renal stones, with regard to ability of ureters to transport the fragments, and need for adjunctive procedures. Patients and methods Between January 2000 and November 2004, 60 children (24 girls and 36 boys) with a total of 72 stones of the upper urinary tract were treated by ESWL using a PCK V5 lithotriptor. Patients with anatomical abnormalities and staghorn stones were excluded from the study. Stone size ranged from 4 to 28 mm. Mean number of shock waves was 1430 (range 600–2000) per ESWL session and mean energy used for stone disintegration was 12 kV (range 6–18). At 24–48 h after ESWL, a plain film or renal ultrasound was obtained to evaluate stone fragmentation. Results Our stone-free rate after one session of ESWL was 92.8% and 81.2% for patients with stones smaller than 10 mm and stones between 10 and 28 mm, respectively. Seven (11.6%) patients developed distal ureteral steinstrasse, and the stone size was 15–20 mm and 20–28 mm in two and five patients, respectively. The steinstrasse completely cleared under meticulous follow-up. No patient needed an adjunctive procedure, such as a double-J stent or nephrostomy tube placement, or ureteroscopic stone manipulation. Conclusion A child's ureter is capable of transporting the fragments after lithotripsy. Interventional procedures should be a last resort. Expectant management is usually adequate even in patients who develop steinstrasse after ESWL.

The performance of commercial photodiodes for dosimetry in mammography

This paper reports the results of an investigation carried out to determine the suitability of using a commercially available photodiode for dosimetry in mammography thereby providing a low cost dosemeter with a direct reading of the radiation dose. A mammographic X-ray generator (Siemens Mammomat 1000) with tube potential in the 23-30 kV range and a constant potential X-ray generator (Pantak) with a W/Mo anode/filter combination were used in this study. The results showed that the photodiode response is highly linear within mammographic dose ranges and that the energy dependence was <3% at tube potentials of 25-30 kV. Good agreement was observed between the incident air kerma measured with both the photodiode and the ionization chamber. These results show the viability of using the photodiode as a dosemeter system in mammography.

Effectiveness of electro-acupuncture compared to sedo-analgesics in relieving pain during shockwave lithotripsy

The aim of this study was to compare the clinical efficacy of electro-acupuncture (EA) with the combination of tramadol+midazolam (TM) for pain relief during outpatient extracorporeal shockwave lithotripsy (ESWL). A total of 35 patients (20 men, 15 women) with stones located in the pelvicalyceal system of the kidney were randomized prospectively to undergo lithotripsy with a third generation lithotriptor (Stone Lith, smart PCK) after receiving either EA (n=17) or TM (n=18) for sedation and analgesia. EA treatment was applied to patients by the same licensed acupuncturist 30 min prior to ESWL in group EA. Tramadol (1.5 mg/kg) 30 min before the start of lithotripsy and midazolam (0.06 mg/kg) 5 min prior to ESWL were given as a sedo-analgesic intravenously to group TM. During ESWL, blood pressure, heart rate, pain and sedation levels were measured at baseline and every 15 min thereafter. The pain intensity perceived during lithotripsy was evaluated using a visual analog scale (VAS). There was no statistical differences in the diameters of the stones and age of the patients between groups (P=0.590; P=0.568, respectively). In the EA group, the median of maximum energy level achieved was 16.0 kV (range 10-23 kV), while it was 18.0 kV (range 10-20 kV) in the TM group. There was no statistically significant difference between the maximum energy levels applied to the patients during ESWL (P=0.613). The median numbers of shockwaves were 2,114 (range 1,100-3,800) and 2,200 (range 1,500-3,200) in the EA and TM groups, respectively. In the TM group, the numbers of shockwaves used were higher than in group EA during ESWL. However, this difference was not significant (P=0.732). VAS scores were consistently lower in the EA group compared with the TM group throughout the ESWL procedure. The median VAS score was 5.0 (range 1-10) in the EA group while it was 8.0 (range 2-10) in the TM group. The patients who underwent EA had lower median scores of VAS than patients who took only conservative treatment, but this difference was not significant (P=0.245). When both groups were compared for stone-free rates, no significant difference was found [82.3% (14/17) for group EA, 88.8% (16/18) for group TM] (P=0.658). Durations of ESWL procedures were similar in both groups [median 27.4 min (range 15.7-34.3) in group EA vs 27.1 min (range 16.1-33.6) in group TM] (P=0.517). No side effects was seen in any patient who received EA. Side effects such as mild orthostatic hypotension and dizziness occurred in patients given sedo-analgesia, but these were not severe enough to require any patient to be excluded from the study. Our study shows that EA is an effective method for inducing sedation with analgesia without any demonstrable side effects.

Application of commercial MOSFET detectors for in vivo dosimetry in the therapeutic x-ray range from 80 kV to 250 kV

The purpose of this study was to investigate the dosimetric characteristics (energy dependence, linearity, fading, reproducibility, etc) of MOSFET detectors for in vivo dosimetry in the kV x-ray range. The experience of MOSFET in vivo dosimetry in a pre-clinical study using the Alderson phantom and in clinical practice is also reported. All measurements were performed with a Gulmay D3300 kV unit and TN-502RDI MOSFET detectors. For the determination of correction factors different solid phantoms and a calibrated Farmer-type chamber were used. The MOSFET signal was linear with applied dose in the range from 0.2 to 2 Gy for all energies. Due to fading it is recommended to read the MOSFET signal during the first 15 min after irradiation. For long time intervals between irradiation and readout the fading can vary largely with the detector. The temperature dependence of the detector signal was small (0.3% °C−1) in the temperature range between 22 and 40 °C. The variation of the measuring signal with beam incidence amounts to ±5% and should be considered in clinical applications. Finally, for entrance dose measurements energy-dependent calibration factors, correction factors for field size and irradiated cable length were applied. The overall accuracy, for all measurements, was dominated by reproducibility as a function of applied dose. During the pre-clinical in vivo study, the agreement between MOSFET and TLD measurements was well within 3%. The results of MOSFET measurements, to determine the dosimetric characteristics as well as clinical applications, showed that MOSFET detectors are suitable for in vivo dosimetry in the kV range. However, some energy-dependent dosimetry effects need to be considered and corrected for. Due to reproducibility effects at low dose levels accurate in vivo measurements are only possible if the applied dose is equal to or larger than 2 Gy.

High voltage photoconductive switches of amorphous silicon for electroactive polymer actuators

Dielectric elastomers exhibit remarkable elastic and electroactive properties. Upon kilovolt range electrical stimulation, the polymeric actuator deforms with very large mechanical strain. We made thin film amorphous silicon (a-Si:H) photoconductive switches on 50-μm thick flexible polyimide films to control the polymer actuation. 155–310-nm thick a-Si:H films were grown by plasma enhanced chemical vapor deposition at 150 °C substrate temperature. The photoconductive switches were made with 250-, 500- or 1000-μm gaps formed by electrodes of evaporated aluminum. When properly passivated, the switches have a dielectric breakdown strength of ∼10 kV/mm across the gap. We present optical and electrical characteristics of these a-Si:H switches in the 1–10 kV range. The integration of a nine-element a-Si:H high voltage photoconductor switch array with a nine-element electroactive polymer actuator array is demonstrated. The time constants measured on this array agree well with the values modeled using the a-Si:H photoconductor characteristics.

High voltage photoconductive switches of amorphous silicon for electroactive polymer actuators

Dielectric elastomers exhibit remarkable elastic and electroactive properties. Upon kilovolt range electrical stimulation, the polymeric actuator deforms with very large mechanical strain. We made thin film amorphous silicon (a-Si:H) photoconductive switches on 50-μm thick flexible polyimide films to control the polymer actuation. 155–310-nm thick a-Si:H films were grown by plasma enhanced chemical vapor deposition at 150 °C substrate temperature. The photoconductive switches were made with 250-, 500- or 1000-μm gaps formed by electrodes of evaporated aluminum. When properly passivated, the switches have a dielectric breakdown strength of ∼10 kV/mm across the gap. We present optical and electrical characteristics of these a-Si:H switches in the 1–10 kV range. The integration of a nine-element a-Si:H high voltage photoconductor switch array with a nine-element electroactive polymer actuator array is demonstrated. The time constants measured on this array agree well with the values modeled using the a-Si:H photoconductor characteristics.

On anode effects in explosive emission diodes

Planar explosive emission diodes operating at moderate pulse energy densities of a few joules per square centimeter conform normally with the Child–Langmuir law in its basic unipolar form. Several recent publications [D. A. Shiffler et al., IEEE Trans. Plasma Sci., 30, 1232, (2002)] suggest, however, that the bipolar flow develops in the diode at comparatively low electron current densities of ∼10 A/cm2 and pulse durations of ∼50 ns. Understanding the nature of a charged particle flow (unipolar or bipolar) in explosive emission diodes is of paramount importance for interpretation of the experimental results. We have therefore investigated the operation of a diode having velvet cathodes at voltages in the 40–170 kV range, electron current densities of 4–16 A/cm2, beam power densities of 0.3–2.2 MW/cm2, and pulse energy densities of 0.5–3.3 J/cm2. Within these latitudes of operating parameters, unipolar flow was always observed and bipolar flow was achieved only when specific measures for generation of anode plasma were introduced. Prolonged operation of the CsI coated cathodes may lead to a deposition of thin CsI films on the anode and, potentially, other elements of the high voltage structure. In interpreting the plasma images, care must therefore be taken to distinguish between the light coming from the anode plasma and the luminescence from CsI films.