Microsecond Pulse Length Research Articles

Frequency-doubled Nd:YAG MOPA laser system with programmable rectangular pulses up to 200 microseconds

A compact frequency-doubled diode-pumped Nd:YAG master-oscillator power-amplifier laser system with programmable microsecond pulse length has been developed. Analog pulse shaping of the output from a single-frequency continuous-wave Nd:YAG oscillator, and subsequent amplification, allowed the generation of rectangular pulses with pulse lengths on the order of the Nd:YAG fluorescence lifetime. Temporally flat-top pulses of 1064 nm light with 520 mJ pulse energy, 2.6 kW peak power, and 200 μs duration, with linewidth below 10 kHz, were obtained at a repetition rate of 2 Hz. Second harmonic generation in a LBO crystal yielded pulses of 262 mJ and 1.3 kW peak power at 532 nm. The peak power can be maintained within 2.9% over the duration of the laser pulse, and long-term intensity stability of 1.1% was observed. The spatially flat-top beam at 1064 nm used in the amplifier is converted to a Gaussian beam at 532 nm with beam quality factor M2 = 1.41(14) during the second harmonic generation. This system has potential as a pump source for Ti:sapphire, dye, or optical parametric amplifiers to generate tunable high-power single-frequency radiation for applications in precision measurements and laser slowing.

Ionoacoustic detection of swift heavy ions

The maximum energy loss (Bragg peak) located near the end of range is a characteristic feature of ion stopping in matter, which generates an acoustic pulse, if ions are deposited into a medium in adequately short bunches. This so-called ionoacoustic effect has been studied for decades, mainly for astrophysical applications, and it has recently found renewed interest in proton therapy for precise range measurements in tissue. After detailed preparatory studies with 20 MeV protons at the MLL tandem accelerator, ionoacoustic range measurements were performed in water at the upgraded SIS18 synchrotron of GSI with 238U and 124Xe ion beams of energy about 300 MeV/u, and 12C ions of energy about 200 MeV/u using fast beam extraction to get 1 microsecond pulse lengths. Acoustic signals were recorded in axial geometry by standard piezo-based transducers at a 500 kHz mean frequency and evaluated in both the time and frequency domains. The resulting ranges for the different ions and energies were found to agree with Geant4 simulations as well as previous measurements to better than 1%. Given the high accuracy provided by ionoacoustic range measurements in water and their relative simplicity, we propose this new method for stopping power measurements for heavy ions at GeV energies and above. Our experimental results clearly demonstrate the potential of an ionoacoustic particle monitor especially for very intense heavy ion beams foreseen at future accelerator facilities.

Treatment With Irreversible Electroporation for Pancreatic Cancer Resulting in Massive Gastrointestinal Hemorrhage From a Splenic Artery Pseudoaneurysm

Irreversible electroporation (IRE) therapy utilizes high voltage small microsecond pulse lengths to create nanopores in cell membrane and induce apoptosis. In recent years, it has been utilized as a treatment for patients with pancreatic cancer. It can be administered either intraoperatively, laproscopically, or percutaneously. Prior reported complications included gastrointestinal bleeding (typically ulcers in the duodenum), bile/pancreatic leak, portal vein thrombosis, perforation, pancreatitis and arrhythmias. Reported here is a case of a massive delayed GI bleed from a splenic artery pseudoaneurysm with fistula to the distal duodenum post IRE treatment. Case Presentation 56 year old female with unresectable pancreatic adenocarcinoma who had received chemotherapy, radiation, plastic stent placement, and intraoperative IRE four months prior to development of hematemesis followed by hematochezia. Upon presentation, the patient was tachycardic with maroon colored stool on rectal exam. Her hemoglobin had dropped from 12.6 to 8.7. EGD done at that time revealed no lesions or blood. CT imaging showed a 4cm x 3cm collection containing gas and liquid in the IRE treatment bed concerning for possible abscess. The patient improved with antibiotics, had no further bleeding, and was discharged home. One month later, she had recurrent hematemesis followed by hematochezia. She presented in hemorrhagic shock and required an emergent EGD which again failed to reveal any blood or lesions. Shortly after this EGD, she had a third bleeding event that prompted a CT angiography. A pseudoaneurysm of the splenic artery was found to be abutting the bowel distal to the ligament of Treitz with intraluminal blush consistent with active bleed. The patient was taken for emergent embolization. She had no recurrent bleeding in the following 3 months of follow up. Delayed massive gastrointestinal hemorrhage as the result of a splenic artery pseudoaneurysm to small bowel fistula following IRE treatment had not previously been reported in the literature. The proposed pathophysiology of this complication includes direct treatment effect on the splenic artery in addition to post treatment inflammation/fluid collection creating an environment for contact and fistula formation between the artery and the small bowel. Clinical awareness of this possible complication could support the use of CT angiography prior to endoscopic evaluation in the appropriate patient population.Figure: Splenic artery aneurysm (arrow) with associated inflammation in the area of the proximal jejunum.Figure: Sagittal view of splenic artery pseudoaneurysm (arrow).

Use of irreversible electroporation in unresectable pancreatic cancer.

Irreversible electroporation is a non-thermal injury ablative modality that has been in clinical use since 2008 in the treatment of locally advanced soft tissue tumors. It has been reported to be utilized intraoperatively, laparoscopically or percutaneously. The method of action of IRE relies on a high voltage (maximum 3,000 volts) small microsecond pulse lengths (70 to 90 microseconds) to induce cell membrane porosity which leads to slow/protracted cell death over time. One of the largest unmet needs in oncology that IRE has been utilized is in locally advanced (stage III) pancreatic cancer. Recent studies have demonstrated the safety and palliation with encouraging improvement in overall survival. Its inherent limitation still remains tissue heterogeneity and the unique settings based on tumor histology and prior induction therapy. There remains a high technical demand of the end-user and the more extensive knowledge transfer which makes the learning curve longer in order to achieve appropriate and safe utilization.

Temporal evolution of atmosphere pressure plasma jets driven by microsecond pulses with positive and negative polarities

Current-voltage characteristics, discharge images, and optical spectra of atmospheric pressure plasma jets (APPJs) are studied using a microsecond pulse length generator producing repetitive output pulses with different polarities. The experimental results show that the APPJs excited by the pulses with positive polarity have longer plume, faster propagation speed, higher power, and more excited species, such as , O, He, , than that with the negatively excited APPJs. The images taken using an intensified charge-coupled device show that the APPJs excited by pulses with positive polarity are characterized by a bullet-like structure, while the APPJs excited by pulses with negative polarity are continuous. The propagation speed of the APPJs driven by a microsecond pulse length generator is about tens of km/s, which is similar to the APPJs driven by a kHz frequency sinusoidal voltage source. The analysis shows that the space charge accumulation effect plays an important role during the discharge. The transient enhanced electric field induced by the accumulated ions between the needle-like electrode and the nozzle in the APPJs excited by pulses with negative polarity enhances electron field emission from the cathode, which is illustrated by the bright line on the time-integrated images. This makes the shape of the APPJ excited using pulses with negative polarity different from the bullet-like shape of the APPJs excited by pulses with positive polarity.

Evaluation of thermal injury to liver, pancreas and kidney during irreversible electroporation in an in vivo experimental model.

Irreversible electroporation (IRE) is a new technique for tumour cell ablation that is reported to involve non-thermal-based energy using high voltage at short microsecond pulse lengths. In vivo assessment of the thermal energy generated during IRE has not been performed. Thermal injury can be predicted using a critical temperature model. The aim of this study was to assess the potential for thermal injury during IRE in an in vivo porcine model. In vivo continuous temperature assessments of 86 different IRE procedures were performed on porcine liver, pancreas, kidney and retroperitoneal tissue. Tissue temperature was measured continuously throughout IRE by means of two thermocouples placed at set distances (0·5 cm or less, and 1 cm) from the IRE probes within the treatment field. Thermal injury was defined as a tissue temperature of 54°C lasting at least 10 s. Tissue type, pulse length, probe exposure length, number of probes and retreatment were evaluated for associations with thermal injury. In addition, IRE ablation was performed with metal clips or metal stents within the ablation field to determine their effect on thermal injury. An increase in tissue temperature above the animals' baseline temperature (median 36·0°C) was generated during IRE in all tissues studied, with the greatest increase found at the thermocouple placed within 0·5 cm in all instances. On univariable and multivariable analysis, ablation in kidney tissue (maximum temperature 62·8°C), ablation with a pulse length setting of 100 µs (maximum 54·7°C), probe exposure of at least 3·0 cm (maximum 52·0°C) and ablation with metal within the ablation field (maximum 65·3°C) were all associated with a significant risk of thermal injury. IRE can generate thermal energy, and even thermal injury, based on tissue type, probe exposure lengths, pulse lengths and proximity to metal. Awareness of probe placement regarding proximity to critical structures as well as probe exposure length and pulse length are necessary to ensure safety and prevent thermal injury. A probe exposure of 2·5 cm or less for liver IRE, and 1·5 cm or less for pancreas, with maximum pulse length of 90 µs will result in safe and non-thermal energy delivery with spacing of 1·5-2·3 cm between probe pairs.

Suppressing high-power microwave dielectric multipactor by the sawtooth surface

This paper theoretically and experimentally researches the effect of a periodic sawtooth surface on vacuum multipactor. Dynamic calculation and particle-in-cell simulation are applied to analyze the electron impact energy and multipactor scenario on the periodic isosceles triangular surface with different slope angles and heights. It has been discovered that, with the slope angle increasing, the impact energy significantly decreases to be lower than the first crossover energy of the secondary yield curve, leading to suppressed multipactor. S-band high power microwave dielectric breakdown experiment, with microsecond pulse length, was conducted. It is confirmed by experiment that the periodic sawtooth surface with sufficiently large slope angle (such as 45°) can effectively increase the breakdown threshold of about 2 compared to the flat surface.

The effect of grooved surface on dielectric multipactor

The effect of periodic rectangular grooves on vacuum multipactor has been theoretically and experimentally investigated. Dynamic calculation is applied to research the electron trajectory and impact energy under groove surface. Two-dimensional electromagnetic particle-in-cell simulation is used to analyze and compare multipactor scenario, statistic energy, and secondary emission yield on the flat surface with that on the corrugated surface. It has been found by computational and simulative analysis that grooved surface can explicitly suppress multipactor in the developmental stage of multipactor. S-band high power microwave (HPM) dielectric breakdown experiment under vacuum, with microsecond pulse length was conducted. It was confirmed by experiment that periodic grooves perpendicular to the major electric field can effectively increase transmitted power.

Second Harmonic Operation at 460 GHz and Broadband Continuous Frequency Tuning of a Gyrotron Oscillator

We report the short-pulse operation of a 460 GHz gyrotron oscillator both at the fundamental (near 230 GHz) and second harmonic (near 460 GHz) of electron cyclotron resonance. During operation in a microsecond pulse length regime with 13-kV beam voltage and 110-mA beam current, the instrument generates several watts of power in two second harmonic modes, the TE/sub 2,6,1/ at 456.15 GHz and the TE/sub 0,6,1/ at 458.56 GHz. Operation in the fundamental modes, including the TE/sub 0,3,1/ mode at 237.91 GHz and the TE/sub 2,3,1/ at 233.15 GHz, is observed at output powers up to 70 W. Further, we demonstrate broadband continuous frequency tuning of the fundamental modes of the oscillator over a range of more than 2 GHz through variation of the magnetic field alone. We interpret these results in terms of smooth transitions between higher order axial modes of the resonator. The 460 GHz gyrotron is currently being processed for continuous duty operation, where it will serve as a microwave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) studies at 16 T (700 MHz /sup 1/H), a field strength which is two-fold higher than has been accessible with previous technology.

A microsecond-pulse-length, frequency-stabilized virtual-cathode oscillator using a resonant cavity

Experiments at the microsecond pulse length have demonstrated that the virtual-cathode oscillator generates narrowband (0.3% bandwidth) microwave pulses when the virtual cathode is surrounded by a resonant cavity and is driven by an appropriate electron beam. This result is a significant departure from the behavior of a free-running virtual-cathode oscillator where the frequency depends on the (current density)/sup 1/2/. The long-pulse experimental results are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development of arc ion sources for heavy ion fusion

An overview of the development of arc ion sources for heavy ion fusion is presented. Two approaches to heavy ion fusion (HIF)-the RF linac-storage ring approach and the induction linac approach-are described. RF linac schemes require low emittance and moderate current levels, because the beam is accumulated in storage rings before being focused on target. The induction linac approach requires low emittance and high current, because this is a single-pass approach to HIF and one wishes to limit the number of beams in the machine. The RF scheme generally uses long pulse sources together with a buncher of RFQ. The induction linac approach requires sources in the microsecond pulse length range, with good optics being maintained during the pulse. Emphasis is on the induction linac approach pursued at the Lawrence Berkeley Laboratory. >

Microsecond operation of a general purpose pulsed proton gun

A magnetically insulated pulsed proton gun is described that provides an extracted parallel beam of annular cross section for microsecond pulse lengths. In initial tests, time-averaged currents of 5 kA and current densities of 50 A/cm2 were achieved with ≲200 kV voltage. The behavior of the gun appears to be determined by instabilities of the cathode electron cloud growing over time scales of the order of 0.5 msec to give ion current-density enhancements greater than a factor of 50 above the Child-Langmuir limit.

Recent advances in high power microwave electron accelerators for physics research

Abstract There have been some large steps forward in the microwave electron linear accelerator art. More power, precision and versatility than heretofore are now available for use in the search for further fundamental knowledge in nuclear structure. A number of very high current linear accelerators of great precision are described, among which are the following 1. a 10-MeV synchrotron injector having 200 mA of pulse current with an energy spread of 1% and a divergence of less than 1 milliradian; 2. a 35-MeV with pulse currents up to 500 mA and 30 microsecond pulse length offering 2 × 10 12 neutrons per pulse ; 3. a very new 130-MeV linear accelerator for the U.S. National Bureau of Standards. A breakthrough in the problem of “beam blow-up” and its possible origin in TM 110 mode excitation is described. New accelerating structures have been employed to advantage in the elimination of this problem. A new and potentially useful R.F. power tube called the Amplitron is just emerging, and its attractive features are described. Techniques are now available for microwave linear accelerators of arbitrarily great precision and power output.