The efficiency of ablation of metals by scanning beam of pulsed irradiation with nanosecond-range fiber Yb:YAG laser

The results of experiments on ablation of targets made of stainless steel and aluminum by a scanning beam of nanosecond pulses at intensity up to 109 W/cm2 are presented. It was found that the overlap of the impact zones during irradiating leads to an increase in the ablation depth in proportion to the area of overlap of the irradiation spots. This is due to increase in overlap irradiation spots degree, zones with a large number of pulse effects are formed on surface, which increases the depth of the melt bath and leads to the ejection of larger particles. An increase in ablation depth of aluminum increase with increase of the interval between pulses up to 10 ms and overlapping of the irradiation spots. The shape of the ejected particles changes from spherical, when formed from a melt, to an irregular shape, when the target is mechanically destroyed by an internal shock wave. The size and velocity distribution of the ejected particles was determined, and on the basis of these data, the laser radiation shielding coefficients were calculated depending on the degree of overlapping of the irradiation spots. It was found that the main mechanism for the decrease in the efficiency of ablation by a scanning beam of radiation is the backflow of microparticles deposited on the target surface. The analysis of the energy balance of the aluminum ablation process is carried out.

CONTROLLING OPTICAL BEAMS USING MUTUALLY COMPLEMENTARY SCANNERS

Devices based on scanners (deflectors, scanning devices) with mutually complementary characteristics can be a solution to control problems of an optical scanning beam in a number of applications. The paper considers optical-mechanical, electronic, acousto-optical, electro-optical, as well as microelectromechanical (MEMS), micro-optical electromechanical (MOEMS) scanners as components of combined scanning devices. The qualitative indicators of composite scanning, including the speed, resolution, control accuracy of the optical beam, the dimensions of the scanned space, etc., are given. Along with the advantages, the difficulties arising in the implementation of combined scanning are noted.

Multifunctional ionization chamber and its electronic path for use on the medical accelerator "Prometeus"

The article describes the proposed new multifunctional ionization chamber (MIC) designed to measure dose profiles when the medical accelerator "Prometheus" is operating in the scanning "pencil beam" mode. A digital image acquisition detector (DIDE) with a tissue-equivalent water phantom is used to calibrate the accelerator before a radiation therapy session. The application of the CPPI on the beam of a proton accelerator operating in the mode of beam splitting into spots with a scanning beam is considered. The CDPI detector allows for a few accelerator pulses in on-line mode to see how the energy release of each spot is distributed over the area of the irradiated target, which is the actual calibration of the accelerator before the proton therapy session. During the proton therapy session, it is planned to install the MIC directly in front of the patient. The MIC chamber contains two ionization chambers operating simultaneously — a pad chamber (PC) operating on gas or "warm liquid" and a strip ionization chamber operating only on gas (SC). At the accelerator "Prometheus" it is proposed to use a MIC, which will be used in the mode of operation by the method of active scanning with a "pencil" proton beam. The use of the MIC operation is intended to control the density of the beam intensity during the irradiation of the "target" in the patient during the proton therapy session. In case of violation of the planned operating mode of the accelerator and the beam goes beyond the parameters preset before the session, the deviation detection control system (SDMS) will turn off the accelerator. The device of the readout electronics (SE) of the MIC and SKOO cameras is described. This proposed detector, including the MIC and SKOO camera and the reading electronics serving it, will improve the quality of the therapeutic beam supply, due to the accurate determination of the absorbed dose density supplied by the scanning beam to each spot of the irradiated target, and therefore the generated high dose distribution field will correspond to the irradiated volume of the patient and will increase the safety and control of patient exposure to the target. The PC included in the MIC is designed on a "warm liquid" (or gas) and is a high-precision ionization chamber with coordinate sensitivity over the width of the irradiated target. The SC included in the MIC operates on gas and controls the direction of the incident beam to a given spot in the target. A version of the charge-sensitive preamplifier (QCD) and the SE system designed for experimental verification of the MIC prototype has been developed. The SCOO circuit working in conjunction with the MIC camera allows you to control the predetermined parameters of the irradiation of the patient's target boundaries and turns off the accelerator if these parameters deviate from the initially specified ones.