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We show the laser-driven acceleration of unprecedented, collimated (2 mrad) and quasi-monoenergetic (dE/E = 25%) electron beams with energy up to 50 MeV at 1 kHz repetition rate. The laser driver is a multi-cycle (15 fs) 1 kHz OPCPA system, operating at 26 mJ (1.7 TW). The scalability of the driver laser technology and the electron beams reported in this work pave the way towards developing high brilliance X-ray sources for medical imaging, innovative devices for brain cancer treatment and represent a step forward to the realization of a kHz GeV electron beamline., 9 pages, 3 figures, 1 table

Ultrafast electron dynamics drive phenomena such as photochemical reactions, catalysis, and light harvesting. To capture such dynamics in real-time, femtosecond to attosecond light sources are extensively used. However, an exact match between the excitation photon energy and a characteristic resonance is crucial. High-harmonic generation sources are exceptional in terms of pulse duration but limited in spectral tunability in the VUV range. Here, we present a monochromatic femtosecond source continuously tunable around 21 eV photon energy utilizing the second harmonic of an OPCPA laser system to drive high-harmonic generation. The unique tunability of the source is verified in an experiment probing the interatomic Coulombic decay in doped He nanodroplets across the He absorption bands. Moreover, we achieved intensities sufficient for driving non-linear processes using a tight focusing of the VUV beam. We demonstrated it on the observation of collective autoionization of multiply excited pure He nanodroplets., Comment: 12 pages, 5 figures, submitted to Nat. Commun

An upgraded version of the water-jet plasma X-ray source suitable for handling the 100 W power of the driving 1 kHz optical parametric chirped-pulse amplification (OPCPA) L1 Allegra laser system has been installed and commissioned at ELI Beamlines. Hard X-ray generation with the L1 Allegra laser is demonstrated for the first time., ELI Beamlines is a rapidly progressing pillar of the pan-European Extreme Light Infrastructure (ELI) project focusing on the development and deployment of science driven by high-power lasers for user operations. This work reports the results of a commissioning run of a water-jet plasma X-ray source driven by the L1 Allegra laser, outlining the current capabilities and future potential of the system. The L1 Allegra is one of the lasers developed in-house at ELI Beamlines, designed to be able to reach a pulse energy of 100 mJ at a 1 kHz repetition rate with excellent beam properties. The water-jet plasma X-ray source driven by this laser opens opportunities for new pump–probe experiments with sub-picosecond temporal resolution and inherent synchronization between pump and probe pulses.

A dual-output thin-disk picosecond laser operating at 100 W with 1 kHz repetition rate is reported in this Letter. By electronically adjusting the amplitude of the optical seed pulses that are injected into the laser cavity, the energy extracted from the gain medium can be shared between two pulses. Amplified double pulses are subsequently spatially separated into two independent beams by a fast Pockels cell, compressed in one common compressor, and frequency-doubled with ∼70% efficiency. This approach significantly decreases strain on the optics, as well as nonlinear effects, and is advantageous for power scaling.

Thin disk regenerative amplifiers have suitable characteristics to pump picosecond OPCPA. By optimizing the trigger pulse to the MZM, a dual-output laser operating at 100 W, 1 kHz repetition rate, compressed to the transform limit is reported.

We report on the progress in developing a high energy, 1 kHz OPCPA auxiliary laser (F-SYNC) which is intended to supplement the L1-Allegra beamline. The auxiliary pulses are synchronized to the main laser with femtosecond precision and arbitrary delay.

We report on the performance improvements of the L1 Allegra laser system. The pump laser upgrade with the thin-disk multipass amplifier allows for the energy ramp-up, while a newly developed synchronized auxiliary laser system provides the capability of the pump-probe experiments at arbitrary delays.

We report on a 1 kHz, 515 nm laser system, based on a commercially available 230 W average power Yb:YAG thin-disk regenerative amplifier, developed for pumping one of the last optical parametric chirped pulse amplification (OPCPA) stages of the Allegra laser system at ELI Beamlines. To avoid problems with self-focusing of picosecond pulses, the 1030 nm output pulses are compressed and frequency doubled with an LBO crystal in vacuum. Additionally, development of a thermal management system was needed to ensure stable phase matching conditions at high average power. The resulting 515 nm pulses have an energy of more than 120 mJ with SHG efficiency of 60% and an average RMS stability of 1.1% for more than 8 h.

The L1 Allegra is an OPCPA-based, high average power, high repetition rate laser system pumped by thin-disc based regenerative amplifiers currently under development at the ELI-Beamlines center in Czech Republic. The repetition rate is 1 kHz, pulse duration is below 15 fs and the wavelength centered around 820 nm with a maximum design pulse energy of 100 mJ. To avoid problems with self-focusing, a large portion of the system was placed inside vacuum, including the compressors and second-harmonic crystals for the last three 1030 nm pump lasers, the final three OPA stages, and the chirped mirror compressor. In order to reach the designed output energy of the whole system, the pump lasers need to be efficient, stable, and providing enough pump power for each of the amplification stages. Pulse compression of the final three pump lasers as well as efficient conversion to the second harmonic frequency in vacuum has posed several challenges and we report on their solutions and results. The vacuum environment causes difficulties for two main reasons. The first one is laser-induced-contamination (LIC) degrading the optical surfaces of dielectric gratings, mirrors and crystals, due to the presence of degassing components contaminating the vacuum chambers. The second reason is second-harmonic generation crystal mounts heating up, requiring regular phase matching corrections by rotation of the crystal mounts. The LIC problem was solved by regular cleaning of the chambers by means of an RF-plasma source, and the heating problem was solved by implementing active temperature stabilization by means of installing thermo-electric coolers on the crystal holders. To increase the efficiency of the second-harmonic generation, beam profiles of the pump lasers had to be improved. The original Faraday rotators, present in the linearly-designed regenerative amplifiers, caused non-Gaussian beam profiles due to the self focusing inside the rotators. By using KTF crystals inside a new type of rotators, the spatial profile of the pump lasers is more Gaussian, allowing the efficiency of the SHG to be higher, almost by 25%. All the solved problems recently allowed the system to reach a short pulse output energy of 56 mJ, paving a way to reach 100 mJ successfuly in the future.

Vacuum chambers are frequently used in high-energy, high-peak-power laser systems to prevent deleterious nonlinear effects, which can result from propagation in air. In the vacuum sections of the Allegra laser system at ELI-Beamlines, we observed degradation of several optical elements due to laser-induced contamination (LIC). This contamination is present on surfaces with laser intensity above 30 G W / c m 2 with wavelengths of 515, 800, and 1030 nm. It can lead to undesired absorption on diffraction gratings, mirrors, and crystals and ultimately to degradation of the laser beam profile. Because the Allegra laser is intended to be a high-uptime source for users, such progressive degradation is unacceptable for operation. Here, we evaluate three methods of removing LIC from optics in vacuum. One of them, the radio-frequency-generated plasma cleaning, appears to be a suitable solution from the perspective of operating a reliable, on-demand source for users.

Stable picosecond supercontinuum generated in long crystals is an excellent means of seeding broadband, high-energy CPA systems. The generated output energy and spectrum can be almost three times as stable as the pump for a wide range of input pulse parameters. In this work, we show this is an intrinsic property for crystals longer than the filament and for a range of input energy values. We present a description of the stability mechanism in both the visible and infrared regions together with experimental data that support the theoretical explanation.

We report on the results of the recent experiments of the XUV ELIps reflection and ellipsometry station at ELI Beamlines. Using the High Harmonic Generation source and a 1 kHz in-house developed laser, experiments in known samples such as GaN, STO and SiC were done and will be discussed.

In this paper, the authors present the characterization experiments of a 20 fs vacuum ultraviolet beam from a high harmonic generation source. The beam hits a silicon sample and passes a triple reflection gold polarizer located inside an ultrahigh vacuum chamber. The polarizer’s Malus curve was obtained; the total acquisition time for each point of the curve was 30 s. This aims to be the first vacuum ultraviolet time-resolved user station dedicated to ellipsometry. The high harmonic beam is generated by a 12 mJ, 1 kHz, 20 fs, in-house-developed laser and detected by a back-illuminated charge-coupled device.In this paper, the authors present the characterization experiments of a 20 fs vacuum ultraviolet beam from a high harmonic generation source. The beam hits a silicon sample and passes a triple reflection gold polarizer located inside an ultrahigh vacuum chamber. The polarizer’s Malus curve was obtained; the total acquisition time for each point of the curve was 30 s. This aims to be the first vacuum ultraviolet time-resolved user station dedicated to ellipsometry. The high harmonic beam is generated by a 12 mJ, 1 kHz, 20 fs, in-house-developed laser and detected by a back-illuminated charge-coupled device.

We present latest progress and experimental capabilities of user-oriented XUV beamline based on high harmonic generation (HHG). The focal length of the HHG was recently extended to 5 meters, upscaling the overall XUV photon flux.

We report on the current state of readiness of the 1 kHz, 15 fs L1 ALLEGRA laser system for long term reliable operation driving user experiments based on HHG. The highly automated system, based on short pulse OPCPA, currently generates pulses with energy of 30 mJ at central wavelength of 820 nm.

We present experimental setup for user-oriented high-harmonic source with a description of different focusing and interaction geometries. Infrared rejection system and resulting extreme ultraviolet beam characterization are also described. Beamline is designed to be driven with 100 mJ

Three main paths are being developed within the ELI research program for transforming driving laser pulses into bursts of bright short wavelength radiation: high-order harmonic generation in gases, plasma X-ray sources and sources based on relativistic electron beams accelerated in laser plasma. For each of these research areas, dedicated beamlines are built to provide a unique combination of X-ray sources to the scientific community. The application of these beamlines has a well-defined balance between fundamental science and applications in different fields of science and technology. Here we summarize the current status of those user beamlines and we introduce new diagnostics devices developed within the implementation phase of the project, namely compact XUV spectrometer and beam profiler that is using only one fixed detector and an imaging Michelson interferometer with increased sensitivity for low density gas jet characterization.

We report on the status of the Allegra laser beamline currently being commissioned at the ELI-Beamlines facility. The beamline will be used to drive X-ray sources for research in material science, molecular physics, and biophysics, where a high repetition rate is essential. Designed for user operation, significant attention is given to stability, reliability, and automation of the system. A photo of the beamline is shown in Fig. 1(a).

The Allegra femtosecond laser system is the main driver for high harmonic and plasma x-ray secondary sources at ELI-Beamlines operating at a 1 kHz rep rate. The system is based on OPCPA technology and consists of seven amplification stages pumped by thin-disk picosecond lasers. It is designed to reach 30 mJ output in the first phase of operation and to be ramped up to 50 mJ by engaging an additional pump laser. The amplified pulse is compressed to sub-20fs by an array of chirped mirrors and higher order dispersion is pre-compensated for by a Dazzler AOPDF in the front-end. In this paper we present the overview of Allegra system and the current status of deployment with a special focus on the high average power OPCPA in vacuum.

A passive peak intensity stabilization method based on optimal combination of Kerr nonlinearity and linear dispersion is presented. By analyzing the Kerr nonlinearity with a 3D numerical model it was found that if a Gaussian laser pulse acquires a certain amount of nonlinear phase and is consequently over-compressed to have sight negative chirp, the uctuations in peak intensity caused by energy uctuations are reduced. Thus, even if the energy of laser pulses uctuates the peak intensity can be stabilized. The simulations for realistic pulse parameters show an increase in peak intensity stability by well-over an order of magnitude. We demonstrate this process experimentally using a thin disk pump laser at ELI{Beamlines

We report on the status of the re-commissioning of a high energy OPCPA laser system with programmable spectrum that serves as a frontend for a 10 PW laser at ELI-Beamlines. The OPCPA chain was developed by a consortium of National Energetics and Ekspla along with scientists of ELI-Beamlines.1 The laser system, consisting of three picosecond OPCPA stages, pulse cleaner, Offner stretcher, and 5 nanosecond OPCPA stages pumped by Nd:YAG lasers with programmable pulse shape (NL944, Ekspla), allows for precise spectral shaping while achieving high nonlinear conversion efficiency. Employing a subsequent Nd:glass power amplifiers (PA), the system was demonstrated to yield>1 kJ of energy, while maintaining broad spectrum of > 13 nm (FWHM). After recommissioning the OPCPA frontend in Dolni Břežany, an output energy of 4.3 J, flat beam-profile and good far-field quality has been demonstrated. The spectral shape has been optimized to support > 15 nm bandwidth and >1.5 kJ, consistent with 10 PW operation of the fully integrated laser system after compression.

The ITER main vacuum system consists of six torus exhaust pumps integrated in the Cryostat through dedicated housings at the building B1 level. The Port Cell area outside the cryopump plug is affected by neutrons streaming through the housing structure and diagnostics penetrations. The aim of the study presented in this paper is to perform a complete assessment of the nuclear responses in the Cryopump Port Cell #12 by means of the MCNP-5 Monte Carlo code in a full 3-D geometry. The results of the neutronic analyses provide guidelines for the design and maintenance of the embedded components, ensuring their structural integrity and proper operation. Radiation transport calculations have been carried out to determine the radiation field inside the Port Cell through 3-D neutrons and gamma maps. Nuclear heating induced by neutrons and photons and the absorbed dose during the ITER lifetime on steel and silicon have been estimated in the Port Cell area, in order to assess the nuclear loads that the diagnostics and related electronic components have to withstand. Furthermore, the impact of the gamma-rays emitted by neutron-activated water circulating in the Primary Heat Transfer System have been evaluated on the Port Cell environment: 3-D maps of the gamma flux, nuclear loads on steel/silicon during plasma operation are provided.

The dynamics of the H2O molecule density in a metal gas-discharge chamber filled with low-pressure water vapor or its mixtures with noble gases was investigated by manometric and spectral methods. Regimes both with and without discharge excitation were studied. In the absence of a discharge, the molecule density dynamics is governed by the heterogeneous interaction of molecules with the chamber walls. In the presence of a discharge, in addition to the heterogeneous interaction, fast plasmachemical molecule dissociation also contributes to the initial stage of H2O molecule loss. The role of heating of the chamber walls is discussed.

State-of-the-art physics experiments are pushing the development of lasers with ultra-high peak power pulses. 4 PW pulses have been produced with TiSa [1] and 10 PW with the same gain medium is scheduled at LULI (Apollon) and at ELI-NP. The other approach is to use Nd-doped glass as gain medium, whose interest is in its capability of delivering higher energy at the expense of a longer pulse duration. Based on this gain material combined with an OPCPA based front-end, a kJ-10 PW class laser has been designed and built. The front-end, consisting of picosecond OPCPA, temporal pulse cleaning and nanosecond OPCPA, delivers pulses with excess of 4 Joules at 5 Hz with a shaped spectrum to pre-compensate for gain distortions in Nd:glass power amplifiers. Two liquid-cooled, mixed glass power amplifiers, namely PA1 and PA2, are used for further amplification. Up to now, they have been activated demonstrating 70 J at 1 shot a minute after PA1 and 1 kJ at 1 shot every 7 minutes for PA2. The Fourier limit of the spectrum is 150 fs meaning 6 PW capability after compression. This energy level has been obtained with only 3 Joules seed energy, from the OPCPA and partial activation of PA2. Scaling of this result suggests that more than 1.7 kJ should be obtained leading to 10 PW after compression while the output spectrum will remain compatible with 150 Fs thanks to the OPCPA spectral tailoring capability.

We report on the status of the Allegra laser beamline, which is designed to provide ub-20 fs pulses with tens of mJ of energy with exceptionally high contrast at a 1 kHz repetition rate.

We discuss the performance of several stages of high energy, high average power SHG and OPCPA with picosecond pulses in vacuum as part of the ALLEGRA laser at ELI-Beamlines.

We measure parameters of a stable supercontinuum generated in long YAG crystal, driven by 3 ps pulses at 1030 nm and evaluate the impact of initial conditions on supercontinuum stability.

We present a broadband optical parametric chirped pulse amplification (OPCPA) system delivering 4 J pulses at a repetition rate of 5 Hz. It will serve as a frontend for the 1.5 kJ

For most extreme light applications, a reliable and stable driver laser is crucial to successful experiments. As lasers grow in energy and peak power they become increasingly complex and more failure modes are introduced to the system as a whole. For this reason it is prudent to develop a laser with simplicity, repeatability, and durability in mind. With the wide commercial availability of high quality, inexpensive fiber components, much of the required pulse conditioning for seeding high energy laser systems can take place entirely in fiber. This allows for much of the laser front end to be compact, alignment-free, and computer controlled with potentially dramatic savings in cost and space on the optical table. Here we explore some of the current trends in fiberbased front ends for high peak power laser systems. The requirements for any given high peak power laser are always quite different and fiber front ends are enormously customizable, so here we present two basic versions of fiber front ends which are used at the ELI-Beamlines facility which resemble other common fiber front end architectures.

A novel method for measuring the number density of water molecules in low-temperature plasma is developed. The absolute intensities of rotational lines of the (0,0) band of the OH(A 2Σ–X 2П) transition are used. Lines with sufficiently large rotational quantum numbers referring to the so-called “hot” group of molecules produced by electron-impact dissociative excitation of water molecules are chosen for measurements. The excitation rate of a process with a known cross section is determined by measuring the parameters of plasma electrons by means of the probe method. The measured number densities of molecules are compared with those in the initial plasma-forming mixture. The time evolution of the particle densities in plasma is investigated. The problems of the sensitivity and applicability of the absolute spectral method are considered.

The 10PW OPCPA/glass laser under construction for ELI-Beamlines will exhibit output pulses delivered every minute with an energy of 1.5kJ within 150fs duration. We will show results of the sub-sections as well as output performances.

Overview of progress in construction and testing of the laser systems of ELI-Beamlines, accomplished since 2015, is presented. Good progress has been achieved in construction of all four lasers based largely on the technology of diode-pumped solid state lasers (DPSSL). The first part of the L1 laser, designed to provide 200 mJ

We report on the status of the high repetition rate, high energy, L1 laser beamline at the ELI-Beamlines facility. The beamline is based on picosecond optical parametric chirped pulse amplification (OPCPA) of pulses from a mode-locked Ti:Sapphire oscillator and has a target energy/repetition rate of 100 mJ/1 kHz with < 15fs pulse duration. The OPCPA pump lasers use thin disk technology to achieve the high energy and average power required to pump such a high energy, high repetition rate broadband amplifier. Here we report on the progress in beamline development and discuss the technical challenges involved in producing such a system and their solutions. A major focus of the laser development is reliable, robust operation and long term stability; mechanical, optical, and control system architecture design considerations to achieve our goals of long term stability are discussed.

We present an active cavity pointing stabilization system based on a novel method that tracks the cavity mode position directly on the thin disk gain medium itself. Here, the overlap of the lasing cavity with the pump, visible as a depletion within the pumped area, is most crucial to the stability of the laser. Short term stability as well as long term stability are significantly increased enabling day long operation, directly from a cold start of the laser system, without the need for thermalization and manual alignment.

The validation of spectroscopic method for localization of the water plume around a coolant leak in ITER plasma chamber based on the excitation of coolant particles in plasma near wall is presented. The sensitivity of OH hydroxyl and Xe as minor admixtures to cooling water detection in different types of plasma discharges relevant to ITER tested in stellarator L-2M and hollow cathode glow discharge. The limit of sensitivity of water detection in hot plasma using OH* hydroxyl band is estimated as 3 × 10 −5 Pa m 3 s −1 and ∼1 × 10 −6 Pa m 3 s −1 for Xe. For glow discharge conditions measured sensitivity of OH* detection is equal to 3 × 10 −5 Pa m 3 s −1 .

The ITER tokamak will be fuelled at a time averaged rate of up to 200 Pam 3 s −1 requiring neutralised gas in the divertor to be pumped to balance the fuelling and remove the fusion helium and other impurities in the exhaust. This is achieved on ITER using large bespoke cryo-sorption pumps. In this paper design evolution of the ITER divertor pumping system is outlined from the 1998 configuration to the current design. Details of the new, 6 direct pump, system design which will be used in the build of ITER are given. The operating modes of the new system configuration for different plasma scenarios are described and the performance of the new system is analysed and compared with previous baselines.

Results are presented from L-2M stellarator experiments on testing a possible method for detection of water microleakages in the cooling system of the first wall and vacuum chamber of ITER. The method consists in the spectroscopic detection of spectral lines of the OH hydroxyl, which forms via the dissociation of water molecules in plasma. Emission in the spectral band of 305–310 nm can be detected even at water leakage rates less than 10−4 Pa m3/s. Chemical reactions between water and boron compounds on the vacuum chamber wall delay the detection of leakages up to ∼2000 s. A similar phenomenon can be expected when a leakage will occur in ITER, where the materials suggested for the first wall (Be, Li) can also chemically react with water.

We present a stable supercontinuum (SC) generated in a bulk YAG crystal, pumped by 3 ps chirped pulses at 1030 nm. The SC is generated in a loose focus geometry in a 13 cm long YAG crystal, allowing for stable and robust single-filament generation. The SC energy stability exceeds that of the pump laser by almost a factor of 3. Additionally, we show that the SC spectrum has long-term stability and that the SC is coherent and compressible by compressing the portions of SC spectra close to the corresponding Fourier limit. This makes the picosecond-pulse-driven SC a suitable stable seed for OPCPA amplifiers.

We report on the design and performance of a fiber-based, multi-channel laser amplifier seed pulse distribution system. The device is designed to condition and distribute low energy laser pulses from a mode-locked oscillator to multiple, highly synchronized, high energy amplifiers integrated into a laser beamline. Critical functions such as temporal pulse stretching well beyond 100 ps/nm, pulse picking, and fine control over the pulse delay up to 300 ps are all performed in fiber eliminating the need for bulky and expensive grating stretchers, Pockels cells, and delay lines. These functions are characterized and the system as a whole is demonstrated by seeding two high energy amplifiers in the laser beamline. The design of this system allows for complete computer control of all functions, including tuning of dispersion, and is entirely hands-free. The performance of this device and its subsystems will be relevant to those developing lasers where reliability, size, and cost are key concerns in addition to performance; this includes those developing large-scale laser systems similar to ours and also those developing table-top experiments and commercial systems.

We present a stable supercontinuum generation in a YAG crystal, driven by 3 ps pulses at 1030 nm. The supercontinuum is demonstrated to be coherent and compressible by Dazzler and prism compressor to below 15 fs.

We report on a broadband OPCPA system, pumped at 515 nm by frequency doubled Yb:YAG thin disk lasers. The system delivers 11.3 mJ pulses at a central wavelength of 800 nm with a spatial beam quality of Msup2/sup= 1.25 and25% pump-to-signal conversion efficiency. The broadband pulses were demonstrated to be compressible to 12 fs using a chirped mirror compressor.

We report on a frequency-doubled picosecond Yb:YAG thin disk regenerative amplifier, developed as a pump laser for a kilohertz repetition rate OPCPA. At a repetition rate of 1 kHz, the compressed output of the regenerative amplifier has a pulse duration of 1.2 ps and pulse energy of 90 mJ with energy stability of σ < 0.8% and M2 < 1.2. The pulses are frequency doubled in an LBO crystal yielding 42 mJ at 515 nm.

Results of testing of a possible method for location of water microleakages in the cooling system of the first wall and vacuum chamber of ITER are presented. The method consists in spectroscopic detection of the emission lines of atoms and ions of the Xe additive dissolved in water. These lines are excited when the water with dissolved Xe contacts the plasma. The high electron cyclotron resonance heating (ECRH) power deposited in a relatively small plasma volume in the L-2M stellarator (P = 0.5 MW, V = 0.24 m3, and the specific heating power ∼2 MW/m3) makes it possible to achieve plasma parameters close to those in the edge plasma of ITER for different operating modes, including the H-mode with an edge transport barrier. In test experiments, several lines of Xe ions were revealed suitable for detection of xenon in plasma with parameters close to those in the edge plasma of ITER at leakage rates at a level of ∼10−6 Pa m3 s−1 and spatial resolution of ∼0.5 cm.

We present a high-energy picosecond Nd:YAG-amplification system seeded with pulses of a femtosecond Yb:KGW oscillator. The Nd : YAG amplifier is intended to pump an optical parametric chirped-pulse amplification (OPCPA) system. Generation of high contrast pulses with energy of 600 mJ and pulse duration of 100 ps is demonstrated.

This paper takes the ITER fueling requirements and current knowledge of gas balance and exhaust from operating tokamaks to predict all likely gas inputs into the ITER Vacuum systems. Areas where gas dynamics modeling is relevant to the ITER design are highlighted. The design and operation of the ITER vacuum system gives an element of segregation of different gas flows and species. This paper analyses the time dependent gas segregation in the vacuum system resulting from different temperature dependences of cryogenic sorption and condensation processes of different gas species. As a specific example, the optimal transfer of 41 Ar through the vacuum system is studied with respect to its decay and the resulting effects on the design of system components.

Experimental determination of the sensitivity of the method for detection of water microleakages in the cooling systems of the plasma vacuum chambers of complex electrophysical devices (such as tokamaks, fuel elements of nuclear reactors, and plasmachemical reactors) is considered. It was shown that the spectroscopic method for detection of water microleakages by using the hydroxyl radiation spectrum makes it possible to detect leakages at a level of 10−5 Pa m3 s−1. The spatial resolution of the method allows one to localize defects with an accuracy of several centimeters.