J. Zebrowski

Spatial structure and energy spectrum of ion beams studied with CN detectors within a small PF device

Abstract The paper reports the application of Solid-State Nuclear Track Detectors to study the pulsed plasma-ion streams emitted from plasma-focus (PF) type discharges, which were performed within a low-energy PACO device constructed at Instituto de Fisica Arroyo Seco. The PACO device was operated under static initial gas conditions or with dynamic gas puffing. Studies of the structure of the fast deuteron beams were carried out within an energy range from 80 keV to about 2 MeV . Studies of ion energy and an ion angular distribution were also performed. The measurements showed that the fast deuterons are emitted in many “narrow” micro-beams, as in other larger PF devices. The anisotropy of the deuteron angular distribution was explained by the stochastic character of the formation of local ion sources within the PF discharge column.

Use of Cherenkov-type detectors for measurements of runaway electrons in the ISTTOK tokamak.

Gas, fluid, or solid Cherenkov-type detectors have been widely used in high-energy physics for determination of parameters of charged particles, which are moving with relativistic velocities. This paper presents experimental results on the detection of runaway electrons using Cherenkov-type detectors in the ISTTOK tokamak discharges. Such detectors have been specially designed for measurements of energetic electrons in tokamak plasma. The technique based on the use of the Cherenkov-type detectors has enabled the detection of energetic electrons (energies higher than 80 keV) and determination of their spatial and temporal parameters in the ISTTOK discharges. Obtained experimental data were found in adequate agreement to the results of numerical modeling of the runaway electron generation in ISTTOK.

Application of intense plasma-ion streams emitted from powerful PF-type discharges for material engineering

This paper concerns various applications of powerful ion- and plasma-streams generated by high-current pulse discharges realized within different plasma-focus (PF) facilities. General characteristics of the emitted plasma-ion streams are summarized. The possibility of application of special arrangements, e.g. cryogenic targets, CD2 or metal wires, hydrogen or deuterium getters, special alloy targets, etc, is described. The paper presents results of different experiments oriented on the interaction of the pulsed plasma-ion streams with various material targets placed inside PF-360 device (at IPJ in Swierk) and PF-1000 facility (at IPPLM in Warsaw). Attention is paid to plasma–target interactions and influence of the material targets on emission characteristics of the PF-type discharges. The diagnostics includes current and voltage measurements, optical photography and spectroscopy, x-ray emission observations and fast-neutron measurements (from deuterium discharges). Particular attention is paid to time-resolved spectroscopic studies. The use of pulsed ion- and plasma-streams for modifications of different materials, e.g. those of particular interest for the construction of nuclear fusion reactors, is described.

Mass- and energy-analyses of ions from plasma by means of a miniature Thomson spectrometer

The paper presents an improved version of a miniature mass-spectrometer of the Thomson-type, which has been adopted for ion analysis near the dense plasma region inside a vacuum chamber. Problems connected with the separation of ions from plasma streams are considered. Input diaphragms and pumping systems, needed to ensure good vacuum inside the analyzing region, are described. The application of the miniature Thomson-type analyzer is illustrated by ion parabolas recorded in plasma-focus facility and rod plasma injector experiment. A quantitative analysis of the recorded ion parabolas is presented. Factors influencing accuracy of the ion analysis are discussed and methods of the spectrometer calibration are described.

Development of a diagnostic technique based on Cherenkov effect for measurements of fast electrons in fusion devices

A diagnostic technique based on the Cherenkov effect is proposed for detection and characterization of fast (super-thermal and runaway) electrons in fusion devices. The detectors of Cherenkov radiation have been specially designed for measurements in the ISTTOK tokamak. Properties of several materials have been studied to determine the most appropriate one to be used as a radiator of Cherenkov emission in the detector. This technique has enabled the detection of energetic electrons (70 keV and higher) and the determination of their spatial and temporal variations in the ISTTOK discharges. Measurement of hard x-ray emission has also been carried out in experiments for validation of the measuring capabilities of the Cherenkov-type detector and a high correlation was found between the data of both diagnostics. A reasonable agreement was found between experimental data and the results of numerical modeling of the runaway electron generation in ISTTOK.

Characteristics of four-channel Cherenkov-type detector for measurements of runaway electrons in the ISTTOK tokamak.

A diagnostics capable of characterizing the runaway and superthermal electrons has been developing on the ISTTOK tokamak. In previous paper, a use of single-channel Cherenkov-type detector with titanium filter for runaway electron studies in ISTTOK was reported. To measure fast electron populations with different energies, a prototype of a four-channel detector with molybdenum filters was designed. Test-stand studies of filters with different thicknesses (1, 3, 7, 10, 20, 50, and 100 μm) have shown that they should allow the detection of electrons with energies higher than 69, 75, 87, 95, 120, 181, and 260 keV, respectively. First results of measurements with the four-channel detector revealed the possibility to measure reliably different fast electrons populations simultaneously.

Cherenkov-type diagnostics of fast electrons within tokamak plasmas

This paper presents a summary of the most important results of fast electron measurements performed so far within different tokamaks by means of Cherenkov-type detectors. In the ISTTOK tokamak (IPFN, IST, Lisboa, Portugal), two measuring heads were applied, each equipped with four radiators made of different types of alumina-nitrate poly-crystals. A two-channel measuring head equipped with diamond radiators was also used. Within the COMPASS tokamak (IPP AS CR, Prague, Czech Republic) some preliminary measurements have recently been performed by means of a new single-channel Cherenkov-type detector. The experimental data from the TORE SUPRA tokamak (CEA, IFRM, Cadarache, France), which were collected by means of a DENEPR-2 probe during two recent experimental campaigns, have been briefly analyzed. A new Cherenkov probe (the so-called DENEPR-3) has been mounted within the TORE SUPRA machine, but the electron measurements could not be performed because of the failure of this facility. Some conclusions concerning the fast electron emission are presented.

New plasma-focus experiments without and with additional targets

Summary form only given. The paper reports on results of recent Plasma Focus (PF) experiments performed with medium-scale machines operated at 60-180 kJ. The first series of PF discharges was carried out within a PF-150 machine at 60 W, 28 kV. The device was equipped with Mather-type electrodes of 100 mm and 50 mm in diameter, and 200 mm in length. Attention was paid to current-sheath (CS) and PF pinch dynamics, which was investigated with a high-speed framing camera, consisted of two VR channels and two soft X-ray modules. Exposure of VR frames was about 1 ns, and that of X-ray frames was about 0.8 ns. Synchronization was realized with fast photodiodes registering the CS radiation. Electrical signals were stored with a TDS784A digital oscilloscope, and all the frames were elaborated with an image capturing and processing system (AICPS). Measurements were also performed with X-ray pinhole cameras and neutron counters. Comparison of the VR pictures and corresponding X-ray images revealed correlation between fine structures formed inside the pinch column. The second series of experiments within PF-150 machine was carried out with solid targets made of thin metal- or carbon-wires, fixed on the z-axis at the electrode outlet. Using the same diagnostic equipment, it was observed that the interaction of the low-mass CS layer with the wire target produces an almost homogenous plasma column, which does not expand considerably during the CS collapse process. At the same time the X-ray emission along the z-axis is considerably higher that that emitted in the radial directions. The third series of PF experiments was performed within a PF-360 machine equipped with the Mather-type electrodes of 170 mm and 120 mm in diameter, and 300 mm in length. The system was operated at 110 kJ, 30 kV, or 176 kJ, 35 kV. Several additional diagnostic techniques were applied to study time-integrated and time-resolved characteristics of the charged particle emissions. To increase a neutron yield from fusion reactions the use was made of cryogenic targets in the form of heavy-ice (D/sub 2/O) layers deposited upon special cryogenic targets. Considerable differences in the neutron yield have been observed in dependence on the experimental conditions applied. These new experiments are described, and the results are compared with those of previous PF studies.

Studies of pulsed plasma-ion streams during their free propagation and interaction with SIC-targets

Summary form only given. The paper reports on experimental studies of deuteriumplasma streams during their free propagation in a vacuum chamber and their interaction with a silicon-carbide (SiC) target. The plasma streams were generated by the RPI-IBIS coaxial multi-rod accelerator, which (after the pulse injection of pure deuterium) was powered from a 60-μF condenser bank charged initially to 30 kV. In a so-called medium operational mode an average value of the deuteron energy was about 40 keV [1].The reported optical measurements were carried out trough a side-on optical window, using a quartz collimator and a quartz optical-cable, which was coupled with a Mechelle®900 spectrometer. It could be operated in a wavelength range from about 300 nm to 1100 nm, at different exposition times (from 100 ns to 50 ms) with a chosen delay time (ranging from 100 ns to 1 ms). The spectrometer was equipped with a PCO SensiCam camera and software needed for an analysis of recorded optical spectra. Particular attention was paid to detailed spectroscopic measurements of free-propagating plasma streams and plasma produced at the surface of a SiC target, which was placed at a distance of 20 cm from the electrode ends, and irradiated by powerful (1-5 MW/cm2) deuterium-plasma streams. The main aim was to record and identify spectral lines of carbon and silicon ions. On the basis of the spaceand time-resolved spectroscopic measurements of a DE line, it was estimated that the highest deuterium-plasma density was about 1017 cm-3, while the average value was 3 x 1016 cm-3. Surface changes of the irradiated SiC targets were analyzed with a scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS). It was shown that plasma-ion fluxes of energy density of about 2-7 J/cm2 are sufficient for material studies. The advantages of the RPI-IBIS system are relatively high energies of the emitted deuterons (ranging up to several hundreds keV), and high power loads upon the target in ion micro-beams spots. The presented results are of importance for basic plasma physics and for fusion-oriented material studies.

Detection method for direct studies of fast electrons via Cherenkov effect in the ISTTOK tokamak

Measurements of the energetic particles (electrons and ions) are one of the main scientific tasks for various experimental devices generating high-temperature plasmas. The significance of such studies is proportional to the amount of information about different processes occurring inside plasma, in which such charged particles are involved.