M. Jakubowski

Diagnostics of Fast Electrons within Castor Tokamak by Means of a Modified Cherenkov-Type Probe

The paper reports on experimental studies performed within the CASTOR tokamak, which was operated at IPP in Prague, Czech Republic, during the last experimental campaign carried out in autumn 2006. The main aim was to implement a new diagnostic technique for measurements of energetic (>80 keV) electrons within the tokamak edge plasma region. The technique was based on the use of a Cherenkov-type probe similar to the first prototype detector, which was tested during the previous experiments with the CASTOR device. In particular, the distributions of fast electrons in a standard scenario at different values of plasma current Ip, and toroidal magnetic field BT are determined.

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.

Cherenkov Detector For Measurements Of Fast Electrons In CASTOR‐Tokamak

The paper reports on capabilities of an improved version of the Cherenkov detector designed for measurements of fast electrons. The described technique enables the identification of electron beams, the measurements of their temporal characteristics, as well as the estimation of their spatial properties to be performed. Results obtained in the last experimental campaign with the CASTOR facility show good measuring capabilities of such a detection system. The radial distributions of fast‐electron streams at different plasma densities, as well as the electron fluency dependences on discharge currents and toroidal magnetic fields are also presented.

Development of a Cherenkov-type diagnostic system to study runaway electrons within the COMPASS tokamak

Direct measurements of fast electrons, which are produced in high-temperature plasma and escape from tokamak-type facilities, are of particular interest for ITER and future fusion devices, where intense runaway electrons (RE) can significantly damage the first wall components. Therefore, the runaway control and mitigation based on credible measuring methods should be developed already in present devices. A team from the National Centre for Nuclear Research (NCBJ), Poland, developed special probes equipped with Cherenkov-type detectors for measurements of the fast electrons within edge plasmas of tokamaks. Studies of the fast runaway electrons were extensively carried out at the COMPASS tokamak at the Institute of Plasma Physics (IPP) in Prague during experimental campaigns in 2014–2016. In order to investigate an electron-beam energy distribution a three-channel probe equipped with the Cherenkov-type detectors sensitive to electrons of different energies has been constructed. The measurements performed by means of these detectors showed that the first fast electron peak appears usually in the current ramp-up phase, even before the hard X-rays (HXR) pulse. Some electron signals can also be observed during subsequent HXR emissions. However, the most distinct electron peaks in all energy channels appear mainly during the plasma disruption. A correlation of Cherenkov signals with the MHD activity was also studied.

Development of the Cherenkov-type diagnostic system to study runaway electrons within tokamaks

Measurements of fast electrons produced in plasma a nd escaping from tokamak-type facilities are of particular interest for diagnostics, because such electrons deliver information about processes occurring inside bulk plasma. A team from the National Centre for Nuclear Research (NCBJ, former IPJ) proposed Cherenkov-type probes for measurements of fast electrons in tokamaks because of high spatial- and temporal-reso lution of such detectors. The paper presents feasibility and design studies o f the Cherenkov-type probes, a development of the measuring head constructions designed for di fferent tokamak devices. In order to lower an energy threshold of the electron detection the auth ors applied radiators with the highest values of the refractive index. Different radiator materia ls, such as aluminium nitride (AlN) and CVD diamond crystals were applied. Several versions of measuring heads and different manipulators, e.g., a movable vacuum-tight shaft or a fast-moving reciprocating probe, have been manufactured and used. The practical application of the Cherenkov probes required also a consideration of the optical fibres and photomultip liers spectral characteristics. In order to investigate an energy distribution of fast electron beams the multi-channel probes have been designed and manufactured.

Signal acquisition in Cherenkov-type diagnostics of electron beams within tokamak facilities

The paper presents feasibility and design studies of Cherenkov-type probes, a development of the measuring head construction designed for different tokamak devices, and in particular the acquisition of optical signals to a data storage system. In order to lower the energy threshold of the electron detection the authors applied radiators with the highest values of the refractive index. Different radiator materials, such as aluminium nitride and CVD diamond were applied. Several versions of measuring heads and different manipulators, e.g., a movable vacuum-tight shaft or a fast-moving reciprocating probe, were manufactured and used. The practical application of the Cherenkov probes required also a consideration of spectral characteristics of optical fibres and photomultipliers. The Cherenkov radiation, as generated inside the radiators, is lead out through separate fibres (optical cables) to the atmospheric pressure side. The emitted radiation in the blue (near ultraviolet) spectrum range should be collected and delivered through appropriate optical cables to a control room, amplified within photomultipliers and recorded in a digital form. In order to investigate an electron energy distribution the multi-channel probes have also been designed and applied.