Frank Leipold

Antenna design for fast ion collective Thomson scattering diagnostic for the international thermonuclear experimental reactor

Fast ion physics will play an important role for the international thermonuclear experimental reactor (ITER), where confined alpha particles will affect and be affected by plasma dynamics and thereby have impacts on the overall confinement. A fast ion collective Thomson scattering (CTS) diagnostic using gyrotrons operated at 60 GHz will meet the requirements for spatially and temporally resolved measurements of the velocity distributions of confined fast alphas in ITER by evaluating the scattered radiation (CTS signal). While a receiver antenna on the low field side of the tokamak, resolving near perpendicular (to the magnetic field) velocity components, has been enabled, an additional antenna on the high field side (HFS) would enable measurements of near parallel (to the magnetic field) velocity components. A compact design solution for the proposed mirror system on the HFS is presented. The HFS CTS antenna is located behind the blankets and views the plasma through the gap between two blanket modules. The viewing gap has been modified to dimensions 30x500 mm(2) to optimize the CTS signal. A 1:1 mock-up of the HFS mirror system was built. Measurements of the beam characteristics for millimeter-waves at 60 GHz used in the mock-up agree well with the modeling.

Ultrasound enhanced plasma surface modification at atmospheric pressure

Abstract Efficiency of atmospheric pressure plasma treatment can be highly enhanced by simultaneous high power ultrasonic irradiation onto the treating surface. It is because ultrasonic waves with a sound pressure level (SPL) above ∼140 dB can reduce the thickness of a boundary gas layer between the plasma and the material surface, and thus, many reactive species generated in the plasma can reach the surface before they are inactivated and can be efficiently utilised for surface modification. In the present work, glass fibre reinforced polyester plates were treated using a dielectric barrier discharge and a gliding arc at atmospheric pressure to study adhesion improvement. The effect of ultrasonic irradiation with the frequency diapason between 20 and 40 kHz at the SPL of ∼150 dB was investigated. After the plasma treatment without ultrasonic irradiation, the wettability was significantly improved. The ultrasonic irradiation during the plasma treatment consistently enhanced the treatment efficiency. The principal effect of ultrasonic irradiation can be attributed to enhancing surface oxidation during plasma treatment. In addition, ultrasonic irradiation can suppress arcing, and the uniformity of the treatment can be improved.

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR.

Resolving the bulk ion region of millimeter-wave collective Thomson scattering spectra at ASDEX Upgrade.

Collective Thomson scattering (CTS) measurements provide information about the composition and velocity distribution of confined ion populations in fusion plasmas. The bulk ion part of the CTS spectrum is dominated by scattering off fluctuations driven by the motion of thermalized ion populations. It thus contains information about the ion temperature, rotation velocity, and plasma composition. To resolve the bulk ion region and access this information, we installed a fast acquisition system capable of sampling rates up to 12.5 GS/s in the CTS system at ASDEX Upgrade. CTS spectra with frequency resolution in the range of 1 MHz are then obtained through direct digitization and Fourier analysis of the CTS signal. We here describe the design, calibration, and operation of the fast receiver system and give examples of measured bulk ion CTS spectra showing the effects of changing ion temperature, rotation velocity, and plasma composition.

Modification of the collective Thomson scattering radiometer in the search for parametric decay on TEXTOR

Strong scattering of high-power millimeter waves at 140 GHz has been shown to take place in heating and current-drive experiments at TEXTOR when a tearing mode is present in the plasma. The scattering signal is at present supposed to be generated by the parametric decay instability. Here we describe the heterodyne detection system used to characterize the newly discovered signal measured at TEXTOR, and we present spectral shapes in which the signal can appear under different conditions. The radiation is collected by the receiver through a quasi-optical transmission line that is independent of the electron cyclotron resonance heating transmission line, and so the scattering geometry is variable. The signal is detected with 42 frequency channels ranging from 136 to 142 GHz. We demonstrate that the large signal does not originate from gyrotron spurious radiation. The measured signal agrees well with independent backscattering radiometer data.

Temporally resolved plasma composition measurements by collective Thomson scattering in TEXTOR (invited).

Fusion plasma composition measurements by collective Thomson scattering (CTS) were demonstrated in recent proof-of-principle measurements in TEXTOR [S. B. Korsholm et al., Phys. Rev. Lett. 106, 165004 (2011)]. Such measurements rely on the ability to resolve and interpret ion cyclotron structure in CTS spectra. Here, we extend these techniques to enable temporally resolved plasma composition measurements by CTS in TEXTOR, and we discuss the prospect for such measurements with newly installed hardware upgrades for the CTS system on ASDEX Upgrade.

Heterodyne detector for measuring the characteristics of elliptically polarized microwaves

We present the design and results of an innovative device which is capable of determining the three characteristic parameters of elliptically polarized light (ellipticity, angle of ellipticity and direction of rotation) for microwave radiation at a frequency of 105 GHz. The device consists of two perpendicularly oriented rectangular pickup waveguides to measure the electrical field in two orthogonal directions. The microwave frequency is mixed down by a heterodyne technique to frequencies on the order of 200 MHz. An oscilloscope is used to determine the relative amplitudes of the electrical fields and the phase shift between them, from which the three characteristic parameters are calculated. As an essential part of the commissioning, the device was used to characterize the polarizers of the transmission line for the collective Thompson scattering (CTS) experiment at ASDEX Upgrade. The measurements were also used to benchmark the modeling of the microwave propagation through the transmission line. Comparison between measurements and calculated wave characteristics show good agreement and are presented.

Elevation angle alignment of quasi optical receiver mirrors of collective Thomson scattering diagnostic by sawtooth measurements

Localized measurements of the fast ion velocity distribution function and the plasma composition measurements are of significant interest for the fusion community. Collective Thomson scattering (CTS) diagnostics allow such measurements with spatial and temporal resolution. Localized measurements require a good alignment of the optical path in the transmission line. Monitoring the alignment during the experiment greatly benefits the confidence in the CTS measurements. An in situ technique for the assessment of the elevation angle alignment of the receiver is developed. Using the CTS diagnostic on TEXTOR without a source of probing radiation in discharges with sawtooth oscillations, an elevation angle misalignment of 0.9° was found with an accuracy of 0.25°.

Sterilization of packed matter by means of low temperature atmospheric pressure plasmas

Summary form only given. The decontamination of material in closed containers by means of atmospheric pressure plasmas is investigated. The target is Listeria monocytogenes, a bacterium which causes listeriosis and can be found in plants and food. The non-pathogenic species, Listeria innocua, is used for these experiments. Microscopy glass slides were inoculated with Listeria innocua by spraying. The slides were placed in a plastic bag. The plastic bag was filled with a gas mixture of 97.5% Ar + 2.5% O2 and sealed. The sealed bag was placed between the electrodes of a dielectric barrier discharge. The exposure time was varied between 1 and 30 min. A reduction of Listeria innocua of more than log 2.4 (detection limit) was obtained after a treatment time of 2 min. The electrode temperature of the DBD was found to be below 30 °C.

Gyrotron Collective Thomson Scattering Diagnostics of Fast Ions in Textor and Asdex Upgrade

Summary form only given. A critical need exists for confined fast ion diagnostics in tokamak fusion experiments, particularly for fusion product alpha particles in ITER and future fusion burning experiments. To develop this diagnostic capability and in support of current fast ion plasma physics research, collective Thomson scattering (CTS) diagnostics have been implemented at TEXTOR and ASDEX Upgrade (AUG) tokamaks using available gyrotron infrastructure with the addition of sensitive scattered signal receiver systems. At TEXTOR a 180 kW, 110 GHz gyrotron and a 42 channel. 6 GHz bandwidth heterodyne receiver has achieved up to 100 CTS scattered spectra per plasma shot with 4 ms time and 10 cm spatial resolution. Large scattering angles (~160deg) with steerable optics enable observation of fast ion spatial and field orientation anisotropies. Studies of fast ion dynamics behavior with neutral beam injection (NBI) and ion cyclotron heating have commenced, resulting in unique observations of fast ions redistributions during sawteeth and slow down after NBI turn off. At AUG a 1 MW, 105 GHz mode of a two-frequency gyrotron with a 50 channel, 10 GHz bandwidth receiver is becoming operational for CTS diagnostics with resolutions similar to TEXTOR. Precise gyrotron frequency measurements, notch filter timing, transmission line alignments, and receiver field of view mappings inside the tokamak have been accomplished using novel beam profile instrumentation. AUG-CTS commissioning progress will be presented. Plasma measurements in AUG are expected to provide new insights into fast ion physics and to further validate gyrotron CTS as a fast ion diagnostic tool for ITER.