Plasma impurities observed by a pulse height analysis diagnostic during the divertor campaign of the Wendelstein 7-X stellarator

The paper reports on the optimization process of the soft X-ray pulse height analyzer installed on the Wendelstein 7-X (W7-X) stellarator. It is a 3-channel system that records X-ray spectra in the range from 0.6 to 19.6 keV. X-ray spectra, with a temporal and spatial resolution of 100 ms and 2.5 cm (depending on selected slit sizes), respectively, are line integrated along a line-of-sight that crosses near to the plasma center. In the second W7-X operation phase with a carbon test divertor unit, light impurities, e.g., carbon and oxygen, were observed as well as mid- to high-Z elements, e.g., sulfur, chlorine, chromium, manganese, iron, and nickel. In addition, X-ray lines from several tracer elements have been observed after the laser blow-off injection of different impurities, e.g., silicon, titanium, and iron, and during discharges with prefill or a gas puff of neon or argon. These measurements were achieved by optimizing light absorber-foil selection, which defines the detected energy range, and remotely controlled pinhole size, which defines photon flux. The identification of X-ray lines was confirmed by other spectroscopic diagnostics, e.g., by the High-Efficiency XUV Overview Spectrometer, HEXOS, and high-resolution X-ray imaging spectrometer, HR-XIS.

Experimental demonstration of an electromagnetic pulse mitigation concept for a laser driven proton source

The targets that are used to produce high-energy protons with ultra-high intensity lasers generate a strong electromagnetic pulse (EMP). To mitigate that undesired side effect, we developed and tested a concept called the "birdhouse." It consists in confining the EMP field in a finite volume and in dissipating the trapped electromagnetic energy with an electric resistor. A prototype was tested at a 10 TW 50 fs laser facility. The recorded average EMP mitigation ratio is about 20 for frequencies from 100 MHz to 6 GHz. The EMP mitigation ratio attains the level of 50 in the frequency range of 1-2 GHz where microwave emission is maximal. We measured the intensity of proton emission in two directions: along the laser propagation direction and along the edge of the proton beam. We observed that the "birdhouse" induces a two-fold increase of the intensity in the center of the proton beam and a two-fold reduction of the intensity on its edge. We did not observe any modification of the proton beam normalized spectrum.

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited)

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

Resonant third harmonic generation of KrF laser in Ar gas

Investigations of emission of harmonics from argon gas jet irradiated by 700 fs, 5 mJ pulses from a KrF laser are presented. Harmonics conversion was optimized by varying the experimental geometry and the nozzle size. For the collection of the harmonic radiation silicon and solar-blind diamond semiconductor detectors equipped with charge preamplifiers were applied. The possibility of using a single-crystal CVD diamond detector for separate measurement of the 3rd harmonic in the presence of a strong pumping radiation was explored. Our experiments show that the earlier suggested 0.7% conversion efficiency can really be obtained, but only in the case when phase matching is optimized with an elongated gas target length corresponding to the length of coherence.

Shot-to-shot reproducibility in the emission of fast highly charged metal ions from a laser ion source

The generation of fast highly charged metal ions with the use of the sub-nanosecond Prague Asterix Laser System, operated at a fundamental wavelength of 1315 nm, is reported. Particular attention is paid to shot-to-shot reproducibility in the ion emission. Au and Pd targets were exposed to intensities up to 5 × 10(16) W∕cm(2). Above the laser intensity threshold of ∼3 × 10(14) W∕cm(2) the plasma is generated in a form of irregular bursts. The maximum energy of protons constituting the leading edge of the fastest burst reaches a value up to 1 MeV. The fast ions in the following bursts have energy gradually decreasing with the increasing burst number, namely, from a value of about 0.5 MeV∕charge regardless of the atomic number and mass of the ionized species.

Laser generation of Au ions with charge states above 50+

Results of recent studies on highly charged Au ion generation, using the intense long pulses of the PALS high power iodine laser (lambda=1.315 microm, E(L)=800 J400 ps), operating under variable experimental conditions (1omega, 3omega, varying target thickness and changing focus positions), are presented. Both the ion collectors and the ion electrostatic analyzers were applied for the identification of ions in a large distance from the target. The time-of-flight collector signals were treated by a means of peak deconvolution assuming a shifted Maxwell-Boltzmann form of the constituent ion current peaks. Attention was paid to the influence of pulse precursor, which becomes evident, especially, if using thinner targets and 1omega. The results for 3omega point to the presence of several groups of ions with the highest recorded charge state Au(53+).

Semiconductor and thermoluminescent dosimetry of pulsed soft X ray plasma sources

A multichannel detection system having a dynamic range of approximately 1 x 10(-9) Gy --20 Gy was developed with the use of commercially produced Si-photodiodes and TLDs for accurate measurement of X ray energy emitted from plasma-focus facility and from laser-produced plasmas. The proof of linearity of the employed detectors accomplished by a comparison of their responses to a broad band spectrum of X rays emitted from plasmas, is reported. It is demonstrated that TLDs irradiated with no protective filter show an incorrect response due to overloading in the sub-keV range and repopulation of dosimetric peaks induced by the UV radiation. The measurement of the power of undesirable secondary X ray sources driven by the primary plasma inside the interaction chamber was performed on the basis of analysis of space dependence of X ray intensity with respect to the assumed r(-2) decrease in the intensity far away from the plasma.

Fast proton generation from ultrashort laser pulse interaction with double-layer foil targets

The results of studies of fast-proton generation from foil targets irradiated by 1-ps laser pulse at 10(17) W/cm (2) are presented. It is shown that a considerable increase in proton energy and current is possible when a double-layer foil target containing a high- Z layer and a low- Z hydrogen-rich layer is used instead of a single-layer target. Proton energies and current increase with the Z of the high- Z layer and depend essentially on the target and the layer thicknesses. Above 10(9) forward-emitted protons of energy >100 keV have been recorded within a cone angle <3 degrees.