Solid Surface Plasma Research Articles

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part A: concepts and questions

It is estimated that pilot plants and reactors may experience rates of net erosion and deposition of solid plasma facing component (PFC) material of 103–105 kg yr−1. Even if the net erosion (wear) problem can be solved, the redeposition of so much material has the potential for major interference with operation, including disruptions due to so-called ‘unidentified flying objects (UFOs)’ and unsafe dust levels. The potential implications appear to be no less serious than for plasma contact with the divertor target: a dust explosion or a major UFO-disruption could be as damaging for an actively-cooled deuterium-tritium (DT) tokamak as target failure. It will therefore be necessary to manage material deposits to prevent their fouling operation. This situation appears to require a fundamental paradigm shift with regard to meeting the challenge of taming the plasma–material interface: it appears that any acceptable solid PFC material will in effect be flow-through, like liquid–metal PFCs, although at far lower mass flow rates. Solid PFC material will have to be treated as a consumable, like brake pads in cars. ITER will use high-Z (tungsten) armor on the divertor targets and low-Z (beryllium) on the main walls. The ARIES-AT reactor design calls for a similar arrangement, but with SiC cladding on the main walls. Non-metallic low-Z refractory materials such as ceramics (graphite, SiC, etc) used as in situ replenishable, relatively thin—of order mm—claddings on a substrate which is resistant to neutron damage could provide a potential solution for the main walls, while reducing the risk of degrading the confined plasma. Separately, wall conditioning has proven essential for achieving high performance. For DT devices, however, standard methods appear to be unworkable, but recently powder droppers injecting low-Z material ∼continuously into discharges have been quite effective and may be usable in DT devices as well. The resulting massive generation of low-Z debris, however, has the same potential to seriously disrupt operation as noted above. Powder droppers provide a unique opportunity to carry out controlled studies on the management of low-Z slag in all current tokamaks, independent of whether their protection tiles use low-Z or high-Z material.

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part B: required research in present tokamaks

The companion part A paper (Stangeby et al 2022) reports a number of independent estimates indicating that high-duty-cycle DT tokamaks starting with pilot plants will likely experience rates of net erosion and deposition of solid PFC, plasma facing component, material in the range of 103 to 104 kg yr−1, regardless of the material used. The subsequent redeposition of such large quantities of material has the potential for major interference with tokamak operation. Similar levels and issues will be involved if ∼continuous low-Z powder dropping is used for surface conditioning of DT tokamaks, independent of the material used for the PFC armor. In Stangeby et al (2022) (part A) it is proposed that for high-duty-cycle DT tokamaks, non-metallic low-Z refractory materials such as ceramics (graphite, SiC, etc) used as in situ replenishable, relatively thin—of order mm—claddings on a substrate which is resistant to neutron damage could provide a potential solution for protecting the main walls, while reducing the risk of degrading the confined plasma. Assessment of whether such an approach is viable will require information, much of which is not available today. Section 6 of part A identifies a partial list of major physics questions that will need to be answered in order to make an informed assessment. This part B report describes R&D needed to be done in present tokamaks in order to answer many of these questions. Most of the required R&D is to establish better understanding of low-Z slag generation and to identify means to safely manage it. Powder droppers provide a unique opportunity to carry out controlled studies on the management of low-Z slag in current tokamaks, independent of whether their protection tiles use low-Z or high-Z material.

High-order harmonic generation from a solid-surface plasma by relativistic-intensity sub-100-fs mid-infrared pulses.

High-order harmonic generation (HHG) in plasmas induced by ultrashort, relativistic-intensity laser pulses on solid surfaces can provide an efficient source of attosecond pulses and opens routes toward new regimes of laser-matter interactions, x-ray generation, laser particle acceleration, and relativistic nonlinear optics. However, field intensities in the range of Irel∼1019 W/cm2 are typically needed to achieve the relativistic regime of HHG in experiments with near-infrared laser pulses. Here, we show that, in the mid-infrared range, due to the λ-2 scaling of Irel with the driver wavelength λ, relativistic HHG can be observed at much lower levels of laser field intensities. High-peak-power 80-fs, 3.9-μm pulses are focused in our experiments on a solid surface to provide field intensities in the range of 1017 W/cm2. Remarkably, this level of field intensities, considered as low by the standards of relativistic optics in the near infrared, is shown to be sufficient for generation of high-order harmonics with signature properties of relativistic HHG-beam directionality, spectra with extended plateaus, and a high HHG yield sustained for both p- and s-polarized driver fields.

Polarization peculiarities of femtosecond laser induced harmonic generation from solid surface plasma

Polarization characteristics of harmonic generation from solid surface plasma were studied using a femtosecond CPA Ti:S–Nd:glass laser. The ratios between the harmonics intensities produced by p-polarized and s-polarized pump were 25 and 6 for the second and fifth harmonics, respectively, at pump intensities ( I) below 10 17 W cm −2. The same ratios decreased to 2.4 and 1.5 for I>2×10 17 W cm −2. This decrease was explained by the rippling caused by Rayleigh–Taylor-like instability at the critical density surface, and Faraday rotation effect due to the influence of the spontaneous magnetic fields generated in plasma. Theoretical analysis of s- and p-polarization-induced harmonic generation was presented using LPIC code. The blueshift of the harmonics has been also observed. It was found that the blueshift increases with the harmonic order.

Present Research Status on Divertor and Plasma Facing Components for Fusion Power Plants

ABSTRACTThis paper is a review of the present status of research activities on the divertor and the first wall for fusion power plants in Japan. There are several key issues as follows; 1) plasma facing materials, 2) fabrication techniques, 3) thermal hydraulics, and 4) component design. It can be said that solid surface plasma facing components are the present reference design of fusion power plants in Japan. Based on plasma conditions, it turns out that sputtering erosion of plasma facing materials could not be negligible, and some armor would be necessary. From this view point, tungsten is one of the most promising materials. This paper reviews the present engineering status on tungsten as a plasma facing material for fusion power plants.

Measurements of blue shifts due to collisionless absorption in harmonic generation from subpicosecond laser-produced plasmas.

Harmonic generation from solid surface plasmas is studied using a subpicosecond Nd:glass laser system. For a 45 degrees angle of incidence, the speculars up to the fifth harmonics are blue shifted when the laser intensity exceeds 2 x 10(16) W cm(-2). The second harmonic is blueshifted by approximately 16 A, and the fifth harmonic is blue shifted by approximately 51 A for p polarization at the intensity of 1 x 10(17) W cm(-2). We observed the blue shift of the fifth harmonics and found that the magnitude of blue shift is higher compared with that for the second harmonics. The blue shift is interpreted as a collisionless absorption due to the anomalous skin effect. It is also found that the divergence of harmonics preserves a smaller divergence when using a shorter pulse length for the driving laser.

Enhancement of high-order harmonic generation efficiency by double pulse lasers

We have investigated the efficiency of the fifth order harmonics from solid surface plasma by varying the electron density gradient using two 2.2 ps, 1054 nm laser pulses. A prepulse irradiates the Al deposited target 0-20 ps prior to the arrival of the main Nd: Glass laser pulse in order to modify the electron density gradient. It is found that 5 ps time difference between the prepulse and main pulse results in an increase in the third harmonic signal by a factor of 2-3, which then decreases for time delays greater than 10 ps, and the fourth and fifth harmonic signal also increase. Experiments and calculations are presented that demonstrate a strong dependence of the conversion efficiency on the plasma scale length.

Enhancement of high-order harmonic generation efficiency from solid-surface plasma by controlling the electron density gradient of picosecond laser-produced plasmas

We have investigated the efficiency of generating the fifth-order harmonic from plasma at the surface of a solid by varying the electron density gradient of the picosecond laser-produced plasmas. In order to modify the plasma scale length, we carried out experiments in which a 2.2 ps prepulse irradiates the solid target 0-20 ps prior to the arrival of the main Nd:glass laser pulse with the same pulse duration. The chirped pulse amplification Nd:glass laser system delivered pulses up to 100 mJ energy at 1054 nm wavelength. Although third-harmonic radiation can also be observed when a single beam is used, a /spl sim/5 ps time delay results in an increase in the harmonic signal by a factor of 2-3, which then decreases after 10 ps. We also have carried out more detailed model-calculations using the simulations. As a result, it is found that it is possible to generate high harmonics efficiency by controlling the density gradient using two laser pulses.

Dynamical Behavior of Higher Harmonic Generation from a Picosecond-Laser-Produced Solid Surface Plasma

We have studied experimentally the efficiency of the harmonics from solid surface plasma by varying the electron density scale length produced during the interaction of a 2.2 ps, 100 mJ, 1054 nm, p-polarized laser pulse with deposited Al targets. Although third harmonic radiation can also be observed when a single beam is used, an approximately 5 ps time difference between the prepulse and the main pulse results in an increase in the harmonic signal by a factor of 2–3, which then decreases after 10 ps. The results for the 4th- and 5th-order harmonic generation are also similar. We have also calculated the electron density scale length by using the one-dimensional (1D) hydrodynamic code HYADES. The calculation results are in good agreement with our experimental results. It is found that it is possible to generate high efficiency harmonics by controlling the density gradient using a double laser pulse.

The spatial distribution of high-order harmonics from solid surface plasmas

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a 2.2-ps 1054-nm laser pulse at maximum intensities of /spl sim/2/spl times/10/sup 17/ W/spl middot/cm/sup -2/. It is found that the divergence of the harmonics gradually increases at intensities between I/spl lambda//sup 2/=2/spl times/10/sup 15/ and 2/spl times/10/sup 16/ W/spl middot/cm/sup -2//spl middot//spl mu/m/sup 2/. We experimentally observe that the divergence of the harmonics decreases as the order of the harmonic increases. We also found that the divergence of the third harmonics emission is smaller for 2.2-ps pulsewidth for the driving lasers as compared with those for 100-ps pulsewidth lasers.

High-order harmonic generation from a solid surface plasma by using a picosecond laser

Investigations have been performed on harmonic generation from solid-vacuum interfaces using a prepulse-free 2.2-ps 1054-nm laser pulse at maximum intensities of /spl sim/1/spl times/10/sup 15/ W/spl middot/cm/sup -2/. The harmonic intensities were strongest for p-polarized pump and dropped to about 50% in the case of s-polarized pumps. The ratio I/sub 4w/I/sub 2w/ between the intensities of the fourth and second harmonics was 0.25 at I/spl lambda//sup 2/=6/spl times/10/sup 14/ W/spl middot/cm/sup -2//spl middot//spl mu/m/sup 2/, which is relatively large when compared with the value predicted by simulations. The harmonics are collimated in the specular direction, with a transverse mode and divergence comparable to that of the driving laser.