Intense Soft X-ray Research Articles

Modeling X‐Ray Loops and EUV “Moss” in an Active Region Core

The soft X-ray intensity of loops in active region cores and the corresponding footpoint, or moss, intensity observed in the EUV remain steady for several hours of observation. The steadiness of the emission has prompted many to suggest that the heating in these loops must also be steady, although no direct comparison between the observed X-ray and EUV intensities and the steady heating solutions of the hydrodynamic equations has yet been made. In this paper we perform these simulations and simultaneously model the X-ray and EUV moss intensities in one active region core with steady uniform heating. To perform this task, we introduce a new technique to constrain the model parameters using the measured EUV footpoint intensity to infer a heating rate. Using an ensemble of loop structures derived from magnetic field extrapolation of photospheric field, we associate each field line with an EUV moss intensity, then determine the steady uniform heating rate on that field line that reproduces the observed EUV intensity within 5% for a specific cross-sectional area, or filling factor. We then calculate the total X-ray filter intensities from all loops in the ensemble and compare this to the observed X-ray intensities. We complete this task iteratively to determine the filling factor that returns the best match to the observed X-ray intensities. We find that a filling factor of 8% and loops that expand with height provides the best agreement with the intensity in two X-ray filters, although the simulated SXT Al12 intensity is 147% the observed intensity and the SXT AlMg intensity is 80% the observed intensity. From this solution we determine the required heating rate scales as 0.29L−0.95. Finally, we discuss the future potential of this type of modeling, such as the ability to use density measurements to fully constrain filling factor, and its shortcomings, such as the requirement to use potential field extrapolations to approximate the coronal field.

Development and applications of multimillijoule soft X-ray lasers

We review development of multimillijoule X-ray lasers and of applications of these new laboratory sources carried out recently at the PALS facility. A backbone of this development is the neon-like zinc laser providing saturated output at 21.2 nm, with up to 10 mJ of energy per pulse. This represents currently the most energetic soft X-ray laboratory source. Recent improvements in its operation include better control of the beam shape, and more complete understanding of the prepulse pumping. The laser at 21.2 nm has been employed for a number of application experiments reviewed in this paper. They include transmission measurements of intense soft X-ray radiation, studies of fundamental processes of soft X-ray ablation, ablation micropatterning, feasibility study of soft X-ray Thomson scattering from dense plasmas, visualization of nanometric transient perturbation of optical surfaces, measurements of ablation rates of foils heated by IR pulses, and studies of 2D plasma hydrodynamics in the regime of sequential illumination.

X-ray photochemistry in iron complexes from Fe(0) to Fe(IV) – Can a bug become a feature?

Under intense soft X-ray irradiation, we have observed time-dependent changes in the soft X-ray spectra of virtually all the Fe coordination complexes that we have examined, indicating chemical transformation of the compound under study. Each compound, with oxidation states ranging from Fe(IV) to Fe(0), has been studied with either Fe L-edge spectroscopy or N K-edge spectroscopy. We find that very often a well-defined spectroscopic change occurs, at least initially, which is apparently capable of straightforward interpretation in terms of X-ray induced photoreduction, photooxidation or ligand photolysis. We briefly discuss the probable chemical nature of the changes and then estimate the rate of chemical change, thereby establishing the necessary radiation dose. We also demonstrate that the photochemistry not only depends on the Fe oxidation state but also the coordination chemistry of the complex. It seems that a proper understanding of such X-ray photochemical effects could well greatly assist the assignment of soft X-ray spectra of uncharacterized metal sites.

Characterization of soft X-ray undulator for the Siam Photon Source

An undulator for production of intense soft X-rays has been constructed for the Siam Photon Source (SPS). The device, named U60, is being characterized and prepared for installation. It is a pure permanent magnet planar undulator, consisting of 41 magnetic periods, with 60 mm period length. Utilization of the undulator radiation in the photon energy range of 30–900 eV is planned. Magnetic field mapping of the undulator has been carried out using a Hall probe. The measured magnetic fields are used to estimate field errors and perturbations of the undulator on the electron beam dynamics in the storage ring. Compensation schemes for the perturbation are investigated.

Method for experimental determination of Z dependence of impurity transport on JET

The prediction of impurity peaking in future fusion devices such as ITER necessitates the study of the dependence on Z of the impurity transport in present devices. In this paper we describe a novel technique to determine the transport of impurities with different atomic numbers independently. A technique has been developed that allows simultaneously the measurement of the transport of Ne and Ar in the same discharge while minimizing the systematic errors in the spectroscopic measurements. The reproduction of the charge-exchange measured densities, absolute vaccum ultra-violet line intensities and absolute soft x-ray intensity is achieved in an impurity transport simulation. The method used to estimate the errors on the transport coefficients of neon (Ne) and argon (Ar) is presented. In the plasma region where the diffusion and convection coefficients are determined for hybrid discharges, the transport of Ne and Ar is observed to exceed neoclassical predictions. In the same regions, the diffusion coefficients of both impurities are similar. The convection coefficients are also comparable for Ne and Ar. The peaking of Ne and Ar density profiles are comparable during the period where the intermittent slow reconnecting n = 1 mode is stable in these hybrid discharges.

Micromachining of Inorganic Materials using Laser Plasma Soft X-Rays

We have investigated nanomachining of inorganic materials using laser plasma soft X-rays. The soft X-ray was generated by irradiating Ta targets with pulsed Nd:YAG laser light. The laser plasma soft X-rays were focused using an ellipsoidal mirror, which is designed so as to focus soft X-rays at around 10 nm efficiently. The focused soft X-rays were incident to the surfaces of inorganic materials such as silica glass, LiF, CaF2 and LiNbO3. It is found that these materials are ablated by soft X-ray irradiation. In particular, silica glass can be ablated at rates of 0.2-150 nm/shot, which can be controlled by the intensity of soft X-rays. It is remarkable that silica can be ablated smoothly with a roughness of 1 nm after 10 shots of soft X-ray irradiation at a rate of 50 nm/shot. In order to demonstrate nanomachining of silica glass, we used contact masks on top of silica glass plates fabricated by electron beam lithography technique. The silica glass plates were irradiated with laser plasma soft X-rays through the windows of the contact masks. We found that nanofabrication of trenches with a width of 70 nm are performed clearly.

Dawn of nonlinear optics in the soft X-ray region

This article explores the generation of intense soft X-ray pulses and its application to nonlinear multiphoton processes. Using such nonlinear processes, the temporal width of the 42 eV soft X-ray pulse was measured directly by an autocorrelation technique. A train of attosecond frequency-pulses was also characterized by frequency-resolved autocorrelation. Intense high harmonics produced by the phase-matching technique enable the observation of these nonlinear optical processes

Resonant diffusive radiation in random multilayered systems

We have theoretically shown that the yield of diffuse radiation generated by relativistic electrons passing random multilayered systems can be increased when a resonant condition is met. Resonant condition can be satisfied for the wavelength region representing visible light as well as soft X-rays. The intensity of diffusive soft X-rays for specific multilayered systems consisting of two components is compared with the intensity of Cherenkov radiation. For radiation at photon energy of 99.4eV, the intensity of resonant diffusive radiation (RDR) generated by 5MeV electrons passing a Be/Si multilayer exceeds the intensity of Cherenkov radiation by a factor of ≈60 for electrons with the same energy passing a Si foil. For a photon energy of 453eV and 13MeV electrons passing Be/Ti multilayer generate RDR exceeding Cherenkov radiation generated by electrons passing a Ti foils by a factor ≈130.

Dissociative ATI of H2 and D2 in intense soft x-ray laser fields

The dissociative photoionization processes of H2 and D2 in an intense, short-pulsed soft x-ray field (λ = 29.6 nm, 3 × 1012 W cm−2, 15 fs) generated as the 27th high-order harmonic of a Ti:sapphire laser is investigated for the first time by time-of-flight mass spectrometry. The ejected H+ fragment ions from H2 are distributed in the kinetic energy range of 3–20 eV peaked at 10.0 eV, and the yield exhibits a nonlinear dependence of the order of 1.7(1) on the light field intensity of the 27th harmonic. Similarly, the D+ fragment ions from D2 are distributed in the 3–20 eV range peaked at 10.3 eV, and the yield exhibits a nonlinear dependence of the order of 1.6 ± 0.2. The kinetic energy distributions of H+ and D+ and the nonlinearity in their yields are interpreted well by the two coexisting ionization processes: (i) one-photon dissociative ionization and (ii) dissociative two-photon above-threshold-ionization (ATI) mainly into the repulsive 2pπu state.

Small GOES flares with intense hard X-ray emission

Large solar flares with intense soft X-ray emission (i.e., high GOES class) generally tend to show a strong hard X-ray emission. However, there are examples of low GOES class events with unusually strong hard X-ray emission. In this paper, we analyse the morphology and physical parameters of such small GOES intensity flares with strong hard X-ray emission, using Yohkoh SXT images and photometric data obtained from INTERBALL-TAIL RF15-I X-ray Photometer. We observe a great variety in the soft X-ray morphology of such flares (a large diversity of loop configurations). Some of these flares do not differ greatly in their morphology from large intense flares, but most flares are generally compact. In spite of their low intensities in soft X-rays, the significant hard X-ray emission is observed by INTERBALL up to 30–60 keV. We briefly discuss some of the possible causes of the soft and hard X-ray emission ratio of these events.

On the Origins of Solar EIT Waves

Abstract : Approximately half of the large-scale coronal waves identified in images obtained by the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory from 1997 March to 1998 June were associated with small solar flares with soft X-ray intensities below C class. The probability of a given flare of this intensity having an associated EIT wave is low. For example, of ~8,000 B-class flares occurring during this 15 month period, only 1% were linked to EIT waves. These results indicate the need for a special condition that distinguishes flares with EIT waves from the vast majority of flares that lack wave association. Various lines of evidence, including the fact that EIT waves have recently been shown to be highly associated with coronal mass ejections (CMEs), suggest that this special condition is a CME. A CME is not a sufficient condition for a detectable EIT wave, however, because we calculate that 5 times as many front-side CMEs as EIT waves occurred during this period, after taking the various visibility factors for both phenomena into account. In general, EIT wave association increases with CME speed and width.

Above-threshold double ionization of helium with attosecond intense soft x-ray pulses

We theoretically study a process in which helium is doubly ionized by absorbing two soft x-ray $(91.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ photons from attosecond, intense high-order harmonic sources. We directly solve the time-dependent Schr\"odinger equation and obtain electron energy distribution in the double continuum. Our results show that between the two peaks in electron energy spectra, expected for usual sequential ionization, an additional component (anomalous component) is present. The total two photon above-threshold double ionization yield including the anomalous component is explained by sequential processes: the two electrons are ejected one by one, absorbing a single photon each. This observation rejects the intuition that nonsequential double ionization would be responsible for the anomalous component. With the help of simple semiclassical stochastic models, we discuss two possible origins of the anomalous component, namely, postionization energy exchange and second ionization during core relaxation, of which the latter is more plausible: the ionization interval is so short that the second electron is ejected while the two electrons are still exchanging energy.

Efficient absorption and intense soft X-ray emission from gas cluster plasmas irradiated by 25-ps laser pulses

A highly efficient absorption is observed in multipicosecond laser pulse irradiation of argon and nitrogen gas cluster targets at moderate intensities of /spl sim/10/sup 15/ W/cm/sup 2/ using a 1.5 J/25 ps Nd : phosphate glass laser (/spl lambda/=1.054 /spl mu/m). Measurements of laser light absorption, side scattering, and extreme-ultraviolet (XUV) emission spectrum are performed for argon and nitrogen gases puffed through sonic or hypersonic nozzles in the pressure range from 10 to 60 bar. High values of 80%-85% of laser light absorption have been measured in both nitrogen and argon gas cluster plasmas. The X-ray line emission from argon plasma was /spl sim/ 30-40 times stronger than that from the nitrogen plasma, which is related to the formation of larger sized clusters in argon. The very high laser light absorption observed in these multipicosecond laser pulse irradiation experiments may be understood to be occurring due to intense heating of the near solid density clusters in the initial part of the laser pulse creating a plasma of a high degree of ionization, which continues to absorb the rest of the laser pulse energy through collisional processes during its expansion to subcritical densities.

Behaviors of Electron Heat Transportation in HT-7 Sawtoothing Plasma

It is found that in HT-7 ohmic plasma, main energy loss comes from electron heat conduction, hence quantitative data of electron heat diffusivity is a very important issue for investigation of electron heat transportation behavior in different target plasmas so as to get high performance plasma. A time-to-peak method of the heat pulse propagation originating from the sawtooth activity on the soft x-ray intensity signal has been adopted to experimentally determine electron heat diffusivity χeHP on the HT-7 tokamak. Aiming to improve the signal-to-noise (S/N) ratio of the original signal to get a stable and reasonable electron heat diffusivity χeHD value, some data processing methods, including average of tens of sawteeth, is discussed. The electron heat diffusivity χeHP is larger than χePB which is determined from the balance of background plasma power. Based on variation of the measured electron heat diffusivity χeHP, performances of different high confinement plasmas are analyzed.

Comparison of the absolute soft X-ray intensity between a cryogenic radiometer and an ion chamber

Absolute intensities of monochromatized soft X-rays from the synchrotron radiation source were measured sequentially using both a cryogenic radiometer and a multi-electrode ion chamber at a photon energy of 150.4 eV. The absolute intensities obtained by these methods were mutually compared to check reliability for the different measuring methods and agreed with each other within the uncertainty of about 10%.

A Statistical Study of the Correlation between Magnetic Helicity Injection and Soft X‐Ray Activity in Solar Active Regions

The correlation between magnetic helicity injection across the photosphere and soft X-ray activity in the solar corona is statistically investigated for seven active regions appearing in the years 1997-2000. The magnetic helicity flux into the solar corona is analyzed by the induction equation method, using magnetograms observed by the Michelson Doppler Imager on SOHO and by the vector magnetograph at the National Astronomical Observatory of Japan. Soft X-ray activity is evaluated from the data observed by the Yohkoh soft X-ray telescope (SXT). Soft X-ray activity of active regions in nonflare phases is found to correlate better with unsigned magnetic helicity flux than with the simple integration of the magnetic helicity flow. In addition, several magnetic variables, e.g., magnetic flux and electric current flux, are investigated, and it is confirmed that any fluxes given by the area integration of magnetic variables are well correlated with soft X-ray activity. However, for the magnetic helicity flow, not only the whole area flux but also the local intensity correlates well with the soft X-ray intensity. The relation between the spatial structure of the magnetic shear and soft X-ray activity is also investigated, and it is revealed that structural complexity in the magnetic shear tends to increase the efficiency of energy liberation in the solar corona. These results indicate that the magnetic helicity injected from the photosphere is relevant to the heating process in the solar corona, although several magnetic variables, not only magnetic helicity, could be related to that.

Intense femtosecond soft X-ray pulse; generation and application

Intense femtosecond soft X-ray pulse; generation and application

A Quantitative Study of the Homologous Flares on 2000 November 24

We present an examination of multiwavelength observations of three X-class homologous flares that occurred on 2000 November 24. By investigating the behavior of the two-ribbon flares in Hα and the ultraviolet (1600 A), we found that the temporal variation of the distance between the two ribbons shows a good correlation with the soft X-ray light curve. From this finding we can derive the relation dISXR(t)/dt ∝ Vrib(t), where ISXR(t) is the soft X-ray intensity and Vrib(t) is the separation velocity of the two ribbons. This relation is similar to the well-known empirical law, the Neupert effect. We also measured the rise time, velocity of the plasmoid/filament ejection, and separation velocity of the two ribbons for each of these homologous flares. Since the magnetic reconnection model predicts that each of these physical parameters has a dependence on the coronal magnetic field strength Bc, we derived the relative Bc between the three flares from each of the parameters. We compared the relative Bc values, which are derived from those parameters, and found that they are roughly equal. Our results successfully support the magnetic reconnection model. Moreover, the relative hard X-ray maximum intensity in each flare is consistent with the relative Bc derived above, if we assume that the hard X-ray intensity is proportional to the energy release rate as implied by the Neupert effect.

Ultrasoft x-ray emission from a linear ECR plasma source

Ultrasoft x-ray emission in the energy range 10–100 eV is observed from a linear electron cyclotron resonance (ECR) plasma system. ECR plasma is produced in a short cylindrical stainless steel (SS) chamber with a central axial magnetic field of 875 G, generated by two coils placed axially to have the first two ECR surfaces inside the experimental chamber. Plasma is produced in a pressure range of 2 × 10−5 mbar with hydrogen. Also, plasma is produced with net microwave power varying from 160 to 800 W at 2.45 GHz frequency from a magnetron. The waveguide from the magnetron to the vacuum vessel supports rectangular TE10 mode and the electromagnetic wave propagation in plasma is in extraordinary (X) mode. The ultrasoft x-ray emission is observed with a vacuum photodiode. The results clearly indicate a decrease in x-ray emission on increasing the working pressure and an increase in emission with an increase in the input microwave power. The experiments also ensure that the presence of a metal target enhances the intensity of soft x-rays generated in the ECR plasma system.

Application of CVD diamonds as dosimeters of soft X-ray emission from plasma sources

The thermoluminescent properties of polycrystalline chemical vapour deposition (CVD) diamond, as free-standing CVD cutting tool material, type CVDITE-CDM (De Beers Company), were studied with respect to its use in the dosimetry of soft X-ray emission from laser-produced plasma. The range of linearity for 5.9-keV radiation was measured to be only two orders of magnitude, ranging from a sensitivity threshold of ∼0.01 to ∼2 Gy. In this linearity range, the sensitivity of CVD diamonds is about 65 times lower than the sensitivity of TLD-100 dosimeters. The unpolished (grained) face of CVD diamonds shows ∼1.5-times higher thermoluminescence (TL) response after irradiation than the polished face, in the high-temperature range, but the polished face shows slightly higher TL response in the low-temperature range. A strong TL sensitivity to the blue portion of the visible light spectrum was measured. Simultaneous irradiation of TLD-100 dosimeters and CVD diamonds by soft X-rays emitted from a laser-produced plasma showed that CVDITE-CDM diamonds can be applied as detectors of intense soft X-ray radiation.