Search Results (11 total)

The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.

In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.

The spatial brightness profiles of emission lines for the K-like through He-like ionization states of Fe, Ge, and Ni have been measured during a set of experiments in which Fe and Ge were introduced into FTU tokamak plasmas by using the laser blowoff technique. Nickel was an intrinsic impurity observed during these experiments that was sputtered from the inconel limiter. The brightness profiles were measured by spatially scanable, photometrically calibrated vaccum ultraviolet and x-ray spectrometers that covered the 1 to 1700 Å region. Simulations of these profiles and the time evolution of the laser blowoffs were performed with the MIST transport code using several sets of atomic physics compilations [ADPAK (originally in MIST), Arnaud and Raymond (AR92), Arnaud and Rothenflug (AR85), Mazzotta et al., and Mattioli (an extension to Mazzotta)]. The goal was to determine which set of available rates could best simulate the measured spatial brightness profiles and the charge state balance in the plasma. The Mazzotta et al. (for Fe and Ni), the Mattioli (for Ge), and the AR92 (for Fe only) rates adequately simulated the He-, Li-, Be-, Na-, Mg-like ionization states. The F- to B-like charge states could not be simulated by these compilations unless the relevant dielectronic rates were multiplied by a factor of 2. The ADPAK rates could not adequately predict any of the charge states of Fe, Ge, or Ni.

This work presents and interprets, by means of detailed atomic calculations, observations of L-shell (n=3→n=2) transitions in highly ionized molybdenum, the main intrinsic heavy impurity in the Frascati tokamak upgrade plasmas. These hot plasmas were obtained by additional electron cyclotron resonance heating (ECRH), at the frequency of 140 Ghz, during the current ramp-up phase of the discharge. Injecting 400 kW on axis and 800 kW slightly off axis, the peak central electron temperature reached 8.0 and 7.0 keV, respectively, for a time much longer than the ionization equilibrium time of the molybdenum ions. X-ray emissions from rarely observed high charge states, Mo³⁰⁺ to Mo³⁹⁺, have been studied with moderate spectral resolution (λ/Δλ∼150) and a time resolution of 5 ms. A sophisticated collisional-radiative model for the study of molybdenum ions in plasmas with electron temperature in the range 4–20 keV is presented. The sensitivity of the x-ray emission to the temperature and to impurity transport processes is discussed. This model has been then used to investigate two different plasma scenarios. In the first regime the ECRH heating occurs on axis during the current ramp up phase, when the magnetic shear is evolving from negative to zero up to the half radius. The spectrum is well reproduced with the molybdenum ions in coronal equilibrium and with a central impurity peaking. In the second regime, at the beginning of the current flat top when magnetic shear is monotonic and sawtoothing activity is appearing, the lowest charge states (Mo³³⁺ to Mo³⁰⁺), populated off axis, are affected by anomalous transport and the total molybdenum profile is found to be flat up to the half radius. We conclude with the presentation of “synthetic spectra” computed for even higher temperature plasmas that are expected in future experiments with higher ECRH power input.

Experimental neon spectra (in the 10-nm region), from the tokamak Tore Supra and the reversed field pinch experiment RFX, have been simulated. The spectra include lines from three neon ionization states, namely Ne⁷⁺, Ne⁶⁺, and Ne⁵⁺ ions. Collisional radiative models have been built for these three Ne ions, considering electron collisional excitation and radiative decay as populating processes of the excited states. These models give photon emission coefficients for the emitted lines at electron density and temperature values corresponding to the experimental situations. Impurity modelling is performed using a one-dimensional impurity transport code, calculating the steady-state radial distribution of the Ne ions. The Ne line brightnesses are evaluated in a post-process subroutine and simulated spectra are obtained. The parts of the spectra corresponding to a single ionization state do not depend on the experimental conditions and show good agreement with the simulated single ionization state spectra. On the other hand, the superposition of the three spectra depends on the experimental conditions, as a consequence of the fact that the ion charge distribution depends not only on the radial profiles of the electron density and temperature, but also of the impurity transport coefficients. Simulations of the Ne spectra (including transport) give confidence in the atomic physics calculations; moreover, they allow the determination of the transport coefficients in the plasma region emitting the considered ionization states.

In a multiple-keV-temperature deuterium plasma such as that in the Joint European Torus (JET), C vi emission arises from excitation processes that, in the absence of neutral-hydrogen-beam heating, occur near the plasma boundary. Distortion to the C vi Lyman decrement, such as the Ly-β:Ly-γ:Ly-δ ratio, and enhancement of high series members (around 1s-12p) are observed. In this situation, large influxes of D⁰ atoms are anticipated at the plasma boundary when the plasma is limited by the carbon inner wall. The intensities of the Lyman-series members are interpreted in terms of electronic excitation of C⁵⁺ ions in their ground state as well as charge-exchange collisions between C⁶⁺ ions and thermal D⁰ atoms, either in their ground state or in their excited states (up to the collision limit). The line-of-sight emission is modeled using an impurity-ion transport code requiring knowledge of the electron temperature, electron density, and D⁰ radial profiles. Comparison of the observed relative intensities of the C vi Lyman series Ly-β, Ly-γ, Ly-δ, etc., with the model calculations allows plausible corrections to be made to the effective charge-exchange rate coefficients as a function of the principal quantum number. The observations offer a potential method of measuring local D⁰ concentrations near the boundary in high-temperature, high-D⁰-concentration recycling plasmas.

The total cross section for deuteron photodisintegration has been measured in the γ-ray energy range between 15 and 75 MeV, by use of the monochromatic LADON photon beam of the Frascati National Laboratories and detection of the proton. The results are in substantial agreement with the standard theory and do not provide evidence for contributions of quark degrees of freedom.

The reaction ²(γ,p)n has been studied using a monochromatic and polarized gamma ray beam at energies E_γ=19.8, 29.0, 38.6, and 60.8 MeV. The beam of an intensity ∼4×10⁵ γ/sec was obtained by Compton back scattering of mode-locked laser light off electron bunches in the Adone storage ring. Photoneutron yields were measured at nine neutron angles theta_n≃15, 30, 45, 60, 90, 120, 135, 150, and 165 deg in the center of mass (c.m.) for E_γ=19.8, 29.0, and 38.6 MeV, and at theta_n≃30, 60, 90, 120, and 150 deg c.m. for E_γ=60.8 MeV. The polarization independent component I₀(theta) of the differential cross section and the polarization dependent component PI₁(theta) were deduced and the angular distribution of the azimuthal asymmetry factor Σ(theta)=I₁(theta)/I₀(theta) was obtained. An extensive comparison with theory has been carried out and the inclusion of corrections due to meson exchange currents and to Δ-isobar configurations have been shown to be mandatory at energies E_γ≳40 MeV. Theoretical and experimental implications of intermediate energy deuteron photo- disintegration studies are discussed in some detail.

The asymmetries Σ(θ_c.m.=π / 2) for the reaction ²H (γ,n)p have been measured with use of a monochromatic and linearly polarized γ-ray beam, obtained by backward Compton scattering of laser light against high-energy electrons. Contributions from meson exchange currents must be included in the theoretical calculations to reproduce our data.

We report an observation of electric quadrupole transitions in a laboratory plasma. Two intense lines seen at 58.832 and 57.927 Å in the spectrum of the TFR tokamak are attributed to the E2 transitions 3d¹⁰-3d⁹4s(J=2) of Mo xv. This classification is based on the agreement between experimental and computed values for wavelengths and intensities of these lines in the tokamak plasma. The influence of cascades on intensities is discussed.

Ionization and recombination rate coefficients have been estimated using the effect of saw-tooth electron-temperature relaxations in a tokamak plasma on the emission of impurity ions. The ionization rate coefficient of highly ionized (of the order of 30) molybdenum has been found to be between 1 and 1.5 times Lotz's semiempirical coefficient. For the first time, we have also estimated the recombination (mainly dielectronic) rate coefficient for the same ions, and have found it to be between 0.5 and 1 times the value given by Burgess's formula.