Study of density and potential fluctuations in the TEXT tokamakwith a heavy ion beam probe

A heavy ion beam probe was used to study the characteristics of density and potential fluctuations in the TEXT tokamak. Fluctuations of density and space potential are nearly Boltzmann like, n˜/n~φ˜/kT_e, near the edge of the plasma (0.80.9). The turbulent E&oarr;×B&oarr; radial particle flux is sufficient to account for all of the particle loss from the tokamak. No poloidal asymmetries, within a poloidal angle range of about 70°, are observed in the fluctuation levels. The fluctuation spectral shape, the density potential phase angle, and the fluctuation propagation speed show a strong radial dependence     

Rensselaer heavy ion beam probe diagnostic methods and techniques

The operating principles, measurement capabilities, hardware, and data analysis techniques of heavy ion beam probe diagnostics as used by the Rensselaer Plasma Dynamics Lab are reviewed. The topics that are addressed include; trajectory calculations of the ion beams; how the diagnostic measures plasma density, electron temperature, electric potential, and magnetic vector potential; the energy analyzer used to detect the beam, other hardware used in the experiments, and the basic techniques used in fluctuation studies and related diagnostic issues     

An optical probe for local ion temperature measurements in a plasma

Local measurements of the ion temperature and the spectral emissivity have been performed in the magnetized plasma of the PSI‐2 device using an optical probe. The proper operation of the probe could be demonstrated. The probe collects the light emission from a small separated plasma volume thus enabling to determine the ion temperature from Doppler‐broadening of suited spectral lines. In addition to the technical features stray light and finite Larmor radii effects ‐ limiting the spatial resolution ‐ are discussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dynamics of capture and fusion in heavy ion collisions

The effect of entrance channel on decrease of the complete fusion cross sections and on the yield of reaction products are associated with the quasifission which depends on the mass asymmetry and shell structure of colliding nuclei. In reactions of massive projectile and target nuclei, the competition between complete fusion and quasifission appears at the stage of compound nucleus formation, in addition to the increase of the fission probability. It is shown that the yield of quasifission products may be symmetric or asymmetric in dependence on peculiarities of shell structure of reaction fragments. Marima of mass or charge distributions are connected with the peculiarities of shell structure of reaction fragments.     

Light particles emission and incomplete fusion in heavy ion collisions

Light particles emission in heavy-ion collisions is considered in terms of direct-reaction mechanisms. The extreme high-energy part of the inclusive spectrum of light particles is shown to arise essentially from the stripping mechanism while the usually observed maximum of the spectrum is formed by a superposition of several mechanisms. The significance is emphasized of the heavy-fragment stripping in the synthesis of the exotic superheavy nuclei.     

Simultaneous measurements of fluctuations of ion current,electron temperature,and floating potential with a tunnel probe

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. Two-dimensional kinetic analysis of the ion current distribution on the concave conductors is used to calibrate the probe for making fast dc measurements of ion flux and electron temperature. Qualitative agreement with classical Langmuir-probe measurements is found, but the electron temperature given by the tunnel probe is several times lower. This discrepancy is partly caused by secondary electron emission, and can easily be corrected. Strong eduction of ion-flux and electron-temperature fluctuations is observed during electrode biasing.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Radiative lifetime measurements of metastable levels in Kr3+ using electrostatic ion beam trap

The radiative lifetimes of the metastable 4s²4p^{3 2}P_3/2 and ²P_1/2 levels in Kr³⁺ ions were measured using an electrostatic ion beam trap. The lifetimes were determined by measuring the number of photons emitted during the ²P_3/2 → ⁴S_3/2 and ²P_1/2 → ⁴S_3/2 forbidden transitions of the stored Kr³⁺ ions, as a function of the storage time. The obtained lifetime values for the ²P_3/2 and ²P_1/2 states were 23.3 ± 1.4 and 52.8 ± 3.8 ms, respectively. For the ²P_3/2 state, this is the first experimental investigation of the radiative lifetime. Considering the experimental errors, the obtained lifetimes are consistent with the previously reported theoretical predictions for each state and the experimental result for the ²P_1/2 state.     

Multifragmentation and incomplete fusion in central heavy ion collisions within a schematic phenomenological model

The projectile-energy thresholds for nuclear multifragmentation in the collisions of ¹²C, ²⁰Ne and ⁴⁰Ar with ¹⁹⁷Au have been calculated using a schematic model applicable to central collisions of intermediate energy heavy ions. The properties of the composite nucleus which remains following the incomplete fusion of the projectile and target nuclei are calculated using the master equation approach of Harp and Miller to determine the number of preequilibrium nucleons emitted and the energy and linear momentum removal by these nucleons. This nucleus is followed through an isentropic expansion phase and the probability distribution of clusters is determined using a percolation approach based on a 3-dimensional cubic lattice. The input parameters for the percolation calculation are derived from the results of the preequilibrium calculation. While the calculated thresholds in projectile energy per nucleon decrease rapidly as the projectile mass increases, the excitation energy at which multifragmentation is first predicted to occur is approximately the same for each system, i.e., 4.5 to 5 MeV/u. This result is consistent with the apparent disappearance of residual heavy products at such bombarding energies.     

Synthesis and characterization of graphene quantum dots and their size reduction using swift heavy ion beam

Graphene quantum dots (GQDs) are nanosized fragments of graphene displaying quantum confinement effect. They have shown to be prepared from various methods which include ion beam etching of graphene. However, recently the modification of the GQDs has garnered tremendous attention owing to its suitability for various applications. Here, we have studied the effect of swift ion beam irradiation on the properties of GQDs. The ion beam treatment on the GQDs exhibited the change in observed photoluminescence of GQDs as they exhibited a blue luminescence on excitation with longwave UV (≈365?nm) due to the reduction in size and removal of the ethoxy (–C–O–C–) groups present on the quantum dots. This was confirmed by transmission electron microscopy, particle size analysis, and Fourier transform infrared spectroscopy.     

Temperature and density spectroscopic measurements in different laser-generated plasmas

A pulsed Nd:Yag laser, at intensities of the order of 10¹⁰ W/cm², is employed to irradiate different thick metallic targets (Ti, Fe, Ag, and Ni) placed in vacuum. The obtained non-equilibrium plasmas are investigated with various analytical techniques. An electrostatic ion energy analyzer and different ion collectors are employed to monitor in situ the ions ejected from the plasma and to determine the core plasma temperature, the ion energy distributions and the ion angular emission. An optical spectrometer is employed to analyze the plasma corona emitted light vs. wavelength and to identify the emitted characteristic lines. The optical spectroscopy permitted to evaluate the electron temperatures and densities. Results show strong temperature and density gradients occurring in the laser-generated plasma plume.     

Comparison of probe and microwave measurements of electron density in plasmatic cylinder

The paper deals with a comparison of the measurement of the electron density of a plasmatic cylinder in a weak magnetic field (radius of cyclotron rotation of ionsr _Hi is larger than radius of probea) parallel to the axis of the plasma by means of a cylindrical Langmuir probe and a microwave interferometer in the X band. A Langmuir probe is used to measure the electron density distribution across the cross-section of the plasmatic cylinder, i.e. in the plane perpendicular to the axis of the cylinder. The measured density distribution is used to determine the mean electron density over the cylinder cross-section. This mean electron density is compared with the density found with a microwave interferometer. In evaluating the measurements the concentration of the microwave energy through the antennas of the interferometer in the cross-section of the plasmatic cylinder is considered. The difference of the mean electron density found by the microwave method from the mean density found by the probe method was in the least favourable case 30%. In conclusion, the paper gives a comparison of the probe and microwave measurements of the plasma density in a stronger magnetic field, whenr _Hi a.The authors thank J. Teichmann, C. Sc., R. Klíma, C.Sc. and other members of the department for discussions of the results, V. Kopecký for aid in the probe measurements and the director of the institute, Ing. J. Váa, for his interest in the experiments.     

Transient photodecay measurements as a probe of the density of localized states in amorphous semiconductors

We show that structure in the density of localized states of an amorphous semiconductor beyond the apparently ubiquitous exponential band tails yield deviations from the usual power-law decay of the photocurrent, i(t) ∝ t^α?1, which can be described analytically. With our expression, dispersive-transport results can be deconvoluted to provide a spectroscopy of the localized-state distribution when well-defined defect centers and band tails are simultaneously present.     

Scaling behaviour and correlations in the string fusion model for heavy ion collisions

The KNO scaling, the scaling of multiplicities, the behaviour of fluctuations in the transverse momentum as a function of the multiplicity and the long range correlations in nucleus-nucleus collisions are studied by a Monte Carlo code based on the string fusion model. It is shown that the fusion of strings produces a strong reduction of the long range correlations at high multiplicities, which could be detected experimentally. On the contrary the KNO scaling, the scaling of multiplicities and the behaviour of fluctuations in the transverse momentum are not modified when string fusion is included.On leave of absence from the Department of High Energy Physics, University of St. Petersburg, 198904 St. Petersburg, Russia     

Nuclear stopping as a probe for in-medium nucleon-nucleon cross sections in intermediate energy heavy ion collisions

Using an isospin-dependent quantum molecular dynamics, nuclear stopping in intermediate heavy ion collisions has been studied. The calculation has been done for colliding systems with different neutron-proton ratios in beam energy ranging from 15 MeV/ u to 150 MeV/ u. It is found that, in the energy region from above Fermi energy to 150 MeV/ u, nuclear stopping is very sensitive to the isospin dependence of in-medium nucleon-nucleon cross section, but insensitive to symmetry potential. From this investigation, we propose that nuclear stopping can be used as a new probe to extract the information on the isospin dependence of in-medium nucleon-nucleon cross section in intermediate energy heavy ion collisions.     

Detailed comparison between probe density measurements of glow discharge in argon and in helium

In this article we shall look a bit more closely at some of the fundamental plasma parameters obtained by a cylindrical Langmuir probe within low-pressure electrical gas discharge plasma. The presented measurements were made in argon and in helium glow discharge plasmas. We are mainly concerned with the densities of the charged particles (electrons and ions) within the plasma and the effect of the discharge conditions upon them. The electron density is calculated from the electron current at the space potential and from the integration over the EEDF. The ion density is calculated by using the OML collisionless theory. The parameterization of Laframboise's numerical results is also used for the ion density calculation. In the range of our experimental conditions the results of plasma density, for both gases, tend to show that the ion densities measured with the OML and Laframboise theories exceeds the measured electron densities. The results also show that the plasma electron and ion densities increased with both discharge power and gas pressure.     

Comparison of dose measurements in CT using a photodiode and a small ion chamber

Owing to the advance of multislice computed tomography (CT), the dosimetric protocol currently used in CT has become inadequate. Instead of dosimetry based on the measurement of the Computed Tomography Dosimetry Index (CTDI) using a pencil ion chamber (IC) 100 mm in length, the use of a short IC and the calculation of the dose equilibrium (D_eq) at the location of the chamber are proposed. The objective of this work was to compare the performance of a short IC and a commercial photodiode (BPW34FS) to measure the accumulated dose at the center of the scan length L, D_L(0), and to obtain the equilibrium dose D_eq using the two detectors. The result for L = 100 mm was compared with the result of the pencil chamber. The results indicate that the commercial photodiode is suitable to measure the accumulated dose at the center of the scan length L as compared with the ICs. This methodology allows measurements of the accumulated dose for any desired scan length, allowing measurement of the equilibrium dose D_eq if the phantom is long enough to allow it.     

Application of a gold neutral beam probe and an end-loss energycomponent analyzer to space potential measurements in a tandem mirror

A Gold Neutral Beam Probe (GNBP) has been investigated from an active diagnostic point of view, and found to be the best way to measure potentials of the barrier and central cell plasmas in the GAMMA 10 tandem mirror. The thermal barrier potential was determined directly by the simultaneous measurements of the on-axis space potentials at both of these locations. Two-dimensional space potentials were obtained by adjusting the energy of the probing beams so that measurements were made along different chords through the plasma cross-section. A newly developed diagnostic device, a cylindrical End-Loss Energy Component Analyzer (ELECA), was also used to measure the plug potential, and was suitable for determining the distribution functions of end-loss ions. This diagnostic can also be useful as a compact energy analyzer for a beam probe diagnostic     

Temperature profile and depth of surface melting of a metal irradiated by a high-current ion beam

We have numerically solved the nonlinear thermal conductivity equation using temperature-dependent thermal coefficients to study the evolution of the temperature profile produced in a metal irradiated by a high-current ion beam. We studied the propagation of the thermal front and the heating rate of the metal surface as functions of ion beam characteristics. We determined the dependence of the maximum heating temperature and penetration depth on the maximum value of the ion beam current. Joint Institute for Nuclear Research. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 41, No. 12, pp. 13–20, December, 1998.     

Optical gain on the 476.5 nm Argon-ion laser line in a gas-target excited by a heavy ion beam

Optical gain on the 476.5 nm Ar II 4p–4s ion laser transition has been observed in argon-gas excited by 2.5 ns pulses of 90 MeV³²S ions with a repetition rate of 4883 Hz. The energy per pulse was 23 J. The projectiles were stopped in the target at pressures between 5 and 20 kPa. Gain was determined from a measured transient increase of the intensity of a 476.5 nm probe laser beam sent along the ion beam axis and back reflected by an aluminum foil. The maximum gain observed was (0.4±0.1)×10^–3 at a target-gas pressure of 5 kPa. Control experiments using krypton as target-gas were performed and yielded a null result. The optical gain observed in argon is consistent with the result from an analysis of spectroscopic studies of rare-gas targets excited by heavy ion beams.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday. This work has been funded by the German Federal Ministry for Research and Technology (BMFT) under contract No. 06 TM 310 I, Gesellschaft für Schwerionenforschung (GSI), Darmstadt, and the Tandem accelerator laboratory, Munich