Measurements of the fast electron bremsstrahlung emission during electron cyclotron resonance heating in the HL-2A tokamak

A fast electron bremsstrahlung (FEB) diagnostic technique based on cadmium telluride (CdTe) detector has been developed recently in the HL-2A tokamak for measurements of the temporal evolution of FEB emission in the energy range of 10-200 keV. With a perpendicular viewing into the plasma on the equatorial plane, the hard x-ray spectra with eight different energy channels are measured. The discrimination of the spectra is implemented by an accurate spectrometry. The system also makes use of fast digitization and software signal processing technology. An ambient environment of neutrons, gammas, and magnetic disturbance requires careful shielding. During electron cyclotron resonance heating, the generation of fast electrons and the oscillations of electron fishbone (e-fishbone) have been found. Using the FEB measurement system, it has been experimentally identified that the mode strongly correlates with the electron cyclotron resonance heating produced fast electrons with 30-70 keV.     

A compact, multiangle electron spectrometer for ultraintense laser-plasma interaction experiments

Experiments on the multiterawatt (MTW) laser at the Laboratory for Laser Energetics will study the effect of the focal-spot shape on the forward acceleration and collimation of electrons. A compact electron spectrometer has been developed to record the energy spectra of electrons ejected in the interaction of the laser at multiple angular locations simultaneously. The modular system with replaceable magnets provides an adjustable energy band, currently 0.2-6 MeV. The detector is an array of imaging plates. The device is designed to operate in the high-noise environment (bremsstrahlung and Compton x rays, gamma rays, and scattered electrons), while being compact enough to fit in the 30 cm radius MTW target chamber. The detector geometry and shielding were optimized with the particle/radiation transport code GEANT4. Calibration was performed with beta sources. The required dynamic range, sensitivity, and resolution were confirmed with initial MTW experimental data.     

Temperature diagnostics of ECR plasma by measurement of electron bremsstrahlung

The x-ray bremsstrahlung spectrum emitted by the electron population in a 14.5 GHz ECR plasma source has been measured using a NaI(Tl) detector, and hence the electron temperature of the higher energy electron population in the plasma has been determined. The x-ray spectra for Ne and Ar gases have been systematically studied as a function of inlet gas pressure from 7 × 10(-7) mbar to 7 × 10(-5) mbar and for input microwave power ～1 W to ～300 W. At the highest input power and optimum pressure conditions, the end point bremsstrahlung energies are seen to reach ～700 keV. The estimated electron temperatures (T(e)) were found to be in the range 20 keV-80 keV. The T(e) is found to be peaking at a pressure of 1 × 10(-5) mbar for both gases. The T(e) is seen to increase with increasing input power in the intermediate power region, i.e., between 100 and 200 W, but shows different behaviour for different gases in the low and high power regions. Both gases show very weak dependence of electron temperature on inlet gas pressure, but the trends in each gas are different.     

激光诱导铜产生等离子体羽辉的颜色特性

为了研究等离子体羽的辐射特性,建立了激光诱导辐射模型。通过时空分辨光谱测量技术,测定了激光烧蚀铜诱导产生等离子体羽辉在氧气环境下的发射光谱及其发射强度随时间与空间的分布;利用快速照相法,拍摄了等离子体羽的照片。结果表明:等离子体羽光谱主要由Cu的原子谱线、一价离子谱线组成;其不同区域的颜色不同,且随环境气压而变化。认为等离子体羽的不同区域诱导发光机理不同,中心区域以连续辐射为主;中间区域以原子谱线和一价离子谱线为主;外围区域以原子谱线为主。连续辐射主要来自近靶处高能电子运动而产生的韧致辐射以及电子和一价离子的复合;一价离子谱线主要来自近靶处高能电子与一价离子碰撞传能;原子谱线主要来自电子与原子的碰撞传能以及近靶处电子与一价离子的复合。     

An upgraded x-ray spectroscopy diagnostic on MST

An upgraded x-ray spectroscopy diagnostic is used to measure the distribution of fast electrons in MST and to determine Z(eff) and the particle diffusion coefficient D(r). A radial array of 12 CdZnTe hard-x-ray detectors measures 10-150 keV Bremsstrahlung from fast electrons, a signature of reduced stochasticity and improved confinement in the plasma. A new Si soft-x-ray detector measures 2-10 keV Bremsstrahlung from thermal and fast electrons. The shaped output pulses from both detector types are digitized and the resulting waveforms are fit with Gaussians to resolve pileup and provide good time and energy resolution. Lead apertures prevent detector saturation and provide a well-known etendue, while lead shielding prevents pickup from stray x-rays. New Be vacuum windows transmit >2?keV x-rays, and additional Al and Be filters are sometimes used to reduce low energy flux for better resolution at higher energies. Measured spectra are compared to those predicted by the Fokker-Planck code CQL3D to deduce Z(eff) and D(r).     

An experimental setup for studying specular reflection of hard γ quanta

An experimental setup is described on which the fact of specular reflection of hard γ quanta (E _γ ≤ 5 MeV) from a smooth amorphous surface has been experimentally established. Specular reflection is observed at sliding incidence of γ quanta onto a reflecting surface at angles of a few thousandths of a degree. The setup is composed of a microtron, a system of electron transportation to a bremsstrahlung target, the bremsstrahlung target generating an intense directed flux of electron bremsstrahlung γ quanta), a guiding system forming a ribbon beam of γ radiation with a width of ～1 cm and a thickness of ≈20 μm, a reflector (a liquid mercury surface or a glass plate), and a detection system containing the counting and spectrometric detectors located at a distance of 120 m from the bremsstrahlung target. Some experimental results are presented.     

Influence of the shear flow on electron cyclotron resonance plasma confinement in an axisymmetric magnetic mirror trap of the electron cyclotron resonance ion source

Influence of shear flows of the dense plasma created under conditions of the electron cyclotron resonance (ECR) gas breakdown on the plasma confinement in the axisymmetric mirror trap ("vortex" confinement) was studied experimentally and theoretically. A limiter with bias potential was set inside the mirror trap for plasma rotation. The limiter construction and the optimal value of the potential were chosen according to the results of the preliminary theoretical analysis. This method of "vortex" confinement realization in an axisymmetric mirror trap for non-equilibrium heavy-ion plasmas seems to be promising for creation of ECR multicharged ion sources with high magnetic fields, more than 1 T.     

A collimator for hot plasma x-ray diagnostics in ion sources

We present a collimator configuration for measuring energy resolved x-ray plasma volume bremsstrahlung emitted, e.g., by an ECRIS. Special attention we paid to shielding the detector against interfering Compton scattered radiation and wall bremsstrahlung stemming from the collimator entrance aperture. We estimate the efficiency for shielding of Compton scattered radiation at least attainable by this arrangement.     

Nomograms for Calculating Parameters of Shields against Direct Bremsstrahlung

Nomograms for calculating thicknesses of shields against direct bremsstrahlung in the band of accelerated electron energy of 0.5–15 MeV are given. The nomograms have been measured for the main shielding materials, such as lead, iron, and concrete, with due account of accurate measurements of source spectra and contribution of secondary radiation.     

Ion beam capture and charge breeding in electron cyclotron resonance ion source plasmas

Beam capture of injected ions and charge breeding in electron cyclotron resonance (ECR) charge breeder ion source plasmas has been investigated utilizing an ECR plasma modeling code, the generalized ECR ion source model, and a Monte Carlo beam capture code. Beam capturing dynamics, charge breeding in the plasma, and the extracted charged ion states are described. Optimization of ion beam energy is performed for (1) high beam capture efficiency and (2) high charge state ion beam extractions. A sample case study for ANL-ECR has been performed. Ions entering ECR ion source plasma are slowed down mostly by the background ions. Assuming Maxwellian plasma ions, maximum beam energy loss occurs when the beam velocity is around the background thermal velocity in magnitude. It is also found that beam capture location affects charge state distribution. For instance, with a majority of beam ions captured near the middle of the device higher currents for higher charge states are obtained. The beam ions captured near the entry have a higher probability of backstreaming after they are captured. For this reason, the optimum beam energy of the injected Ar(+) beam ions for charge breeding is generally higher than the optimum input beam energy for maximum beam energy loss.     

Towards a better comprehension of plasma formation and heating in high performances electron cyclotron resonance ion sources (invited)

Further improvements of electron cyclotron resonance ion sources (ECRIS) output currents and average charge state require a deep understanding of electron and ion dynamics in the plasma. This paper will discuss the most recent advances about modeling of non-classical evidences like the sensitivity of electron energy distribution function to the magnetic field detuning, the influence of plasma turbulences on electron heating and ion confinement, the coupling between electron and ion dynamics. All these issues have in common the non-homogeneous distribution of the plasma inside the source: the abrupt density drop at the resonance layer regulates the heating regimes (from collective to turbulent), the beam formation mechanism and emittance. Possible means to boost the performances of future ECRIS will be proposed. In particular, the use of Bernstein waves, in preliminary experiments performed at Laboratori Nazionali del Sud (LNS) on MDIS (microwave discharge ion sources)-type sources, has permitted to sustain largely overdense plasmas enhancing the warm electron temperature, which will make possible in principle the construction of sources for high intensity multicharged ions beams with simplified magnetic structures.     

Operation of the CAPRICE electron cyclotron resonance ion source applying frequency tuning and double frequency heating

The properties of the electromagnetic waves heating the electrons of the ECR ion sources (ECRIS) plasma affect the features of the extracted ion beams such as the emittance, the shape, and the current, in particular for higher charge states. The electron heating methods such as the frequency tuning effect and the double frequency heating are widely used for enhancing the performances of ECRIS or even for the routine operation during the beam production. In order to better investigate these effects the CAPRICE ECRIS has been operated using these techniques. The ion beam properties for highly charged ions have been measured with beam diagnostic tools. The reason of the observed variations of this performance can be related to the different electromagnetic field patterns, which are changing inside the plasma chamber when the frequency is varying.     

Note: measurement of the runaway electrons in the J-TEXT tokamak

The runaway electrons have been measured by hard x-ray detectors and soft x-ray array in the J-TEXT tokamak. The hard x-ray radiations in the energy ranges of 0.5-5 MeV are measured by two NaI detectors. The flux of lost runaway electrons can be obtained routinely. The soft x-ray array diagnostics are used to monitor the runaway beam generated in disruptions since the soft x-ray is dominated by the interaction between runaway electrons and metallic impurities inside the plasma. With the aid of soft x-ray array, runaway electron beam has been detected directly during the formation of runaway current plateau following the disruptions.     

Neutron analysis of the ITER vertical neutron camera

The neutron fields in the collimators of a new design for the vertical neutron camera (VNC) of the ITER have been calculated for a standard isotropic bulk DT neutron source. The neutron and gamma-ray spectra and flux densities at the neutron detector sites have been calculated. The signal-to-background ratio of VNC detectors (²³⁸U-based fission chambers and diamond detectors) has been estimated. The signal-to-background ratios versus the threshold energy were calculated for the diamond detectors operating in the threshold counter mode. The effect of background Γ-ray radiation on the performance of the diamond detectors in the VNC environment has been estimated. The radiation heating of the VNC structural components has been calculated. The serviceability of the VNC with the proposed design has been demonstrated.     

36-segmented high magnetic field hexapole magnets for electron cyclotron resonance ion source

Two high magnetic field hexapoles for electron cyclotron resonance ion source (ECRIS) have successfully fabricated to provide sufficient radial magnetic confinement to the ECR plasma. The highest magnetic field at the inner pole tip of one of the magnets exceeds 1.5 T, with the inner diameter (i.d.)=74 mm. The other hexapole magnet provides more than 1.35 T magnetic field at the inner pole tip, and the i.d. is 84 mm. In this article, we discuss the necessity to have a good radial magnetic field confinement and the importance of a Halbach hexapole to a high performance ECRIS. The way to design a high magnetic field Halbach structure hexapole and one possible solution to the self-demagnetization problem are both discussed. Based on the above discussions, two high magnetic field hexapoles have been fabricated to be utilized on two high performance ECRISs in Lanzhou. The preliminary results obtained from the two ECR ion sources are given.     

Spatially distributed scintillator arrays for diagnosing runaway electron transport and energy behavior in tokamaks

We present details of a new bismuth germanate [Bi(4)Ge(3)O(12) (BGO)] scintillator array used to diagnose the transport and energy behavior of runaway electrons (REs) in DIII-D. BGO exhibits important properties for these compact detectors including high light yield which sufficiently excites photodiode detectors (8500 photons/MeV), high density and atomic numbers of constituent materials which maximizes sensitivity, and relative neutron blindness which minimizes complications in data interpretation. The detectors observe primarily hard x-ray radiation emitted in a forward beamed pattern by RE when they strike first wall materials or bulk ions and neutrals in the plasma, although we also address photoneutron signals. The arrangement of the array enables time resolved location of x-ray emission and associated asymmetries which help identify instabilities and confinement properties of RE. By shielding a subset of detectors with different thicknesses of lead, and with interpretative support of the code EGSNRC, we also measure RE energy, although due to the often distributed nature of RE strike points and the forward beamed character of emitted hard x-rays, we restrict interpretation as a lower bound for RE energy.     

Advanced light ion source extraction system for a new electron cyclotron resonance ion source geometry at Saclay

One of the main goal of intense light ion injector projects such as IPHI, IFMIF, or SPIRAL2, is to produce high current beams while keeping transverse emittance as low as possible. To prevent emittance growth induced in a dual solenoid low energy transfer line, its length has to be minimized. This can be performed with the advanced light ion source extraction system concept that we are developing: a new ECR 2.45 GHz type ion source based on the use of an additional low energy beam transport (LEBT) short length solenoid close to the extraction aperture to create the resonance in the plasma chamber. The geometry of the source has been considerably modified to allow easy maintenance of each component and to save space in front of the extraction. The source aims to be very flexible and to be able to extract high current ion beams at energy up to 100 kV. A specific experimental setup for this source is under installation on the BETSI test bench, to compare its performances with sources developed up to now in the laboratory, such as SILHI, IFMIF, or SPIRAL2 ECR sources. This original extraction source concept is presented, as well as electromagnetic simulations with OPERA-2D code. Ion beam extraction in space charge compensation regime with AXCEL, and beam dynamics simulation with SOLMAXP codes show the beam quality improvement at the end of the LEBT.     

Comparison between off-resonance and electron Bernstein waves heating regime in a microwave discharge ion source

A microwave discharge ion source (MDIS) operating at the Laboratori Nazionali del Sud of INFN, Catania has been used to compare the traditional electron cyclotron resonance (ECR) heating with an innovative mechanisms of plasma ignition based on the electrostatic Bernstein waves (EBW). EBW are obtained via the inner plasma electromagnetic-to-electrostatic wave conversion and they are absorbed by the plasma at cyclotron resonance harmonics. The heating of plasma by means of EBW at particular frequencies enabled us to reach densities much larger than the cutoff ones. Evidences of EBW generation and absorption together with X-ray emissions due to high energy electrons will be shown. A characterization of the discharge heating process in MDISs as a generalization of the ECR heating mechanism by means of ray tracing will be shown in order to highlight the fundamental physical differences between ECR and EBW heating.     

The preliminary tests of the superconducting electron cyclotron resonance ion source DECRIS-SC2

A new compact version of the "liquid He-free" superconducting ECR ion source, to be used as an injector of highly charged heavy ions for the MC-400 cyclotron, is designed and built at the Flerov Laboratory of Nuclear Reactions in collaboration with the Laboratory of High Energy Physics of JINR. The axial magnetic field of the source is created by the superconducting magnet and the NdFeB hexapole is used for the radial plasma confinement. The microwave frequency of 14 GHz is used for ECR plasma heating. During the first tests, the source shows a good enough performance for the production of medium charge state ions. In this paper, we will present the design parameters and the preliminary results with gaseous ions.     

X-ray grating interferometer for materials-science imaging at a low-coherent wiggler source

X-ray phase-contrast radiography and tomography enable to increase contrast for weakly absorbing materials. Recently, x-ray grating interferometers were developed that extend the possibility of phase-contrast imaging from highly brilliant radiation sources like third-generation synchrotron sources to non-coherent conventional x-ray tube sources. Here, we present the first installation of a three grating x-ray interferometer at a low-coherence wiggler source at the beamline W2 (HARWI II) operated by the Helmholtz-Zentrum Geesthacht at the second-generation synchrotron storage ring DORIS (DESY, Hamburg, Germany). Using this type of the wiggler insertion device with a millimeter-sized source allows monochromatic phase-contrast imaging of centimeter sized objects with high photon flux. Thus, biological and materials-science imaging applications can highly profit from this imaging modality. The specially designed grating interferometer currently works in the photon energy range from 22 to 30 keV, and the range will be increased by using adapted x-ray optical gratings. Our results of an energy-dependent visibility measurement in comparison to corresponding simulations demonstrate the performance of the new setup.