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To fulfil the requirements of ESS on beam transmission and emittance growth a detailed knowledge of the physics of beam formation as well as the interaction of the H- with the residual gas is substantial. Space charge compensated beam transport using solenoids for ion optics is in favour for the Low Energy Beam Transport (LEBT) between ion source and the first RFQ. Space charge compensation reduces the electrical self fields and beam radii and therefore emittance growth due to aberrations and redistribution. Transport of H- near the ion source is negatively influenced by the dipole fields required for beam extraction and e--dumping and the high gas pressure. The destruction of the rotational symmetry together with the space charge forces causes emittance growth and particle losses within the extraction system. High residual gas pressure near the extractor together with the high cross section for stripping will influence the transmission as well as space charge compensation. Therefore a detailed knowledge of the interaction of the residual gas with the beam and the influence of the external fields on the distribution of the compensation particles is necessary to reduce particle losses and emittance growth. Preliminary experiments using positive hydrogen ions for reference already show the influence of dipole fields on beam emittance. First measurements with H- confirm these results. Additional information on the interactions of the residual gas with the beam ions have been gained from the measurements using the momentum and energy analyser.
Low energy beam transport (LEBT) for a future heavy ion driven inertial fusion (HIDIF [1]) facility is a crucial point using a Bi+ beam of 40 mA at 156 keV. High space charge forces (generalised perveance K=3.6*10-3) restrict the use of electrostatic focussing systems. On the other hand magnetic lenses using space charge compensation suffer from the low particle velocity. Additionally the emittance requirements are very high in order to avoid particle losses in the linac and at ring injection [2]. urthermore source noise and rise time of space charge compensation [3] might enhance particle losses and emittance. Gabor lenses [4] using a continuous space charge cloud for focussing could be a serious alternative to conventional LEBT systems. They combine strong cylinder symmetric focussing with partly space charge compensation and low emittance growth due to lower non linear fields. A high tolerance against source noise and current fluctuations and reduced investment costs are other possible advantages. The proof of principle has already been shown [5, 6]. To broaden the experiences an experimental program was started. Therefrom the first experimental results using a double Gabor lens (DGPL, see fig. 1 ) LEBT system for transporting an high perveance Xe+ beam will be presented and the results of numerical simulations will be shown.
Investigation of the focus shift due to compensation process for low energy ion beam transport
(2000)
In magnetic Low Energy Beam Transport (LEBT) sections space charge compensation helps to enhance the transportable beam current and to reduce emittance growth due to space charge forces. For pulsed beams the time neccesary to establish space charge compensation is of great interest for beam transport. Particularly with regard to beam injection into the first accelerator section (e.g. RFQ) investigation of effects on shift of the beam focus due to space charge compensation are very important. The achieved results helps to obviate a mismatch into the first RFQ. To investigate the space charge compensation due to residual gas ionization, time resolved measurements using pulsed ion beams were performed at the LEBT system at the IAP and at the CEA-Saclay injektion line. A residual gas ion energy analyser (RGIA) equiped with a channeltron was used to measure the potential destribution as a function of time to estimate the rise time of compensation. For time resolved measurements (delta t min=50ns) of the radial density profile of the ion beam a CCD-camera was applied. The measured data were used in a numerical simulation of selfconsistant eqilibrium states of the beam plasma [1] to determine plasma parameters such as the density, the temperature, the kinetic and potential energy of the compensation electrons as a function of time. Measurements were done using focused proton beams (10keV, 2mA at IAP and 92keV, 62mA at CEA-Saclay) to get a better understanding of the influence of the compensation process. An interpretation of the acquired data and the achieved results will be presented.
High perveance negative ion beams with low emittance are essential for several next generation particle accelerators (i. g. spallation sources like ESS [1] and SNS [2]). The extraction and transport of these beams have intrinsic difficulties different from positive ion beams. Limitation of beam current and emittance growth have to be avoided. To fulfill the requirements of those projects a detailed knowledge of the physics of beam formation the interaction of the H- with the residual gas and transport is substantial. A compact cesium free H- volume source delivering a low energy high perveance beam (6.5 keV, 2.3 mA, perveance K= 0.0034) has been built to study the fundamental physics of beam transport and will be integrated into the existing LEBT section in the near future. First measurements of the interaction between the ion beam and the residual gas will be presented together with the experimental set up and preliminary results.
High perveance negative ion beams with low emittance are essential for several next generation particle accelerators (i. g. spallation sources like ESS [1] and SNS [2]). The extraction and transport of these beams have intrinsic difficulties different from positive ion beams. Limitation of beam current and emittance growth have to be avoided. To fulfill the requirements of those projects a detailed knowledge of the physics of beam formation the interaction of the H- with the residual gas and transport is substantial. A compact cesium free H- volume source delivering a low energy high perveance beam (6.5 keV, 2.3 mA, perveance K= 0.0034) has been built to study the fundamental physics of beam transport and will be integrated into the existing LEBT section in the near future. First measurements of the interaction between the ion beam and the residual gas will be presented together with the experimental set up and preliminary results.
The knowledge of the build up time of space charge compensation (SCC) and the investigation of the compensation process is of main interest for low energy beam transport of pulsed high perveance ion beams under space charge compensated conditions. To investigate experimentally the rise of compensation an LEBT system consisting of a pulsed ion source, two solenoids and a drift tube as diagnostic section has been set up. The beam potential has been measured time resolved by a residual gas ion energy analyser (RGA). A numerical simulation for the calculation of self-consistent equilibrium states of the beam plasma has been developed to determine plasma parameters which are difficult measure directly. The results of the simulation has been compared with the measured data to investigate the behavior of the compensation electrons as a function of time. The acquired data shows that the theoretical rise time of space charge compensation is by a factor of two shorter than the build up time determined experimentally. In view of description the process of SCC an interpretation of the gained results is given.
The determination of the beam emittance using conventional destructive methods suffers from two main disadvantages. The interaction between the ion beam and the measurement device produces a high amount of secondary particles. Those particles interact with the beam and can change the transport properties of the accelerator. Particularly in the low energy section of high current accelerators like proposed for IFMIF, heavy ion inertial fusion devices (HIDIF) and spallation sources (ESS, SNS) the power deposited on the emittance measurement device can lead to extensive heat on the detector itself and can destruct or at least dejust the device (slit or grit for example). CCD camera measurements of the incident light emitted from interaction of beam ions with residual gas are commonly used for determination of the beam emittance. Fast data acquisition and high time resolution are additional features of such a method. Therefore a matrix formalism is used to derive the emittance from the measured profile of the beam [1,2] which does not take space charge effects and emittance growth into account. A new method to derive the phase space distribution of the beam from a single CCD camera image using statistical numerical methods will be presented together with measurements. The results will be compared with measurements gained from a conventional Allison type (slit-slit) emittance measurement device.
For investigation of space charge compensation process due to residual gas ionization and the experimentally study of the rise of compensation, a Low Energy Beam Transport (LEBT) system consisting of an ion source, two solenoids, a decompensation electrode to generate a pulsed decompensated ion beam and a diagnostic section was set up. The potentials at the beam axis and the beam edge were ascertained from time resolved measurements by a residual gas ion energy analyzer. A numerical simulation of self-consistent equilibrium states of the beam plasma has been developed to determine plasma parameters which are difficult to measure directly. The temporal development of the kinetic and potential energy of the compensation electrons has been analyzed by using the numerically gained results of the simulation. To investigate the compensation process the distribution and the losses of the compensation electrons were studied as a function of time. The acquired data show that the theoretical estimated rise time of space charge compensation neglecting electron losses is shorter than the build up time determined experimentally. To describe the process of space charge compensation an interpretation of the achieved results is given.
A LEBT system consisting of an ion source, two solenoids, and a diagnostic section has been set up to investigate the space charge compensation process due to residual gas ionization [1] and to study experimentally the rise of compensation. To gain the radial beam potential distribution time resolved measurements of the residual gas ion energy distribution were carried out using a Hughes Rojanski analyzer [2,3]. To measure the radial density profile of the ion beam a CCD-camera performed time resolved measurements, which allow an estimation the rise time of compensation. Further the dynamic effect of the space charge compensation on the beam transport was shown. A numerical simulation under assumption of selfconsistent states [4] of the beam plasma has been used to determine plasma parameters such as the radial density profile and the temperature of the electrons. The acquired data show that the theoretical estimated rise time of space charge compensation neglecting electron losses is shorter than the build up time determined experimentally. An interpretation of the achieved results is given.
Influence of space charge fluctuations on the low energy beam transport of high current ion beams
(2000)
For future high current ion accelerators like SNS, ESS or IFMIF the beam behaviour in low energy beam transport sections is dominated by space charge forces. Therefore space charge fluctuations (e. g. source noise) can drastically influence the beam transport properties of the low energy beam transport section. Losses of beam ions and emittance growth are the most severe problems. For electrostatic transport systems either a LEBT design has to be found which is insensitive to variations of the space charge or the origin of the fluctuations has to be eliminated. For space charge compensated transport as proposed for ESS and IFMIF the situation is different: No major influence on beam transport is expected for fluctuations below a cut-off frequency given by the production rate of the compensation particles. Above this frequency the fluctuations can not be compensated by particle production alone, but redistributions of the compensation particles helps to compensate the influence of the fluctuations. Above a second cut-off frequency given by the density and the temperature of the compensation particles their redistribution is too slow to reduce the influence of the space charge fluctuations. Transport simulations for the IFMIF injector including space charge fluctuations will be presented together with a determination of the cut-off frequencies. The results will be compared with measurements of the rise time of space charge compensation.
