O. Meusel

Opportunities for Nuclear Astrophysics at FRANZ

The ‘Frankfurter Neutronenquelle am Stern–Gerlach–Zentrum’ (FRANZ), which is currently under development, will be the strongest neutron source in the astrophysically interesting energy region in the world. It will be about three orders of magnitude more intense than the well-established neutron source at the Research Center Karlsruhe (FZK).

High current ion beam RF acceleration and perspectives for an inertial fusion driver

The actual situation with respect to the use of an RF linac driver for heavy ion inertial fusion (HIF) is discussed. At present, there is no high current heavy ion linac under construction. However, in the course of linac projects for e − , p, d, or highly charged ions several developments were made, which may have some impact on the design of a HIF driver. Medium- and low-β superconducting structures suited for pulsed high current beam operation are actually designed and investigated at several laboratories. A superconducting 40 MeV, 125 mA cw linac for deuteron acceleration is designed for the Inertial Fusion Material Irradiation Facility (IFMIF). The Institute for Applied Physics (IAP) is developing a superconducting 350-MHz, 19-cell prototype CH-cavity for β = 0.1. The prototype cavity will be ready for tests in 2004. A superconducting main HIF driver linac would considerably reduce the power losses. Moreover, it would allow for an efficient linac operation at a higher duty factor. The 1.4-AMeV room-temperature High Current Injector HSI at Gesellschaft fur Schwerionenforschung (GSI) has been in routine operation for more than 2 years now. With a mass-to-charge ratio of up to 65, a current limit of 15 mA for U 4+ , and an energy range from 2.2 AkeV up to 1.4 AMeV, this linac is suited to gain useful experience on the way toward the design of a HIF RF driver. The status and technical improvements of that A/q ≤ 65, 91-MV linac are reported. Beam dynamics calculations for Bi 1+ -beams show that powerful focusing elements at the linac front end are the bottleneck with respect to a further increase in beam current. Besides superconducting and pulsed wire quadrupoles, the potential of the Gabor-plasma lenses is investigated.

Characterization of volume type ion source for p, H+2 and H+3 beams

Abstract Recently, there is an increasing need for H 2 + and H 3 + ion sources. One example is ion therapy facilities, where C 4 + and H 3 + ion beams along the linac are of great interest. Another example is a H 2 + test beam for linacs finally operated with intense deuteron beams. At Frankfurt, a simple proton ion source is needed to test a new kind of beam injection into a magnetic storage ring [N. Joshi, et al., Beam transport in toroidal magnetic field, EPAC’08, Genoa, June 2008 〈 http://www.jacow.org 〉 , to be published [1] ; M. Droba, et al., Design studies on a novel stellarator type high current ion storage ring, EPAC’06, Edinburgh, June 2006, pp. 297–299 〈 http://www.jacow.org 〉 [2] ]. This article describes a volume type ion source which can deliver up to 3.05 mA beam current at 10 keV in stable dc operation. It is a hot filament driven ion source which can provide high fractions of p, H 2 + or H 3 + , depending on the operation settings.

Design of nondestructive emittance measurement device for H− beams

For the diagnostics of high-brightness particle beams as necessary for the European Spallation Source, spallation neutron source IFMIF (D1+), and HIDIF (Bi1+) nondestructive methods have to be developed. Conventional beam diagnostics are established, but are mostly destructive methods and suffer from the power density deposited on surfaces like slits or pinhole plates and beam loss. For negative ions a nondestructive method based on photoneutralization can be used to determine the transverse phase space distribution with high time resolution and a large number of phase space points. The interaction of laser light and H− ions produces a small number of neutral atoms as well as detached electrons. The beam of charged particles is separated magnetically from the neutral beam and the distribution of neutral hydrogen is measured. Under the assumption that the influence of the dipole is small enough, the laser induced emittance measurement can be used, e.g., near the entrance of radio-frequency quadrupole. The ...

A transverse electron target for heavy ion storage rings

Electron-ion interaction processes are of fundamental interest for several research fields like atomic and astrophysics as well as plasma applications. To address this topic, a transverse electron target based on the crossed beam technique was designed and constructed for the application in storage rings. Using a sheet beam of free electrons in crossed beam geometry promises a good energy resolution and gives access to the interaction region for spectroscopy. The produced electron beam has a length of 10 cm in ion beam direction and a width in the transverse plane of 5 mm. Therewith, electron densities of up to 109 electrons/cm3 are reachable in the interaction region. The target allows the adjustment of the electron beam current and energy in the region of several 10 eV to a few keV. Simulations have been performed regarding the energy resolution for electron-ion collisions and its influence on spectroscopic measurements. Also, the effect on ion-beam optics due to the space charge of the electron beam wa...

Experimental studies of stable confined electron clouds using Gabor lenses

Based on the idea of D. Gabor [1] space charge lenses are under investigation to be a powerful focussing device for intense ion beams. A stable confined electron column is used to provide strong radially symmetric electrostatic focussing, e.g. for positively charged ion beams. The advantages of Gabor lenses are a mass independent focussing strength, space charge compensation of the ion beam and reduced magnetic or electric fields compared to conventional focussing devices. Collective phenomena of the electron cloud result in aberrations and emittance growth of the ion beam. The knowledge of the behaviour of the electron cloud prevents a decrease of the beam brilliance. Numerical models developed to describe the electron confinement and dynamics within a Gabor lens help to understand the interaction of the ion beam with the electron column and show the causes of non-neutral plasma instabilities. The diagnosis of the electron cloud properties helps to evaluate the numerical models and to investigate the influence of the ion beam on the confined non-neutral plasma.

JACoW : Space-Charge Compensation in the Transition Area Between LEBT and RFQ

The transition from a space-charge compensated beam in the LEBT to an uncompensated beam in the RFQ will influence the beam parameters. In order to investigate the impact of the electric fields on the space charge compensation, an insulated cone is used as a repeller electrode in front of the RFQ. Depending on the time dependent potential of the RFQ rods respectively to the beam potential, the compensation electrons may be prevented from moving into the RF field which oozes out of the RFQ entrance. The simulation studies are performed with the particle-in-cell code bender [1]. The simulations may substantiate measurements at the CW-operated RFQ in Frankfurt University [2] as well as at the foreseen MYRRHA LEBT-RFQ interface. [3]. In this contribution, a study on a LEBT-RFQ interface is shown.

Beam transport and space charge compensation strategies (invited)

The transport of intense ion beams is affected by the collective behavior of this kind of multi-particle and multi-species system. The space charge expressed by the generalized perveance dominates the dynamical process of thermalisation, which leads to emittance growth. To prevent changes of intrinsic beam properties and to reduce the intensity dependent focusing forces, space charge compensation seems to be an adequate solution. In the case of positively charged ion beams, electrons produced by residual gas ionization and secondary electrons provide the space charge compensation. The influence of the compensation particles on the beam transport and the local degree of space charge compensation is given by different beam properties as well as the ion beam optics. Especially for highly charged ion beams, space charge compensation in combination with poor vacuum conditions leads to recombination processes and therefore increased beam losses. Strategies for providing a compensation-electron reservoir at very low residual gas pressures will be discussed.

A Coupled RFQ-IH-DTL Cavity for FRANZ: A Challenge for RF Technology and Beam Dynamics

For the ‘Frankfurt Neutron Source at the Stern-GerlachZentrum’ (FRANZ) facility an inductively coupled combination of a 4-Rod-type Radio-Frequency-Quadrupole (4Rod-RFQ) and an 8 gap interdigital H-type (IH-DTL) structure will provide the main acceleration of an intense proton beam from 120 keV to 2.0 MeV. The RFQ-IH combination with a total length of about 2.3 m will be operated at 175 MHz in cw mode. The expected total power need is around 200 kW. Due to the internal inductive coupling only one RF amplifier is needed, which significantly reduces the investment costs. At present the RFQ is installed separately in the beam line for conditioning up to the design rf power and for measuring the beam quality behind the RFQ. In parallel, the IH-DTL is rf tuned together with a dummy RFQ outside the FRANZ cave. This paper will present the status of the project with emphasis on key questions like beam dynamics constraints, rf tuning issues and technological challenges resulting from the high thermal load in cw operation.

Heavy Ion Linac for Synchrotron Injectors

The heavy ion high intensity injector linacs for rings are characterized by a low duty cycle and by high beam intensities. Main disadvantages are the very low particle velocity after passing the extraction system and terminal as well as the high voltages needed to accelerate heavy ions to a specified energy per nucleon. Especially for a low energy acceleration part the H-mode cavity design has some advantages. An overview of ion sources, 4-rod Radio Frequency Quadrupoles (RFQs) and room temperature H-mode cavities will be given. Specific features like the beam quality, the efficiency, the beam dynamic and energy schemes will be discussed.