Beam Transport Experiments Research Articles

Experiments with low energy ion beam transport into toroidal magnetic fields

Abstract The stellarator-type storage ring for accumulation of multi-Ampere proton, and other intense ion beams with energies in the range of 100 A k e V to 1 A M e V is designed at Goethe university, Frankfurt am Main. The first proposal for beam confinement with high transversal momentum acceptance was presented in EPAC2006. This ring is typically suited for experiments in plasma physics and nuclear astrophysics. The accumulator ring with a closed longitudinal magnetic field is foreseen with a flux density up to 6 − 8 T . For demonstration, experiments were carried out with two toroidal segments operated at room temperature. The beam transport experiments in toroidal magnetic fields were first described in EPAC2008 within the framework of a proposed low energy ion storage ring. The test setup aims at developing a ring injection system with two beam lines representing the main beam line and the injection line. The primary beam line for the experiments was installed and successfully commissioned in 2009. A special diagnostics probe for “in situ” ion beam detection was installed. This modular technique allows online diagnostics of the ion beam along the beam path. In this paper, we present new results on beam transport experiments, with single segment and coupled configuration. The measurements are shown to be in agreement with the numerical model.

Beam transport experiment with a new kicker control system on the HIRFL**Supported by National Natural Science Foundation of China (U1232123)

A kicker control system is used for beam extraction and injection between two cooling storage rings (CSRs) at the Heavy Ion Research Facility in Lanzhou (HIRFL). To meet the requirements of special physics experiments, the kicker controller has been upgraded, with a new controller designed based on ARM+DSP+FPGA technology and monolithic circuit architecture, which can achieve a precision time delay of 2.5 ns. In September 2014, the new kicker control system was installed in the kicker field, and the test experiment using the system was completed. In addition, a pre-trigger signal was provided by the controller, which was designed to synchronize the beam diagnostic system and physics experiments. Experimental results indicate that the phenomena of “missed kick” and “inefficient kick” were not observed, and the multichannel trigger signal delay could be adjusted individually for kick power supplies in digitization; thus, the beam transport efficiency was improved compared with that of the original system. The fast extraction and injection experiment was successfully completed based on the new kicker control systems for HIRFL.

H– beam transport experiments in a solenoid low energy beam transport

The Front End Test Stand (FETS) is located at Rutherford Appleton Laboratory and aims for a high current, fast chopped 3 MeV H(-) ion beam suitable for future high power proton accelerators like ISIS upgrade. The main components of the front end are the Penning ion source, a low energy beam transport line, an radio-frequency quadrupole (RFQ) and a medium energy beam transport (MEBT) providing also a chopper section and rebuncher. FETS is in the stage of commissioning its low energy beam transport (LEBT) line consisting of three solenoids. The LEBT has to transport an H(-) high current beam (up to 60 mA) at 65 keV. This is the injection energy of the beam into the RFQ. The main diagnostics are slit-slit emittance scanners for each transversal plane. For optimizing the matching to the RFQ, experiments have been performed with a variety of solenoid settings to better understand the actual beam transport. Occasionally, source parameters such as extractor slit width and beam energy were varied as well. The paper also discusses simulations based on these measurements.

Construction and Tests of HCX Quadrupole Doublet for Heavy Ion Beam Transport Experiments

A cryostat housing a pair of previously tested quadrupole magnets has been constructed as a prototype focusing cell for the High Current Experiment (HCX) project. The doublet assembly was constructed with a room temperature beam tube. The magnets were reassembled, electrically connected in series, and tested to the short sample limiting current without further training. The doublet was more sensitive to the current ramping rate as compare with the previous performance. In order to build a compact vacuum jacket for the warm bore structure, a novel radiation insulation made of aluminum coated stainless steel foil was selected. The experiences in assembly of the doublet and fabrication of the cryostat are described. The thermal performances of the cryostat are reported.

Ion source and injector experiments at the HIF/VNL

The heavy-ion fusion program is conducting several ion source and injector experiments to support ongoing HIF beam transport experiments and to develop new injector concepts for future fusion drivers. In the area of large diameter surface source, we studied the beam optics, experimented with aperturing, and benchmarked the computer simulation code. Steady progress was made in the merging beamlet experiment. The RF plasma source was optimized to produce high current density beamlets. Computer simulation of merging beamlets had produced a design and the hardware is being fabricated. We are examining a new concept based on accel–decel injection to produce super-high line charge density for application in driving targets for high energy density physics studies. Since trapping of secondary electrons in high current positive ion beams is still a concern for HIF, we consider a backup option using negative ion beams.

Intense beam transport experiments in a multi-bend system at the University of Maryland Electron Ring

We report on the results of beam transport experiments at the University of Maryland Electron Ring (UMER) over a distance of approximately 4 m . The experiments involve a 10 keV , 25– 100 mA , 100 ns electron beam in a lattice consisting of one short solenoid, 24 printed-circuit magnetic quadrupole lenses, and a number of bending and steering dipole magnets. The diagnostics include capacitive beam-position monitors, phosphor screens, slit-wire emittance meters and others. We discuss the conditions required for matching the space-charge-dominated beam into the UMER lattice, as well as emittance measurements and initial studies of longitudinal dynamics in the ring.

A 1.8-MeV K+ injector for the high current beam transport experiment

For the High Current Beam Transport Experiment (HCX) at Lawrence Berkeley National Laboratory, an injector is required to deliver up to 1.8 MV of 0.6 A K+ beam with an emittance of ≈1 π-mm-mrad. We have successfully operated a 10-cm-diameter surface ionization source together with an electrostatic quadrupole (ESQ) accelerator to meet these requirements. The pulse length is ≈4 μs, firing at once every 10–15 s. By optimizing the extraction diode and the ESQ voltages, we have obtained an output beam with good current density uniformity, except for a small increase near the beam edge. Characterization of the beam emerging from the injector included measurements of the intensity profile, beam imaging, and transverse phase space. These data along with comparison to computer simulations provide the knowledge base for designing and understanding future HCX experiments.

Self-pinched transport of an intense proton beam

Ion beam self-pinched transport (SPT) experiments have been carried out using a 1.1-MeV, 100-kA proton beam. A Rutherford scattering diagnostic and a LiF nuclear activation diagnostic measured the number of protons within a 5 cm radius at 50 cm into the transport region that was filled with low-pressure helium. Time-integrated signals from both diagnostics indicate self-pinching of the ion beam in a helium pressure window between 35 and 80 mTorr. Signals from these two diagnostics are consistent with ballistic transport at pressures above and below this SPT pressure window. Interferometric measurements of electron densities during beam injection into vacuum are consistent with ballistic transport with co-moving electrons. Interferometric measurements for beam injection into helium show that the electron density increases quadratically with pressure through the SPT window and roughly linearly with pressure above the SPT window. The ionization fraction of the helium plateaus at about 1.5% for pressures above 80 mTorr. In the SPT window, the electron density is 3 to 20 times the beam density. Numerical simulations of these beam transport experiments produce results that are in qualitative agreement with the experimental measurements.

Statistical fitting analysis of Stark-broadened optically thick Ar II spectra measured in ion beam transport experiments

We present a method for fitting a multi-line spectrum with an emission model that accounts for multiple line-broadening mechanisms and the effects of finite optical depth. This technique includes an analysis of the joint-probability distribution of the model parameters that allows one to determine statistically valid confidence limits on the fitted parameters. We apply this method to a time series of optical Ar II spectra produced when an intense lithium beam (kinetic energy of 9 MeV and current density of 22 kA cm −2) was injected into an argon gas cell. We show that line optical depth effects are important in these data, and by including finite optical depth as well as Stark and Doppler broadening in the model-fitting procedure, we are able to place meaningful constraints on the time-dependent Ar II level populations and the electron density in the gas cell. The values we derive for these quantities are in reasonably good agreement with detailed collisional-radiative models that include the effects of non-thermal electrons and beam ions. Understanding the time-dependent conditions in the gas cell is critical for efficient beam transport.

Transport of a space-charge dominated electron beam in a short-quadrupole channel

Results are presented for electron beam transport experiments in a 1-m-long straight section consisting of a solenoid and five short printed-circuit quadrupoles. A linear computer code for rms envelope matching, SPOT, is used for channel design, while final simulations with more realistic elements are obtained with a $2\frac{1}{2}\mathrm{D}$ version of WARP, a particle-in-cell code. Reasonable agreement is found between calculations and the effective beam envelope obtained from pictures of the beam on a movable phosphor screen. The results validate, within experimental errors, the use of short magnetic quadrupoles for the transport of space-charge dominated beams. The straight section constitutes the prototype matching section for an electron recirculator to be built at the University of Maryland.

Demonstration of low-loss electron beam transport and mm-wave experiments of the fusion-FEM

In the Fusion-FEM electrostatic Free Electron Maser, an electron beam loss current of less than 0.2% is essential for long-pulse operation. At reduced beam current, 3 A instead of the nominal 12 A, we have demonstrated electron beam acceleration and transport through the undulator at current losses less than 0.02%. This paper summarizes the results of beam transport experiments and outlines the simple yet effective beam diagnostics system. The status of the mm-wave cavity and low-power measurements are given. It is shown that cavity characteristics, such as losses, bandwidth, frequency-tuning and feedback coefficient, can be checked in situ via the output window, by injecting a low power test-signal into the mm-wave system and analyzing the reflected signal.

Investigation of nonthermal particle effects on ionization dynamics in high current density ion beam transport experiments

Light ion inertial fusion experiments require the presence of a moderate density background gas in the transport region to provide charge and current neutralization for a high current density ion beam. In this article, we investigate the effects of nonthermal particles such as beam ions or non-Maxwellian electron distributions on the ionization dynamics of the background gas. In particular, we focus on the case of Li beams being transported in an argon gas. Nonthermal particles as well as thermal electrons are included in time-dependent collisional-radiative calculations to determine time-dependent atomic level populations and charge state distributions in a beam-produced plasma. We also briefly discuss the effects of beam ions and energetic electrons on the visible and vacuum ultraviolet (VUV) spectral regions. It is found that the mean charge state of the gas, and hence the electron density, is significantly increased by collisions with energetic particles. This higher ionization significantly impacts the VUV spectral region, where numerous resonance lines occur. On the other hand, the visible spectrum tends to be less affected because the closely spaced excited states are populated by lower energy thermal electrons.

Nonlinear resonances and chaotic behavior in a periodically focused intense charged-particle beam.

It is shown that beam self-field effects induce nonlinear resonances and chaotic behavior in the envelope oscillations of an intense charged-particle beam propagating through a periodic focusing field. The resonance condition is derived and expressed in terms of the vacuum phase advance and the beam perveance. Certain correlations are found between such resonant and chaotic behavior and well-known instabilities in periodically focused high-current ion beams. The predicted nonlinear resonances and chaotic behavior are expected to be observable in beam transport experiments in which there is a mismatch between the beam and the periodic focusing field.

Heavy-ion fusion injector experiments

We report on three experiments performed in connection with the 2MV electrostatic quadrupole (ESQ) injector under construction at Lawrence Berkeley Laboratory. Scaled experiments have been conducted to study possible beam emittance growth due to beam aberrations in an ESQ injector. The experiment uses the SBTE (Single Beam Transport Experiment) accelerator system, quarter-scale ESQ set-up and a potassium ion diode source. Measured emittance growth changes significantly with variations in current and diode energy, in good agreement with theoretical predictions. In addition, beam transport experiments were performed in a 1MV axisymmetric electrostatic aperture column using a zeolite 1″ diameter potassium ion source. Experimental measurements in good agreement with 2-1/2D simulations showed that low-emittance beams can be produced in axisymmetric structures. Finally, ESQ breakdown voltage tests without beam were performed at up to two times the quadrupole working voltage.

Electron beam injector for longitudinal beam physics experiments

An electron beam injector has been constructed to study the longitudinal beam physics in the University of Maryland electron beam transport experiments, including studies of longitudinal beam pulse compression and resistive-wall instability. The injector consists of a variable-perveance gridded electron gun followed by three matching lenses and one induction acceleration module. In the beam pulse compression experiment, it produces a 50 ns, 40 mA and 2.5 to 7.5 keV electron beam pulse with an approximately linear time-dependent velocity tilt. This beam will be injected into an existing 5 m long periodic transport channel with 36 short solenoid lenses. With the given beam parameters and initial conditions, the beam pulse is expected to be compressed by a factor of 3 or greater when reaching the end of the solenoid channel. In the resistive-wall instability experiment, the injector produces an electron beam pulse of 5 to 50 ns in duration, about 100 mA in current and a few kilovolts in energy. This beam will be guided into a resistive-wall channel of a couple of meters in length for study of the longitudinal resistive-wall instability. This paper reports on the design features and the general performance characteritics of the injector system including its mechanical, electrical, and beam-optical properties.

High-current beam transport

A key issue in the development of heavy-ions drivers for inertial fusion is the feasibility of transporting and manipulating the required high beam currents without excessive particle loss and emittance growth. Experimental, theoretical and simulation studies at various laboratories have produced very encouraging results that have alleviated initial concerns on the current limits of periodic beam transport systems. The understanding and control of emittance growth is one of the dominant themes of current research. A brief overview of activities and recent achievements by the major groups working in this field will be presented. The new results on merging of multiple beamlets in the University of Maryland electron beam transport experiment will be discussed in more detail.

Experimental studies of electron beam transport in the maryland periodic solenoid channel

In the University of Maryland electron beam transport experiment, a 5 kV, 0.2 A electron beam from a thermionic electron source (cathode radius r c = 1.27 cm and cathode temperature kT = 0.12 eV) is injected into a periodic focusing channel consisting of 38 solenoid lenses with period length S = 13.6 cm. The magnetic focusing fields of the channel, and hence the phase advance without space charge, can be varied over a wide range. We report emittance measurements and results of the effects of beam mismatch, misalignments and nonlinear lens forces on beam transport and emittance growth.

Multiframe ion pinhole camera for intense ion beam transport and focusing experiments

We have developed a new ion pinhole camera to obtain energy density profiles of a focused multiterawatt ion beam as a function of ion energy. Beam ions that are elastically scattered from a thin gold foil are imaged through six baffled pinholes onto six separate areas of a sheet of CR-39 nuclear track detector. Each of the areas is covered by an aluminum range filter and records an image with a different lower bound on the ion energy. Subtracting images with adjacent cutoff energies results in images with upper and lower bounds on the ion energy. Ion images are read from the CR-39 with VERA, a fully automatic, image-processor-based nuclear track counting system. Images of the Particle Beam Fusion Accelerator-II (PBFA-II) proton beam have been obtained and show that the beam has been focused to achieve a horizontal FWHM of 5.7 mm and has delivered 9 kJ/cm2 to a target.

High Current Beam Transport Experiments at GSI

The status of the high current ion beam transport experiment is reported. 190 keV Ar1+ ions were injected into six periods of a magnetic quadrupole channel. Since the pulse length is > 0.5 ms partial space charge neutralization occurs. In our experiments, the behavior of unneutralized and partially space charge compensated beams is compared. With an unneutralized beam, emittance growth has been measured for high intensities even in case of the zero-current phase advance σ0 < 90° . This initial emittance growth at high tune depression we attribute to the homogenization effect of the space charge density. An analytical formula based on this assumption describes the emittance growth very well. Furthermore the predicted envelope instabilities for σ0 > 90° were observed even after 6 periods. In agreement with the theory, unstable beam transport was also experimentally found if a beam with different emittances in the two transverse phase planes was injected into the transport channel. Although the space charge force is reduced for a partially neutralized beam a deterioration of the beam quality was measured in a certain range of beam parameters. Only in the range where an unneutralized beam shows the initial emittance growth, the partial neutralization reduces this effect, otherwise the partially neutralized beam is more unstable.

The Effect of Induced Charge at Boundaries on Transverse Dynamics of a Space-Charge-Dominated Beam

A particle simulation code has been used to study the effect on transverse beam dynamics of charge induced on focusing electrodes. A linear transport system was assumed. The initial particle distribution was taken to be that of a uniform elliptical beam with a Gaussian velocity distribution. For misaligned, highly space-charge-dominated beams (betatron phase advance per lattice period ≲ 10°), a large oscillation of the rms emittance occurred in a beat pattern. Linearized Vlasov analysis shows the oscillation to be a sextupole oscillation, driven by the beam coherent betatron motion. Emittance growth accompanied the oscillation. Preliminary experimental results from the Single Beam Transport Experiment (SBTE) are consistent with the code results. Addition of a dodecapole nonlinearity to the computational focusing field greatly reduces the oscillation amplitude.