A single-ion implantation (SII) system combining a laser-cooled linear-Paul-trap ion source (LPT-IS) and a two-stage acceleration lens is being developed to produce an array of nitrogen vacancy (NV) centers, which have recently attracted significant attention as potential qubits. The fabrication of the NV center array requires repeated implantations of a single nitrogen molecule ion (N2+) from SII into a diamond specimen with an energy on the order of a 10-keV and spatial precision of several tens of nanometers or less. To satisfy this requirement, N2+ ions with very low emittance must be selectively extracted from the LPT-IS. In this study, the selective extraction of N2+ ions is investigated using a three-dimensional multiparticle simulation by varying magnitudes and temporal profile of the voltage applied to the end plate electrodes of a conventional LPT. The emittance of the N2+ extracted from the LPT-IS is the lowest when N2+ ion is at the leading end of an array of N2+ and Ca+ ions in the string-like crystalline structure formed with sympathetic cooling. We show that a single N2+ ion with required energy can be extracted from the array of N2+ and Ca+ ions passing through the end plate electrode of the LPT-IS, without spoiling the emittance, by increasing the voltage applied to the end plate electrode at the precise required moment.

A detailed experimental study has been conducted to demonstrate the efficient confinement of ions in the popular four-rod configuration of a linear Paul trap without exciting the transverse radio-frequency (rf) quadrupole field. The three-dimensional (3D) ion confinement is achieved with an identical rf voltage applied to the end electrodes. The optimum operating region is visualized in the stability tune diagram, which indicates that a large number of ions can be stored by adjusting a few fundamental parameters. The lifetime of an ion cloud in the present linear trap is over a second (corresponding to a million rf cycles), long enough for various practical applications. It is also shown through 3D numerical simulations that one can easily extract ions from the trap at a low loss rate below 10%. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research

A compact Paul ion trap is employed to replicate the dynamic behavior of a relativistic charged-particle beam traveling in a large-scale accelerator. The nonlinear resonance bands recently identified at the CERN Proton Synchrotron (PS) are successfully reproduced in the tabletop system. The present experimental data suggests that all these resonances could be excited even without unknown error fields that break the high lattice symmetry of the PS. A new type of stability tune diagram appears to explain the basic feature of the experimental observation.

A variety of resonances have been observed experimentally in a linear Paul trap. They are excited by weak nonlinear fields that exist within the trap aperture depending on the mechanical design and misalignments of the electrodes. Those inevitable nonlinear terms in the ion confinement potential couple the axial and transverse degrees of freedom, making the resonance feature quite complicated. High-order resonances are weak as expected, unless driven by strong error fields or the space-charge potential. Systematic numerical simulations are performed to confirm the origin of each resonance line experimentally identified in the tune diagram. The numerical data suggests that a large fraction of the ions getting unstable and lost due to high-order resonances belong initially to the tail of the Gaussian-like ion distribution in phase space, namely, the lost particles have relatively large oscillation energies from the beginning.

In a conventional linear Paul trap (LPT), four electrode rods are placed symmetrically around the trap axis to generate a radio-frequency (rf) quadrupole field for transverse ion confinement. The periodic nature of the external focusing potential can give rise to serious ion losses under a specific condition. The loss mechanism is essentially the same as the coherent betatron resonance well-known in intense beam dynamics [1, 2]. In fact, the collective motion of an ion plasma in the LPT is shown equivalent to that of a charged-particle beam traveling through an alternating-gradient (AG) focusing lattice. In the present study, we perform the direct measurement of low-order coherent oscillation modes in the LPT by detecting image currents induced on the electrodes' surfaces. The four-rod structure of the LPT allows us to pick up weak signals from the dipole and quadrupole oscillations of a plasma bunch. These signals are Fourier analyzed to evaluate the coherent oscillation tune at different initial ion densities. The measured tune of the quadrupole mode is used to deduce the tune depression as a function of ion number stored in the LPT.

Starting from the principle of least action, we derive a general Hamiltonian that describes the collective motion of an intense charged-particle bunch in a drift-tube linear accelerator. The Alvarez-type structure is assumed as an example, but the present theory can readily be extended to other types of conventional linacs. A Hamiltonian formalism of non-neutral plasma in a linear Paul trap is also constructed, which demonstrates clear similarity between the linac system and compact ion-trap system. The physical equivalence between these two dynamical systems can be employed to perform a fundamental design study of high-intensity hadron linacs in a local tabletop environment. For the tabletop experiment on space-charge effects in short proton and heavy-ion bunches, we have designed an ion trap whose overall dimension is less than 10 cm axially and whose aperture size is 1 cm in diameter. The new trap is introduced in the S-POD (Simulator of Particle Orbit Dynamics) apparatus developed at Hiroshima University for "Laboratory Accelerator Physics."

An ion plasma confined in a linear Paul trap (LPT) exhibits the dynamic behavior physically equivalent to that of a charged-particle beam in an alternating-gradient (AG) transport channel. The Simulator of Particle Orbit Dynamics (S-POD) is a compact apparatus designed on the basis of this fact for diverse beam-physics experiments. We have so far employed Ar+ ions that can readily be produced from neutral Ar gas atoms through the electron bombardment process. A space-charge-induced tune shift of up to about 20% of the bare tune can be achieved in Ar+ plasmas. We are now preparing for future S-POD experiment to explore even higher beam-density regions. For this purpose, a large number of Ca+ ions need to be stored in the LPT. Since S-POD is equipped with a powerful laser cooler for Ca+, the use of this ion species vastly expands the density range we can survey. The production of an intense bunch of Ca+ ions is, however, not so easy because of some technical reasons. By optimizing the operating condition of a multi-sectioned LPT, we succeeded in increasing the number of accumulated Ca+ ions to the level comparable to Ar+ ion plasmas. This paper reports on updated results of the experiment.

Confinement properties of a non-neutral electron plasma are investigated with a nested potential in a uniform magnetic field. The main result is that the positive plug potentials can be maintained by applying an appropriate resonant RF field to the plug potentials. Since the nested potentials in a magnetic field can confine both positive and negative charged particles simultaneously, it is expected that the technique will be applied to study electron-positron plasmas experimentally. (C) 2019 The Japan Society of Applied Physics

A systematic experimental study is performed to clarify the parameter dependence of the noise-induced beam instability previously demonstrated by a Princeton group [M. Chung et al., Phys. Rev. Lett. 102, 145003 (2009)]. Because of the weakness of the driving force, the instability develops very slowly, which substantially limits the application of conventional experimental and numerical techniques. In the present study, a novel tabletop apparatus called "S-POD" (Simulator of Particle Orbit Dynamics) is employed to explore the long-term collective behavior of intense hadron beams. S-POD provides a many-body Coulomb system physically equivalent to a relativistic charged-particle beam and thus enables us to conduct various beam-dynamics experiments without the use of large-scale machines. It is reconfirmed that random noise on the linear beam-focusing potential can be a source of slow beam quality degradation. Experimental observations are explained well by a simple perturbation theory that predicts the existence of a series of dangerous noise frequency bands overlooked in the previous study. Those additional instability bands newly identified with S-POD are more important practically because the driving noise frequencies can be very low. The dependence of the instability on the noise level, operating tune, and beam intensity is examined and found consistent with theoretical predictions.

S-POD (Simulator of Particle Orbit Dynamics) is a non-neutral plasma trap system devised at Hiroshima University to investigate fundamental aspects of space-charge-dominated beam dynamics. Either linear Paul traps or Penning traps have been employed in current systems. For a wider range of beam dynamics studies, we are now developing a modified linear Paul trap which has four extra electrodes in addition to quadrupole electrodes. It can control the strengths and time structure of low-order nonlinear fields independently of the transverse linear quadrupole field. We report on an overview of the design and progress.

As the finalization of a hydrogen experiment towards the deuterium phase, the exploration of the best performance of hydrogen plasma was intensively performed in the large helical device. High ion and electron temperatures, T-i and T-e, of more than 6 keV were simultaneously achieved by superimposing high-power electron cyclotron resonance heating onneutral beam injection (NBI) heated plasma. Although flattening of the ion temperature profile in the core region was observed during the discharges, one could avoid degradation by increasing the electron density. Another key parameter to present plasma performance is an averaged beta value . The high regime around 4% was extended to an order of magnitude lower than the earlier collisional regime. Impurity behaviour in hydrogen discharges with NBI heating was also classified with a wide range of edge plasma parameters. The existence of a no impurity accumulation regime, where the high performance plasma is maintained with high power heating > 10 MW, was identified. Wide parameter scan experiments suggest that the toroidal rotation and the turbulence are the candidates for expelling impurities from the core region.

An extensive experimental study is performed to confirm fundamental resonance bands of an intense hadron beam propagating through an alternating gradient linear transport channel. The present work focuses on the most common lattice geometry called "FODO" or "doublet" that consists of two quadrupoles of opposite polarities. The tabletop ion-trap system "S-POD" (Simulator of Particle Orbit Dynamics) developed at Hiroshima University is employed to clarify the parameter-dependence of coherent beam instability. S-POD can provide a non-neutral plasma physically equivalent to a charged-particle beam in a periodic focusing potential. In contrast with conventional experimental approaches relying on large-scale machines, it is straightforward in S-POD to control the doublet geometry characterized by the quadrupole filling factor and drift-space ratio. We verify that the resonance feature does not essentially change depending on these geometric factors. A few clear stop bands of low-order resonances always appear in the same pattern as previously found with the sinusoidal focusing model. All stop bands become widened and shift to the higher-tune side as the beam density is increased. In the space-charge-dominated regime, the most dangerous stop band is located at the bare betatron phase advance slightly above 90 degrees. Experimental data from S-POD suggest that this severe resonance is driven mainly by the linear self-field potential rather than by nonlinear external imperfections and, therefore, unavoidable at high beam density. The instability of the third-order coherent mode generates relatively weak but noticeable stop bands near the phase advances of 60 and 120 degrees. The latter sextupole stop band is considerably enhanced by lattice imperfections. In a strongly asymmetric focusing channel, extra attention may have to be paid to some coupling resonance lines induced by the Coulomb potential. Our interpretations of experimental data are supported by theoretical predictions and systematic multiparticle simulations.

The compact non-neutral plasma trap systems named “S-POD” have been developed at Hiroshima University as an experimental simulator of beam dynamics. S-POD is based either on a linear Paul-trap or on a Penning trap and can approximately reproduce the collective motion of a relativistic charged-particle beam observed in the center-of-mass frame. We here employ the Paul trap system to investigate beam instability induced by small fluctuations of linear focusing-field that can affect intense beam transport over long propagation distance. In the present study, we consider the fluctuation whose structure is independent of lattice periodicity, for example, due to the noise on power sources for quadrupole magnets. It is confirmed that a resonant instability driven by the fluctuation induces slow beam loss.

The existence of autoresonances for m=2 diocotron oscillations of non-neutral electron plasmas in a uniform magnetic field was predicted by particle-in-cell simulations and it was confirmed in experiments. The obtained results show clear deviations from the standard threshold amplitude dependence on the sweep rate. The threshold amplitude approaches a constant at a lower sweep rate when there is a damping force. It was also found that the aspect ratio for the oval cross section of the confined plasma can be controlled by the frequency of the externally applied driving force.

This paper addresses a detailed experimental study of collective instability bands generated near every half-integer tune per lattice period by coherent dipole and quadrupole resonances. Both instabilities appear side by side or overlap each other but are mostly separable because the dipole resonance often creates a narrower stop band accompanied by more severe particle losses. The separation of these low-order resonance bands becomes greater as the beam intensity increases. In principle, the double stop-band structure can be formed even without machine imperfections when the beam's initial phase-space profile is deviated from the ideal stationary distribution. The tabletop ion-trap system called "S-POD" is employed to experimentally demonstrate the parameter dependence of the double stop-band structure. Numerical simulations are also performed for comparison with experimental observations.