arXiv subjects

T. Ido

Publications and source records attributed to T. Ido.

9 recordsLinked to original sources

arXiv2026

Beam-Tracing-Based Quantitative Reconstruction of Density Fluctuations in QUEST Using Doppler Backscattering

A three-channel X-/Ku-band Doppler backscattering (DBS) system has been developed and installed on QUEST for turbulence and electric-field measurements. In spherical tokamaks, the large magnetic-field pitch angle increases the geometric mismatch between the probing beam wave vector and the local magnetic-field vector, reducing the effective perpendicular projection and resulting in a systematic underestimation of the measured scattering intensity. In addition, in QUEST, where low plasma density requires a low-frequency probe beam, beam propagation effects become increasingly significant, further complicating the interpretation of the measured DBS power in terms of local density fluctuation amplitude. To address these issues, a quantitative correction methodology based on the synthetic DBS code SCOTTY was established. All relevant diagnostic response effects were evaluated using SCOTTY along ray trajectories, yielding a correction factor for reconstructing the local turbulence amplitude from the measured scattering signal. The correction factor exhibits strong spatial and frequency dependence, varying by up to an order of magnitude between the plasma core and edge regions, highlighting the necessity of frequency-dependent corrections. By applying the derived correction factor to experimental measurements, quantitative density fluctuation amplitudes were reconstructed from the detected scattering signals. Evaluation of the fluctuation amplitude indicates enhanced turbulence activity in the plasma edge region, where a finite negative radial electric field is inferred. This work demonstrates the first quantitative turbulence evaluation using low-frequency X-/Ku-band DBS measurements in QUEST and establishes a framework for quantitative DBS analysis in spherical tokamaks.

physics.plasm-ph↗

arXiv2026

Overcoming the space-charge dilemma in low-energy heavy ion beams via a multistage acceleration lens system

Low-energy heavy-ion beams are fundamentally limited by severe space-charge divergence, which constrains the transportable beam current to a few microamperes in conventional electrostatic accelerators. This limitation is particularly critical for high-mass ions, where the generalized perveance increases rapidly because of their low velocity. Here, we demonstrate that this apparent space-charge limit can be overcome by shaping the electrostatic potential configuration of an existing multistage accelerator, thereby transforming the acceleration column itself into a combined acceleration-focusing column. By optimizing the interstage voltage configuration, a strong electrostatic lens effect is superimposed on the accelerating field to counteract space-charge-driven expansion. We formulate a generalized design framework that quantitatively maps the transport 'design window' in terms of beam current, ion mass, and acceleration voltage. For gold ions at 64 keV, this approach enables stable transport of beam currents exceeding 100 microA, more than an order of magnitude higher than the conventional limit. Numerical phase-space analysis shows that this improvement is achieved by prioritizing envelope control over emittance preservation, a trade-off intrinsic to space-charge-dominated regimes. Our results establish a universal and practical guideline for high-current heavy-ion beam transport, relevant to fusion plasma diagnostics, ion implantation, and massive molecular ion applications.

physics.plasm-ph↗

arXiv2025

Enhanced beam transport via space charge mitigation in a multistage accelerator for fusion plasma diagnostics

Efficient transport of high-current negative ion beams is critical for accurate plasma potential diagnostics using heavy-ion beam probe (HIBP) systems in magnetically confined fusion plasmas. However, strong space-charge effects often degrade transport efficiency, particularly for heavy ions such as Au-. In this study, we demonstrate a substantial improvement in beam transport by introducing an electrostatic lens effect through optimized voltage allocation in a multistage acceleration system. Numerical simulations using IGUN, supported by experiments with the LHD-HIBP system, show that this approach effectively suppresses space-charge-induced beam divergence and loss. Without requiring mechanical modifications to the beamline, the optimized configuration enables a 2-3 fold increase in Au-beam current injected into the tandem accelerator. Consequently, plasma potential measurements were extended to higher-density plasmas, reaching line-averaged electron densities up to $1.75\times 10^{19}$ m$^{-3}$ with improved signal-to-noise ratio. This technique offers a compact, practical, and highly effective solution for transporting high-current heavy-ion beams under space-charge-dominated conditions. Beyond its impact on plasma diagnostics, the method is broadly applicable to a wide range of accelerator systems, including those used in scientific and industrial applications where high-intensity beam transport is required.

physics.plasm-ph↗

arXiv2023

Roadmap towards the redefinition of the second

This paper outlines the roadmap towards the redefinition of the second, which was recently updated by the CCTF Task Force created by the CCTF in 2020. The main achievements and the open challenges related to the status of the optical frequency standards, their contribution to time scales and UTC, the possibility of their comparison and the knowledge of the Earth's gravitational potential at the necessary level of uncertainty are discussed. In addition, the mandatory criteria to be achieved before redefinition and their current fulfilment level, together with the redefinition options based on a single or on a set of transitions are described.

physics.atom-ph↗

arXiv2018

First observation with global network of optical atomic clocks aimed for a dark matter detection

We report on the first earth-scale quantum sensor network based on optical atomic clocks aimed at dark matter (DM) detection. Exploiting differences in the susceptibilities to the fine-structure constant of essential parts of an optical atomic clock, i.e. the cold atoms and the optical reference cavity, we can perform sensitive searches for dark matter signatures without the need of real-time comparisons of the clocks. We report a two orders of magnitude improvement in constraints on transient variations of the fine-structure constant, which considerably improves the detection limit for the standard model (SM) - DM coupling. We use Yb and Sr optical atomic clocks at four laboratories on three continents to search for both topological defect (TD) and massive scalar field candidates. No signal consistent with a dark-matter coupling is identified, leading to significantly improved constraints on the DM-SM couplings.

physics.atom-ph↗

arXiv2017

Advanced Satellite-based Frequency Transfer at the 10^{-16} Level

Advanced satellite-based frequency transfers by TWCP and IPPP have been performed between NICT and KRISS. We confirm that the disagreement between them is less than 1x10^{-16} at an averaging time of several days. Additionally, an intercontinental frequency ratio measurement of Sr and Yb optical lattice clocks was directly performed by TWCP. We achieved an uncertainty at the mid-10^{-16} level after a total measurement time of 12 hours. The frequency ratio was consistent with the recently reported values within the uncertainty.

eess.SP↗

arXiv2014

Direct comparison of optical lattice clocks with an intercontinental baseline of 9 000 km

We have demonstrated a direct frequency comparison between two $^{87}{\rm Sr}$ lattice clocks operated in intercontinentally separated laboratories in real time. Two-way satellite time and frequency transfer technique based on the carrier phase was employed for a direct comparison with a baseline of 9 000 km between Japan and Germany. A clock comparison was achieved for 83 640 s resulting in a fractional difference of $(1.1\pm1.6) \times 10^{-15}$, where the statistical part is the biggest contribution to the uncertainty. This measurement directly confirms the agreement of the two optical clocks on an intercontinental scale.

physics.atom-ph↗

arXiv2012

Stability Transfer between Two Clock Lasers Operating at Different Wavelengths for Absolute Frequency Measurement of Clock Transition in 87Sr

We demonstrated transferring the stability of one highly stable clock laser operating at 729 nm to another less stable laser operating at 698 nm. The two different wavelengths were bridged using an optical frequency comb. The improved stability of the clock laser at 698 nm enabled us to evaluate the systematic frequency shifts of the Sr optical lattice clock with shorter averaging time. We determined the absolute frequency of the clock transition 1S0 - 3P0 in 87Sr to be 429 228 004 229 873.9 (1.4) Hz referenced to the SI second on the geoid via International Atomic Time (TAI).

physics.atom-ph↗

arXiv2006

Narrow Line Photoassociation in an Optical Lattice

With ultracold $^{88}$Sr in a 1D magic wavelength optical lattice, we performed narrow line photoassociation spectroscopy near the $^1$S$_0 - ^3$P$_1$ intercombination transition. Nine least-bound vibrational molecular levels associated with the long-range $0_u$ and $1_u$ potential energy surfaces were measured and identified. A simple theoretical model accurately describes the level positions and treats the effects of the lattice confinement on the line shapes. The measured resonance strengths show that optical tuning of the ground state scattering length should be possible without significant atom loss.

physics.atom-ph↗