Modeling and control of transverse coupled bunch mode levels in Indus-2 using artificial neural network.

Transverse coupled bunch instability (TCBI) is a major concern at high beam current operations at all synchrotron light sources. Techniques for the mitigation of TCBI include higher order mode tuning of RF cavities, optimization of vacuum chamber designs, increasing the damping rate of beam oscillations, optimization of betatron tune values, and multi-bunch feedback systems. Due to uncertainties, time-variation, and disturbances, the dynamic behavior of accelerators requires further tuning of beam parameters beyond theory-based set points for minimizing the transverse coupled bunch mode (TCBM) instability. In this work, an artificial neural network (ANN) based system is developed to minimize average TCBM levels in the Indus-2 synchrotron light source at the Raja Ramanna Centre for Advanced Technology in Indore, India. The ANN is trained based on various TCBM measurements collected at the Indus-2 for various values of betatron tune and chromaticity in order to learn how to map beam measurements directly to parameters such as optimal betatron tunes and chromaticity values that are sent to a beam feedback control system. The ANN takes as input real-time beam data and is coupled to a feedback controller, thereby creating an adaptive feedback that is able to adjust in real time to variation of the accelerator and beam. We provide a detailed overview of our approach as well as experimental results in which the ANN-guided feedback approach increases the operational beam current of Indus-2 from a limit of ∼170 mA up to ∼230 mA within ∼21 min. We believe that this general method can be useful for a wide range of synchrotron sources operating at high bunch currents.

Application of XANES in gamma dosimetry.

The thermoluminescence material, CaSO4:Dy, is widely used for the dosimetry of ionizing radiation due to its high sensitivity, low fading and wide dose range from µGy to few tens of gray. However, its application is limited at high doses due to non-linear and saturation effects. In this paper, X-ray Absorption Near Edge Structure (XANES) studies at the Dy L3-edge have been carried out on CaSO4:Dy discs exposed to gamma doses in the range 0-1000 Gy. The results show an increase in white line in XANES spectra with gamma dose. Structural change in CaSO4:Dy also has been studied using X-Ray Diffraction (XRD) and has found no structural change up to 1000 Gy. The study indicates that XANES can be used as an alternative dosimetry technique and is useful in the evaluation of absorbed dose in the case of accidental exposure to high radiation in a radiation facility or during a radiological accident.

Measurements of aperture and beam lifetime using movable beam scrapers in Indus-2 electron storage ring.

In this paper, the measurements of vertical and horizontal aperture which are available for stable beam motion in Indus-2 at beam energy 2.5 GeV using movable beam scrapers are presented. These beam scrapers are installed in one of the long straight sections in the ring. With the movement of beam scrapers towards the beam centre, the beam lifetime is measured. The beam lifetime data obtained from the movement of vertical and horizontal beam scrapers are analyzed. The contribution of beam loss due to beam-gas scattering (vacuum lifetime) and electron-electron scattering within a beam bunch (Touschek lifetime) is separated from the measured beam lifetime at different positions of the beam scrapers. Vertical and horizontal beam sizes at scrapers location are estimated from the scraper movement towards the beam centre in quantum lifetime limit and their values closely agree with measured value obtained using X-ray diagnostic beamline.

Measurement of bunch length in Indus-1 storage ring using fast photodiode.

The length of electron bunches in a storage ring is an important parameter for both synchrotron radiation users and accelerator physicists. Several methods are used for measurements of bunch length using electronic and optical instruments. We have measured temporal profile of synchrotron radiation emitted from dipole magnet of Indus-1 by using fast photodiode. Bunch length is calculated by assuming gaussian profile for the particles inside bunch. The results show that bunch length is increasing with the decrease of gap voltage of RF cavity. These measurements were carried out at low beam current; at high voltage results are in close agreement with theory and the values estimated using ZAP code. In the second experiment, the results show that bunch length increases with the increase of beam current inside the bunch, and above threshold current, it follows Chao-Gareyte scaling law. The longitudinal broadband impedance for Indus-1 SRS was estimated using Keil-Schnell criterion and results were compared with theoretical estimated values using ZAP code.

Measurement of parameters in Indus-2 synchrotron radiation source.

The paper presents the measurement of optics parameters in Indus-2 synchrotron radiation source, which include betatron tune, beta function, dispersion function, natural chromaticity, corrected chromaticity, central RF frequency, momentum compaction factor, and linear betatron coupling. Two methods were used for beta function measurement; a conventional quadrupole scan method and a method using the fitting of the orbit response matrix. A robust Levenberg-Marquardt algorithm was used for nonlinear least square fitting of the orbit response matrix. In this paper, detailed methods for the parameter measurements are described. The measured results are discussed and compared with the theoretical values obtained using accelerator simulation code Accelerator Toolbox in MATLAB.