Indus-2 currently operates with maximum beam current of 200mA at 2.5 GeV energy. Measuring the correct orbit position is an important aspect for enhancing the performance of the machine in terms of reduced closed orbit distortion (COD), higher beam life time, reduced machine tune variation and reducing the corrector strengths. This is also important for commissioning of Insertion Devices (Undulators and Wigglers) with reduced chamber aperture. COD can be reduced when we know the offset between electrical axis of Beam Position Indicator (BPI) and magnetic axis of adjacent Quadrupole Magnet (QP). This offset is measured with the help of electron beam itself using a procedure known as Beam Based Alignment (BBA).

Beam Based Alignment (BBA) is a technique for measurement of offset between electrical axis of Beam Position Indicators (BPI) and magnetic axis of adjacent Quadrupole Magnet (QP) using electron beam itself. Measurement and compensation of this offset is required to reduce close orbit distortion (COD). BBA works on the principle of steering the beam to pass through the center of QP leading to no change in the beam position at nearby BPI with change in QP strength. This requires independent setting of currents in all the QP magnets of Indus-2 ring. Since the existing system has several QP magnets in series, separate Active Shunt Power Supplies (ASPS) are used to connect to each QP magnet to control the current passing through them. The control system for BBA controls all such shunt power supplies. The BBA system comprises of ASPS, the control system for all 72 ASPS and a software application implementing the BBA algorithm and for automating the complete process. The system for control & automation of BBA comprises of following components:

1. Equipment Control Stations (ECS) which interface with Active shunt power supplies (ASPS). The ECS are part of the overall control system, following the three layer architecture of Indus-2 control system.
2. Software application for BBA algorithm and process automation.

1. Control System for Active Shunt Power Supplies (ASPS)
This is based on three layer architecture for seamless integration with existing Indus-2 control system. The lowest layer (layer-3) which interfaces to the ASPS comprises of Versa Module Europa bus (VME) based system providing stable bipolar reference signal and read back with 16 bit accuracy. This system caters to 72 ASPS and will also serve for performing linear optics by closed orbit (LOCO) experiments.

2. Software application for BBA algorithm & process automation
The process of finding the offset is very lengthy and tedius thus gets benefitted by full automation. The BBA algorithm is implemented in the SCADA software WinCCOA. This software application consists of a graphical user interface panel as front-end part and a back-end application programming manager (API) to interface to the Matlab application for curve fitting. The architecture of this application is depicted in the figure-1.
This application provides features like configuration file based operation for flexibility, data logging for all measurements and calculation of merit function. The software has provision for selection of individual/all BPIs in one or both transverse planes. The software takes care of faulty/non-working BPIs dynamically thus providing ruggedness.
The GUI software also provides online editing of all the configuration values.

The tight COD resulting from BBA has immensely helped in the commissioning of the three undulators.