Figure 1: Layout of SR Source Indus-1

Electromagnetic radiation generated by bending the path of electrons moving at speeds close to that of light is called synchrotron radiation (SR). It has emerged as a powerful tool for research in several areas such as materials science, chemistry, biology, medicine and semiconductor technology. Considering its widespread utility, it was decided to build two synchrotron radiation sources at Centre for Advanced Technology, Indore1,2. These sources are Indus-1, a 450 MeV electron storage ring for production of VUV radiation and Indus-2, a storage ring of 2.5 GeV for x-rays. In the first phase, the development of the synchrotron radiation source Indus-1 was taken up. This project involved the development of a 450 MeV electron storage ring and also an injector system which supplies electrons to it. The injector system consists of a 20 MeV microtron and a 450/700 MeV synchrotron. This injector system will also serve as the injector for Indus-2. The synchrotron radiation source Indus-1 thus consists of a 20 MeV microtron, a 450 MeV synchrotron and a storage ring is shown in Figure 1. The electrons are generated and accelerated to 20 MeV in the microtron. After extracting the beam from the microtron, it is injected into the synchrotron; in which the energy of the electrons is increased from 20 MeV to 450 MeV. After acceleration to 450 MeV, the electrons are extracted from the synchrotron and then transported to the storage ring Indus-1. This process of production, acceleration and injection is carried out every one second till the stored current becomes 100 mA. In Indus-1, the electron beam keeps circulating for few hours emitting synchrotron radiation continuously in the dipole magnets. This source was successfully commissioned in 1999 and since then it has been routinely operated.

2. Indus-1 storage ring

Indus-1 is a 450 MeV3,4 storage ring designed to provide radiation in the range 30 - 2000 Å. It is a small ring having a circumference of 18.96 m. The critical wavelength of the radiation emitted from its four 1.5 T bending magnets is 61 Å. A 3 T wiggler is planned in this ring to provide the radiation of critical wavelength 31 Å. The flux and brightness of the radiation from these sources are shown in Figure 2(a, b).

The magnetic lattice of the ring consists of 4 super periods, each having one dipole magnet with a field index of 0.5 and two doublets of quadrupoles (Figure3). The field index of 0.5 in the dipole magnets helps in achieving a larger stability range because it provides weak focussing in radial and vertical planes. Each super-period has a 1.3 m long straight section. Such straight section are used for beam injection; one section accommodates the septum magnet and the other diametrically opposite to it accommodates a pulsed kicker magnet. Of the remaining two straight sections, one is used to accommodate an RF cavity and the other, a 3 Tesla wiggler in future. To correct the natural chromaticity of the ring, which arises due to different focusing of different energy particles, is corrected using a pair of sextupoles in each super period. The sextupole field gives rise to nonlinear kicks to the beam and tracking studies are carried out to find out which of the particles remain stable after getting several such kicks.

       
Figure2a: The photon flux as a function of photon energy.         Figure2b: The source brightness as a function of photon energy.

The region of stability is called a dynamic aperture and in Indus-1, since the dynamic aperture in the presence of the sextupoles is much larger than the vacuum chamber aperture, the performance of the ring is not adversely affected. The circumference of Indus-1 ring has been chosen as 2/3 of that of the synchrotron. The RF frequency of 31.619 MHz provides the electron beam with the additional energy equivalent to the energy radiated by the electrons in the form of synchrotron radiation. It thus keeps the electrons moving on a fixed orbit with a fixed energy.

The ring has a wide tuning range and, the present operating tune point is (1.69,1.31). The parameters of Indus-1 at this operating point are given in Table-1. The photograph of Indus-1 storage ring including initial parts of beam lines is shown in figure 4. In order to achieve a beam lifetime of a few hours, the pressure in the vacuum chamber in the presence of 100 mA electron beam has to be in 10-9 mbar range.

The microtron5-8 is designed to give a 20 MeV electron beam with a current of 25 mA in pulses of 1µs duration at a repetition rate of 1-2 Hz. The beam from the microtron is transported to the synchrotron through transfer line-1 (TL-1), which has a length of about 14m. It has 3 quadrupole doublets and one dipole magnet to take care of the beam parameters as required by the injection process. The magnetic lattice of the synchrotron consists of 6 superperiods, each consisting of a dipole magnet to bend the electron beam on a circular path and a pair of focusing and defocusing quadrupole magnets to achieve the required stability and tuning. The maximum magnetic field of the dipole is 1.32T.

Figure 3: The lattice functions of Indus-1

The circumference of the synchrotron is 28.44 m. The electrons are injected into the synchrotron by adopting a multi-turn injection scheme using 1 ms long electron beam pulse from the microtron at a repetition rate of 1 Hz. A compensated bump producing maximum amplitude near the injection septum is produced using three injection kickers. After injecting the beam, the electrons are accelerated to 450 MeV in nearly 200 ms following a linear ramp using an RF cavity operating at 31.619 MHz. Fields in the dipole, quadrupole and steering magnets are synchronously increased during the acceleration. The harmonic number of the ring is three, giving rise to three circulating bunches in the ring. The accelerated beam is extracted by deflecting it by a fast kicker magnet, having a rise time 45 ns. As the separation between two bunches is 32 ns, during the extraction process, one out of three bunches is lost and two bunches are extracted. These two bunches are then transferred to Indus-1 through TL-2. The synchrotron operates at a repetition rate of 1 Hz. The vacuum in the synchrotron is in 10^-7 mbar range. The length of the transfer line-2 is about 25 m. The line has four quadrupole doublets and two dipole magnets to match the beam parameters at the injection point in Indus-1. The dipole magnet before Indus-1 will be kept off while transporting the 700 MeV beam from the synchrotron to Indus-2. We report here the performance of the storage ring Indus-1 after the shut down taken due to water scarcity during the summer of 2002. During this shutdown, some works including modifications of beam position indicators, field measurement of synchrotron magnets and optimization of the microtron were undertaken. The machine was put regular operation on 26^th of September 2002. Till 31^st March’03, 35.89AH of operation has been made. Figure 5 shows a typical operational behavior of Indus-1 during a day.  The maximum stored current achieved after the shutdown is 172mA. The maximum current achieved earlier was 200mA⁸.

Figure 4: Photograph of Indus-1 storage ring with beamlines

Figure5: Typical beam current variation during a day

Figure6: Beam injection pulses and injected current

We have not again attempted to achieve higher currents due RF power supply problems. Figure 6 shows the accumulated current (TR4A) along with the injected beam at the end of TL-2 (TR2A), the injection kicker pulse (TR3A) and the synchrotron extraction kicker pulse (TR1A). Circulating electron bunches may ionize residual gas molecules creating ions. The motion of some of these ions could be stable under certain beam conditions and they can get trapped. If the density of trapped ions is high, the motion of the electron beam can be perturbed and as a consequence, the lifetime of the beam is reduced. These ions can be removed by creating transverse electric field along the beam path. In Indus-1, 20% circumference is occupied with ion clearing electrodes. There is a provision to apply ±1kV DC voltage on these electrodes. The effect of different DC voltages on the beam lifetime has been studied. It is found that the beam lifetime improves as the voltage is increased. In Figure 7, the effect of ±1kV applied to the ion clearing electrode, on beam lifetime with current is shown. The -ve voltage gives a higher beam lifetime compared to the same +ve voltage. The reason for this is not yet fully understood. The beam lifetime which is 40 minutes (with 0 voltage on ion clearing electrodes) is increased to 60 minutes with -1kV DC voltage on the ion clearing electrodes. The beam lifetime in Indus-1 is mainly determined by the vacuum of the ring and particularly that of the cavity section (BAG4). The lifetime will improve further as the ampere-hour operation increases. The earlier maximum lifetime achieved is 75 minutes⁸. The vacuum is monitored in each straight sections by BA Gauges(BAG1-4) and is shown in Figure 8.

  
Figure7: Beam current vs beam lifetime          Figure 8 Vacuum as a function Beam current

The machine generally operates in two shifts on weekdays. The machine is shut down at 11:00 PM. It takes one to two hours to store beam in to the storage ring in the morning. If no major problem is faced, 100mA beam is delivered to the users by 11:00AM. Some of the problems during the machine operation include inadequate air conditioning during the summer months and the line voltage fluctuation from November to February every year. The voltage fluctuations problem will be solved after commissioning of the power conditioning system which has already been installed. At present the information on Indus-1 operation is available in CAT Web on One (URL http://202.141.112.177). The status of machine beam current as displayed is shown in Figure 9.

Figure 9: Web based display machine beam current

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