Booster
lattice | optics | power supplies | injection/extraction | RF system | diagnostics | vacuum system | transfer line 2
The injection system for the ESRF 6 GeV Storage Ring comprises a 200 MeV linear Pre-Injector and a full energy fast cycling Booster synchrotron. The main features of the booster, a 10 Hz cycling frequency and a low natural beam emittance, were chosen to ensure a fast filling rate of the Storage Ring.
| Repetition Rate |
10 Hz
|
| Energy |
6 GeV
|
| Circumference |
300 meters
|
| Emittance @ 6 GeV |
1.2.10-7 mrad
|
Lattice
The magnet lattice structure was designed to obtain an equilibrium emittance of the order of 10-7 p m.rad after acceleration to 6 GeV. Further objectives were : minimisation of the aperture requirements for the injected linac beam (in order to minimise costs for magnets and power supplies), a low sensitivity to magnet alignment errors and space preservation for additional lattice components. It is based on a simple separated function FODO arrangement of magnets. At three straight sections, separated by an azimuthal angle of 120 degrees, two adjacent cells with only one bending magnet create a vanishing dispersion function in the straight regions. One of these sections is dedicated to the installation of the two RF cavities whereas the two others are well suited to accommodate the injection and extraction elements. The location of the dispersion suppresser regions generates a threefold supersymmetry of the booster. Each superperiod is composed of 13 elementary cells and is mirror symmetric with respect to its centre.
Optics
The maximum of the beta functions are b x = 13.4 m and b z = 13.5 m, whereas the dispersion function Dx varies from 80 cm down to 3 cm in the "missing magnet" region. In order to correct the natural chromaticities z x = -14.9 and z z = -12.8, two different sextupole families have been installed near the focusing and defocusing quadrupoles. Due to the time varying sextupole component induced by eddy currents in the dipole vacuum vessels, these natural chromaticities are changed at injection to
z x = -2.4 and z z = -26.4 (with the 0.3 mm thick vacuum vessel wall).
Power supplies
The main power supplies feed independently the dipole circuit, the focusing quadrupole circuit and the defocusing quadrupole circuit. The 10 Hz variation of the fields in these magnets is created by biased sine wave currents. The classical "White circuit" was chosen to resonate the magnets because it minimises the real and reactive power variations seen from the mains and size of the power electronics.
The 10 Hz inverter is a pulse width modulation (PWM) current source driven by a gate turn off (GTO) thyristor bridge.
The two AC quadrupole circuits are frequency and phase locked on the dipole circuit which, running on its natural resonance, is the "heart" of the 10 Hz timing system. The injection and extraction processes are synchronised on the zero cross-detection of the dipole field.
Injection, extraction
The on-axis injection is performed in one turn using a 10 degree pulsed septum magnet (inside vacuum) and a 5 mrad fast kicker magnet (40 ns fall time). After the creation of a local beam bump by three slow bumpers (3 ms half sine wave), the high energy beam is extracted from the booster in one turn using a 1 mrad fast kicker (40 ns rise time) and two pulsed septum magnets (8 mrad and 4.6°).
RF system
Acceleration is performed by two LEP type cavities, each equipped with two windows. The voltage in the cavities is pulsed at 10 Hz. The modulation is performed by driving the RF input power of the 1 MW klystron which feeds the two cavities at 352.2 MHz. The total power varies from 300 W at capture up to 550 kW at the end of the acceleration. The peak voltage in each cavity is 3.65 MV.
Diagnostics
The main diagnostics tool is the beam position monitor (BPM) system which measures the beam positions at the 75 locations where the BPMs (consisting of four button electrodes) are installed. These measurements (performed on 352.2 MHz signals) enable an application program to compute and draw the closed orbit errors at any time during the acceleration cycle. Each BPM block has been aligned relative to the next quadrupole. The absolute accuracy of the full system is better than 0.5 mm.
Eight fluorescent screens spread over the machine circumference can be inserted to check the progression of the beam during the first turn. Similar movable fluorescent screens are used on the two transfer lines to centre the beam all along. The beam intensity is measured by a current transformer which has a band pass from DC to 100 kHz. The tunes are measured using a tune monitoring system which frequency analyses the beam response to Jitter Free excitement created by two "shakers (horizontal and vertical).
Vacuum system
The stainless steel vacuum chamber of the booster is mainly composed of long thin wall vessels (0.3 mm thick, reinforced by ribs) installed in the gaps of the magnets coupled to short thick vessels where the 80 ion pumps (45 l/s) are located. These vessels are connected together by KF type flanges which are coated with enamel in order to provide the electrical insulation which is necessary to prevent current circulation in the vessel along the ring. The vacuum system is split into nine sections which can be isolated by remote controlled valves.
The UHV conditions imposed during the manufacturing process, associated with the distributed pumping are the reasons for the good static pressure which is achieved over three quarters of the ring without any bakeout. Nevertheless, due to the presence in the vacuum system of the laminated septum magnets, the pressure reaches 3 x 10-7 mbar in the extraction region. During a permanent run with beam the pressure in the whole ring is in the 5 x 10-8 mbar range, which does not affect the performance of the machine.
The transfer line which brings the 6 GeV electron beam from the booster to the storage ring is 66m long. It comprises 5 AC booster type dipoles (which are connected in series with the other booster dipoles and are de-facto at the good field level when the 6 GeV beam is extracted), 14 DC quadrupoles and 17 DC steerers. The beam positions are controlled using fluorescent screen monitors. The last vacuum part of the transfer line was baked in order not to pollute the vacuum in the storage ring injection part.