At DESY the Synchrotron Light Source PETRA III offers scientists outstanding opportunities for experiments with hard X-rays of exceptionally high brilliance since 2009. Research activities have been started towards a future upgrade scenario of PETRA III which envisions the conversion of the PETRA ring into a ultra-low emittance hard X-ray radiation source: PETRA IV. The lattice design is aiming for a horizontal emittance in the range between 10 pm rad and 30 pm rad at a beam energy of 6 GeV. Two different approaches have been considered for the lattice design: a design based on a hybrid multibend achromat with an interleaved sextupole configuration based on the ESRF design, and a lattice with a non-interleaved sextupole configuration with a special phase space exchange configuration. We are reporting the current status of the design activities including studies related to the injector.

A record low horizontal emittance of 1 nm-rad was successfully obtained in PETRA III third generation synchrotron light source. A key system that allowed reaching such value includes 20 permanent magnet damping wigglers installed in two long straight sections. The wigglers radiate almost 1 MW at maximum current of hard X-ray radiation which issues a challenge for the design of SR absorbers and vacuum system components for the damping wiggler section. The paper describes in detail the design consideration, manufacturing and experimental performance of absorbers and vacuum system. The first experimental results of PETRA Ill damping wiggler section operation are presented. (C) 2011 Elsevier Ltd. All rights reserved.

In order to fulfill the demands of a high brilliance synchrotron light source like PETRA III different feedback systems are required. The high brilliance is accomplished by high beam current of 100 mA and very small transverse emittances. The current in PETRA is limited by coupled bunch instabilities to rather low values and powerful longitudinal and transverse feedback systems are necessary to achieve the design current. A careful design of the feedback is required in order to avoid any kind of beam quality degradation such as beam blow up due to noise. Additional requirements on signal processing are: very high dynamic range, adaptive signal adjustment, very high sensitivity to beam oscillations, high resolution and very high bandwidth. This contribution will describe the most important components and their properties. Results of the feedback operation will be presented and discussed. The design current of 100mA has been achieved without the indication of emittance growth and the feedback has been operated reliably during the last user period. PRINCIPLE SYSTEM LAYOUT The principle functionality of multibunch feedback systems hasn’t changed since the first digital processing system came into operation at PETRA in the late 80 [1]. In order to damp coupled bunch instabilities, transverse and longitudinal oscillations of each bunch have to be detected, shifted by 90 degree in Phase and feed back to the according bunches. To meet the requirements of modern synchrotron light sources serious improvements in respect of signal processing quality and stability are essential. PETRA is sectioned into octants, and the beam instrumentation electronics is located in eight buildings around the 2.4 km circumference. The instrumentation buildings are referred to as the cardinal points and they are about 300m far apart from each other. The accelerating RF transmitters and the central machine timing system is located in the south, the transverse feedback system is arranged in south-east and the longitudinal feedback system is installed in the east building. A basic requirement for the flawless function of MBFB components that are working together is a set of noiseless machine synchronous reference frequencies. Long term phase stability can only be provided by a PLL source oscillator that must be tracked to the locally detected beam phase. Phase changes caused by RF phasing or by temperature drifts of any components must be compensated to less than 10ps. The required resolution for transverse beam oscillations is in the sub micron range. This sensitivity can only be achieved by compensating for beam position offsets at the MBFB beam position monitors. Saturation detection and attenuation control of the cascaded amplifier stages inside the beam oscillation detector is a necessary feature to assure robust operation of the system over a wide range of bunch currents. DESCRIPTION OF THE MAIN FEEDBACK COMPONENTS The Beam Oscillation Detector The minimum bunch spacing in PETRA III is 8ns, therefore the double sideband spectral components covering multibunch oscillations are periodically repeated with 125MHz. The spectral information of betatron beam oscillations is appearing as AM sidebands around the machines revolution lines. The principle functionality of the beam oscillation detector is the direct-to-baseband conversion of these AM sidebands. The LO frequency used for down conversion was chosen to be 500MHz. (fRF) It is provided by an external module, that is described in the next chapter. As the detectors block diagram in Fig. 1 points, before down conversion, the input signal passes two amplifier and attenuator stages. The first amplifier has very low noise characteristics. Its output level is controlled to be about 20% below 1dB compression. The second amplifier matches the output of the first stage to the down converters RF input level. The attenuator in front of it is normally not varied, but it can be adjusted to less attenuation in order to detect beam oscillations at very low machine currents. The down converter consists of a double balanced high level mixer. In order to allow a compensation for the unwanted revolution harmonics that are produced by a non centred beam at the BPM, the mixers IF output must be capable to operate down to DC. As a consequence of this, the final amplifiers must achieve a linear transfer characteristic from about 100MHz down to DC. The beam oscillation detector device contains three sections for the transverse and the longitudinal plane. The phase detection part has only one amplifier attenuator stage in the RF input path, since the input signal is much higher than for the transverse detectors. The required sensitivity to phase oscillations is gained after down conversion to the base Figure 1: Diagram of a transverse detector channel. TUPD81 Proceedings of DIPAC2011, Hamburg, Germany 494 C op yr ig ht c ○ 20 11 by th e re sp ec tiv e au th or s— cc C re at iv e C om m on sA tt ri bu tio n 3. 0 (C C B Y 3. 0) 08 Feedbacks and Beam Stability band. The beam oscillation detector was decided to be an in house development, because the specifications of a commercially available device did not suit our needs. The Carrier Frequency Generator PLL The basic fundament for different devices such like modulators, synchronous detectors, digital signal processing and sampling devices that are jointly operating at a certain location, is the central generation of all needed reference frequencies. Synchronous references must have a fixed and defined phase relation to each other. The quality of beam oscillation detection depends on the provided reference frequencies used by the detectors down conversion. But also the ADC and DAC clock frequencies should be of pure quality and always phase locked to the devices working together. The central unit that generates these references is a voltage controlled crystal oscillator that is part of a phase locked loop. The crystal has been ordered to match the PETRA RF frequency that is 499.6655 MHz. Its phase noise is about -140 dBc at 1kHz. The adjustable frequency range of about 20 kHz is small and its span should only allow for the maximum RF frequency changes that may occur during machine studies. The machines timing system delivers a 125 MHz reference, but this cannot directly be used because of improper phase drifts related to the locally detected bunch signals. In addition, due to the long transmission distance from the central distribution system, the noise behaviour is not adequate for the MBFB operation. A phase shifter at the 125MHz reference input is used to adjust the whole system to the detected bunch phase. The PLL closed loop cut-off frequency has been chosen to be about 100 Hz. It must be high enough to ensure the proper tracking to phase changes during machine operation procedures like RF phasing or chromaticity measurements. The Beam Position Monitors A high sensitivity as well as best impedance matching to avoid signal reflections can be achieved with strip line monitors. The maxima of magnitude response exists at   l c n fc 4 1 2   (1) Since the detectors input frequency range is 500±62.5 MHz, the BPM consists of l = 15cm long strip lines that are embedded into an elliptical chamber profile. Each direction of the MBFB system has its own strip line monitor that is installed at high β positions in the ring. The Transverse Beam Offset Compensation Bridge To preserve the monitor strip lines from being irradiated by the synchrotron light they are not aligned in the transverse plane. Also they are not orthogonal because of the elliptical BPM profile. In order to monitor for transverse oscillations the sum of opposite lying strip lines must be subtracted. But if the beam is not exactly centred to the BPM this difference will not be zero. A constant beam offset produces spectral components at multiple of the bunch revolution frequency that may arise to very high magnitudes in respect of the oscillation signal. These offsets are limiting the dynamic range and they are leading to saturation within the detector device. A 180 degree hybrid produces the difference signal which is sampled by the ADC. The beam position offsets are being compensated by controlling two pin diode attenuators forming a balanced bridge. The Digital Signal Processing Unit This device consists of a single printed circuit board that covers all digital signal processing including AD and DA conversion. Three of these boards are required for the longitudinal and transverse directions. Each board is equipped with three 16 bit / 130MS/s ADCs and two 16 bit 160MS/s I-Q DACs. Additional lower bandwidth ADCs and DACs are used for external offset and phase controlling and for auxiliary input / output purpose. To be insensitive against mains failures, the current configuration settings are always stored in EEPROM. A standard USB interface is used to be connected to the server PC. Digital signal processing is executed by an ALTERA Stratix II FPGA. Fine adjustments of the ADC and the DAC clock timing can be accomplished during signal observation. The internal “multi bunch offset compensation” is an optional feature that can be enabled in the case of inhomogeneous machine fillings. Individual bunch signal offsets due to beam loading for instance, may be eliminated here. A similar feature is the “multi bunch band pass” that is optional as well. By enabling this, a configurable band pass is inserted for each bunch. This leads to an enhanced isolation of different tune signals that may occur due to coupling effects. Since each bunch can be described as a harmonic oscillator, the MBFB system works by increasing the damping term, implied by the additional DFB in the general equation of motion:         0 2 2     t x t x D D t x FB     (2) Thus the sampled beam oscillation of each bunch must be shifted by π/2 before feeding back to itself. Actually the phase must be adjustable because the signal p

At DESY the PETRA ring is operated as a synchrotron radiation facility with a very low emittance of 1 nm. Regular user operation has started in summer 2010. A summary of observations and measurements of intensity dependent single bunch effects is presented in this report. The longitudinal impedance of the ring is estimated from the measured bunch length versus beam intensity. The results are compared with predictions from the impedance model. Furthermore measurements of the single bunch intensity limit due to the transverse mode coupling instability (TMCI) are reported. The tune and phase shift around the ring have been measured as a function of the beam intensity. At PETRA III tune spectra have been observed with characteristics which have been observed at other storage rings in connection with electron cloud effects. The present status of the observations of potential electron cloud effects is also discussed.

Orbit stability is a crucial and import issue of 3rd generation light sources. Ambient mechanical and electrical noise cause rather large orbit distortions which have to be counteracted by an orbit feedback. Extensive studies of the orbit distortions in PETRA III have shown that the frequencies of the ambient noise lie within a frequency range from about 0.01Hz to 100Hz. In this paper we describe the main components, their properties and the layout of PETRA III's orbit feedback. Furthermore experimental results on short and long term stability will be presented. It will be shown that the required orbit stability of ±0.5 µm in the vertical plane can be maintained over 50 h.

PETRA III is a 3rd generation synchrotron radiation light source which started commissioning in April 2009. Recently, first frequency map measurements have been made using the turn-by-turn capabilities of the beam position monitors and horizontal as well as vertical kicker magnets. The results are in good agreement with expectations from tracking studies performed with SixTrack.

A two-dimensional laser-wire scanner capable of measuring the transverse charge profiles of an electron (or positron) bunch has been constructed at the PETRA accelerator in DESY. The development of the system is explained in this paper, along with descriptions of its photon detector and laser system. Results of transverse profile scans are presented for both horizontal and vertical directions. The measurement error is 1.3% from a multi-scan measurement in the vertical direction, where single scans can be performed in less than 50s.

Since mid-2007, the existing storage ring PETRA at DESY is reconstructed towards a dedicated third generation hard x-ray light source. The reconstruction includes the total rebuilding of one-eights of the storage ring where the FODO lattice of the arcs is replaced by double–bend achromat (DBA) cells. Damping wigglers are installed to reduce the emittance down to the design value of 1 nm rad. In order to fully benefit from this low emittance, beam stability is a crucial issue. This paper presents an overview of the instrumentation and their latest developments.

The former electron and proton preaccelerator PETRA at DESY is currently reconstructed and will be converted into a high brilliant storage-ring-based X-ray source called PETRA III [1]. Commissioning of the machine is scheduled for January 2009. PETRA III will operate at 6 GeV electrons or positrons with 100 mA stored current and a design emittance of 1 nm rad. Top-up operation is planned right from the beginning to reduce changes in heat-load and thermal drifts to a minimum,. Suitable beam diagnostic instrumentation and machine protection systems have to be established to guarantee the low emittance, sub-micron beam stability and save machine operation. To ensure a very high availability of the beam in top-up mode, injector and pre-accelerator diagnostic systems are refurbished as well. This paper presents a complete overview of the instrumentation and their latest developments to achieve these requirements.

At DESY it is planned to convert the PETRA ring into a synchrotron radiation facility, called PETRA III. The wake fields of a tapered transition from the standard vacuum chamber to the small gap chamber of the insertion devices contribute significantly to the impedance budget of PETRA III. The computer codes MAFIA and PBCI have been used to determine the loss and kick parameters of the tapered transition. PBCI is a recently developed parallelized, fully 3D wake field code, which used a purely explicit, split-operator scheme to solve the Maxwell equation in the time domain.

At DESY it is planned to convert the PETRA ring into a synchrotron radiation facility, called PETRA III. The wake fields of a tapered transition from the standard vacuum chamber to the small gap chamber of the insertion devices contribute significantly to the impedance budget of PETRA III. The computer codes MAFIA and PBCI have been used to determine the loss and kick parameters of the tapered transition. PBCI is a recently developed parallelized, fully 3D wake field code, which used a purely explicit, split-operator scheme to solve the Maxwell equation in the time domain.

The project status for the reconstruction of the 2304 m long existing storage ring PETRA II into a third generation low emittance synchrotron radiation source is presented. To reach the design emittance of 1 nmrad at an energy of 6 GeV together with a beam current of at least 100 mA and a supply of 14 straight sections for the installation of undulators, several fundamental measures are foreseen. For the emittance of 1 nmrad a FODO lattice in seven arcs together with damping wiggler sections in two long straights are envisaged. One eighth of the ring gets new magnets arranged to DBA cells to provide space and the required optics for the undulators. A complete new separately supported vacuum system reduces drastically movements of beam position monitors and magnetic elements due to synchrotron radiation heat load. This is an important precondition for the orbit stabilization system. To beat multi bunch instabilities in order to get a high beam current a powerful wideband feedback system will be established. The planned topping up mode guarantees stable beam current and therefore stable thermal conditions for the experiments.

The current PETRA II Laser-Wire system, being developed for the ILC and PETRA III, uses a piezo-driven mirror to scan laser light across an electron bunch. This paper reports on the recently installed electron-beam finding system, presenting recent horizontal and vertical profile scans with corresponding studies.

After mid-2007, the present PETRA storage ring at DESY will be reconstructed towards a dedicated third generation light source operating at 6 GeV. An emittance reduction down to 1 nmmrad can be achieved by means of damping wigglers. 20 permanent magnet wigglers will be installed in two of the long straights of the machine. The wiggler segments are compact fixed gap devices surrounded by iron enclosures to reduce the leakage flux. Each device will provide a damping integral of 4 Tm per segment and generate a synchrotron radiation power of 42 kW. Every wiggler segment will be followed by an SR-absorber to protect all downstream components, the accumulated on-axis power of about 120 kW will be taken up by a final absorber at the damping section end. The wiggler’s magnetic design, field properties and correction schemes have previously been proven by a short prototype. At present, the first full length (4m) prototype wiggler has been assembled and characterized magnetically. Figure 1: Layout of a regular cell in the damping section.