LNF Uses NI LabVIEW and PXI to Develop a New Radio Frequency Data Acquisition System as Part of the SPARC Project

Author(s):
M. Bellaveglia - INFN-LNF
G. Di Pirro - INFN-LNF
A. Gallo - INFN-LNF

Industry:
Research

Products:
NI-Scope, LabVIEW, PXI/CompactPCI

The Challenge:
Monitoring RF impulses (phase and amplitude) during the activities of a linear accelerator and performing phase stabilization of the device.

The Solution:
Using NI PXI hardware and LabVIEW software to realize a digital multi-channel oscilloscope capable of visualizing phase and amplitude signals originated by demodulated RF impulses.

The SPARC (Sorgente Pulsata Auto-amplificata di Radiazione Coerente) project, funded by the Italian Government in 2003 with a three year time schedule, is an R&D activity aimed to develop a high brightness photo-injector for self-amplified spontaneous emission free-electron laser (SASE-FEL) experiments.

As part of the project, the Laboratori Nazionali di Frascati (LNF) required a new acquisition system for RF signal monitoring and synchronization.  The machine is made up of a photo-injector RF gun and three room-temperature accelerating sections driving a permanent magnet adulator to generate radiation in the range of green visible light and its higher harmonics.

The installation of the machine at LNF started on September 2004 and the first RF gun tests and beam parameters measurements started in February 2006. These initial tests provided encouraging results. The final SPARC configuration is composed of an RF gun driven by a laser producing 10ps flat top pulses that hit a photocathode. The resulting 5.5MeV beam is injected into three SLAC type accelerating sections that bring the beam to an energy level of 150MeV to feed a 14m long ondulator. To meet the requirements of the machine, one of the most important tasks is to have a time jitter of the accelerating fields relative to the laser pulse time arrival less than ±1ps (this corresponds to a phase variation of radio frequency fields of ±1).

Control system
The control system is performing well and it is ready to be extended to the SPARC full configuration. The SPARC main server is a Red Hat Enterprise Linux 3 (RHEL3) operating system, and is situated in a Linux Terminal Server Project (LTSP) configuration so that the consoles are identical diskless workstations. The photo-injector device drivers are installed in industrial PCs placed in the bunker. Every front-end driver or console application is written in National Instruments LabVIEW development system. Two machines form a connecting bridge from the front-end industrial PCs to the control room consoles, the data server and the command server. The data server accepts a request of information from the consoles and sends the data from the proper industrial PC. The data can be software variables (that identify the controlled devices), sampled signals, images, or information about the status of the computer itself. The command server elaborates the queue coming from the consoles and, once identified legal commands, it delivers them to the front-end PCs to control the photo-injector devices.

RF Data Acquisition System Signal monitoring
The core of the synchronization system is the demodulation board and the digitizers that interface with an industrial PC, where data analysis and device control is accomplished. The quadrature IF mixer from Pulsar Microwave Corporation is a special device that provides the demodulated in-phase (I) and in-quadrature (Q) components of the signal under measurement. This is very useful to monitor the phase and amplitude of a signal because we can obtain these quantities from raw values applying a simple mathematical algorithm. The waveform is digitized using NI PXI-5105 data acquisition devices that are 12bit 60Msamples/s A/D converters. This system allows a real time monitor of amplitude and phase of the RF pulses along the machine. They are easily displayed on a monitor of the accelerator control room.

Synchronization
In order to implement a shot-to-shot phase monitor at the 10Hz repetition rate of the machine, we decided to analyze the acquired waveform inside the same software application running in the front-end industrial PC. In this way, only a number, representing the phase for each location of interest, is sent to the control room. Moreover, the control system has been designed to perform a “one-click” phase noise measurement along the whole machine.

To reduce the phase noise and to enhance the performances of the photo-injector, we implemented a phase feedback that analyzes the consecutive acquired values and controls a motorized phase shifter to compensate slow drifts. Upgrades to the system allowed us to characterize the time jitter of the laser pulses with two kind of measurements, a high frequency (79.33MHz) measurement that permits the characterization of the IR laser oscillator and a pulse-to-pulse jitter measurement (UV pulses, 10Hz rep. rate) accomplished at the end of the laser amplification chain.  

The measured time jitters are well inside the SPARC requirements and the monitor system has enough resolution and speed to accomplish its task. The system modularity will allow an easy extension to monitor and control the whole machine RF after the final installation. The next goal to achieve is to implement a feedback using the data acquired from the laser to correct time drifts of the UV pulses.

 

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