The control system of the new Piave injector at LNL

G. Bassato, A. Battistella, S. Canella

INFN - Laboratori Nazionali di Legnaro

Legnaro (Padua) - Italy

INFN – LNL Identity Card

CORE RESEARCH ACTIVITIES

1. Nuclear Structure and Dynamics

2. Applications and Interdisciplinary use of ion beams and nuclear methods to biology, medicin and solid state physics

Accelerator Facilities at LNL

The main facility is constituted by a complex of two accelerators:

• the TANDEM XTU (by High Voltage Corp.) installed in 1982 and operating at a maximum Terminal voltage of 16MV.

• the superconducting Linac ALPI, in operation since 1995, capable of accelerating ions up to 700 MeV, at a maximum specific energy of 20 Mev/amu.

Layout of the Tandem-Alpi complex

The Linac ALPI

• The Linac is based on 12 QWR resonators made in bulk niobium operating at 80 MHz followed by 52 QWR resonators made in copper sputtered with niobium operating at 160 MHz.

• The average field gradient is 4.5 MV/m for niobium sputtered cavities and 6 MV/m for niobium bulk cavities at 7W of RF power.

The ALPI resonators

Low-ß resonators Medium and High-ß resonators

Why the PIAVE Project?(PIAVE = Positive Ion Accelerator for Very Low

Energy)

The new machine aims to:

• replace the Tandem as linac injector

• extend the mass of accelerated ions beyond A=100

• increase the beam current by a factor of 10 to 50

PIAVE Design Keys

• An ECR source installed on a 350KV platform produces ion beams with a mass up to 238U and current up to 5 uA.

• A very low velocity structure based on two superconducting RFQs provides the energy gain from = 0.009 to = 0.035

• A set of 8 QWR resonators provides the further energy step required to inject into the linac.

Layout of the PIAVE injector

SRFQ

ECR + LEBTQWR

ALPI

The Piave project challenge

The PIAVE RFQs are the only superconductingRFQs in the world.

L = 138 cm. (RFQ1) L = 75 cm. (RFQ2)Ø inner = 65 cm.

Maximum field gradient = 25 MV/m @ 10W

Top view of the cryostat housing the two RFQs

The Piave control system

Classic three tiers network based architecture (derived from ALPI)

• Linux PCs at the top level for the Operator Interface.

• VME/Vxworks systems used in the intermediate layer (for control and alarm management).

• Embedded controllers, where possible, at the lowest level.

General Control System Architecture

RESONATORCONTROLLER

PASSIVE DEVICE (AMPLIFIER)

POWER METER

RFINTERLOCKS

MOTOR DRIVER

RS232 ADCs, DACs RS232 DIGITAL I/O DIGITAL I/O

ETHERNET LINK

ETHERNET SWITCH

ETHERNET SWITCH

OPTICAL LINK

CONTROL ROOM

ACCELERATOR ROOM

VME SYSTEMS (VXWORKS)

LINUX PCs

Configuration of the VME systems (typical)

• Processor board: Motorola MV2100 (PowerPC)• Serial line controllers: based on 8 channel IP

Modules (Tews TIP866) plugged on a VME carrier VIPC616

• Analog and Digital I/O: XYCOM560 and XYCOM240

• Step motor controllers: home made design based on Altera FPGA (one VME board drives 8 motors)

• Operating system: VxWorks 5.4

An exception to the “standard architecture”:the ECR control system

• Industrial Linux PC on the platform instead of the VME crate.

• PCI cards (Equinox SST-16) to implement the RS232 links instead of the original terminal servers.

Linux industrial PC (local console)

opticalbreak

ECR platform350 Kv

terminalserver

HP3497A

ground voltageECR console

breakoptical

Software overview

• The system is scalable by modifying configuration files.

• The graphic interface is an extension of ALPI MMI and has been developed using X toolkit to reuse and maintain compatibility with the existing code.

• Standard BSD sockets used for network communications.

The control of RFQ resonators

Due to their mechanical structure, the RFQs are sensitive to different sources of disturbances, and in particular to:

• radiation pressure (Lorenz detuning) measured to be around 0.9 Hz(MV/m)2

• fluctuations in helium bath pressure, measured to be around 60 Hz/mBar (RFQ1) and 40Hz/mBar (RFQ2)

The “standard” phase lock method• The resonator operates in a self-excited loop

configuration.• Phase locking is achieved by providing the

resonator, through a vector modulator, with an in-quadrature power proportional to the phase error.

Phase Shifter

Amplifier

CavityVectorModulator

(CPM)

Q Phaseerror

Advantages and limitations

• The CPM-based method offers a very good separation (low cross-talk) between the phase and amplitude control branches (provided all the components are linear).

• The system is very fast, thus allowing to use a high gain in the feedback loop (with QWR resonators we achieved residual phase errors as low as 0.1 degree).

• The main limitation is its operational bandwidth, that is determined by the resonator loaded Q.

Bandwidth and energy considerations

• With a typical Q0 of 5*108 the unloaded bandwidth

results to be about 1 Hz.• It is a common practice, with superconducting

resonators, to broaden the loaded bandwidth by overcoupling the cavity.

• We estimated the bandwidth had to be broadened to +/- 10Hz to cover the fast disturbances induced on the eigenfrequency by the environmental noise.

• At 4 J of stored energy, it requires 250W; an equivalent amount is necessary for phase locking.

What did go wrong?

• While the mechanical vibrations induced by the environmental noise and the Lorentz detuning effect could be treated by the usual CPM-based method, the frequency shifts caused by the variations of the He gas pressure resulted too fast to be compensated by the mechanical tuners.

• Fluctuations of the He gas pressure exceed 10mBar with a typical change rate of +/-2.5 mBar/min.

• In terms of frequency this means an average rate of +/- 150Hz/min. Peaks can be up to 5 times higher.

The VCX alternative

• The resonator is coupled to an external reactance (VCX).

• Phase lock is obtained by controlling, through RF switches, the energy exchanged between the resonator and the reactance itself.

LOOP PHASE SHIFTER

PIN DIODE SWITCH

RESONATOR

VCX

POWERAMPLIFIER

How does the VCX work?

• The VCX is switched ON and OFF at the fixed frequency of 25KHz.

• The duty cycle of switches status is modulated in response to the phase error (phase error = 0 means duty cycle = 50%).

• The tuning range depends on how much the VCX and the resonator are coupled together. In practice, it is limited by the specs of RF switches and by the power that can be dissipated by the coupler.

The VCX solution: pros and contros

• Pro: wider tuning range for a given power level from the amplifier.

• Contro: the resonator phase is perturbed by the VCX switching. The amplitude of this effect depends on the ratio of the tuning window over the resonant frequency.

Phase wobble = +/-1degree for a tuning window of +/- 100Hz

Practical realization

• The VCX has been designed to allow a tuning window of +/- 100Hz.

• At the maximum field, a reactive power of about 10KW is managed by the RF switches (Unitrode UM4010 pin diodes).

• The pin diodes are driven by a high slew rate (7000 V/us) Pulser Unit to minimize the power dissipated during switching.

This work has been carried out in collaboration

with ARGONNE NATIONAL LABORATORY

VCX assembly

• The power board is installed at the bottom of the cryostat and operates submersed in liquid nitrogen.

• The VCX coupler dissipates 110W, an equivalent amount of power is dissipated by the pin diodes.

System integration

• The integration of VCX has been made without removing any of the functions of our standard controller.

• A FPGA based PWM modulator has been designed to drive the VCX.

• The CPM continues to be used for the stabilization of the field amplitude.

• The phase lock mode (CPM/VCX) is a software selectable option.

The RFQs graphic interface

The RFQs phase lock system

Phasereference

AD

C

MO

TO

RC

ON

TR

CP

U

PWM

Main RF loop

Fast Feedback

Slow Feedback

SteppingMotor

M

VCXTuning Plate

RESONATOR

Phase Shifter

PowerAmplifier

RF Switch

FT=10KHz

FT=10Hz

Mixer

VME SYSTEM

Conclusions • The phase feedback system based on VCX

has broadened the range of the electronic tuning enough to allow the mechanical tuners to compensate the transients of gas pressure in the cryogenic plant.

• The accuracy of mechanical tuners, however, continues to play a determinant role in the overall system performance.

• The Lorentz detuning resulted less critical to be controlled than we expected and could be managed through an analog feedback loop.

• The commissioning of the PIAVE injector has arrived to its conclusion.

• The first beam on target is scheduled for

November 2005.

Conclusions (2)