instrumentation & operation aspects uriot didier

URIOT Didier - MYRRHA accelerator 1st International Design Review 11/12/2012 1

Instrumentation &

operation aspects

URIOT Didier


Some general reflections about:

• Injector, • Tuning procedures / commissioning,• MPS,• Impact on diagnostic requirements,• Virtual accelerator

Introduction


Even if the beam current is relatively low, 4 mA, at 30 keV the space-charge is not negligible at all.Therefore, the chopper location and the gas pressure in the LEBT line are to be considered carefully.

Level of pressureBeam losses

due to electron captures

transient timedue to space-charge

compensation

Injector : space-charge compensation issue

Our capacity to produce a pulsed beam as short as possible with reduced transients is essential for the commissioning phase, the first tuning stages, and the reliable routine operation of the accelerator.

6kV chopper


YY’

1.0 10-5 mbar(Krypton)5.0 10-5 mbar(Krypton)

1.0 10-6 mbar 1.5 10-6 mbar 1.5 10-5 mbar

5.0 10-5 mbar(Krypton)

4.5 10-5 mbar(Azote)

1.2 10-5 mbar(Argon)

Space-charge compensation problem is very complex and some experimentations show beam emittance changing depending on pressure. And it’s especially true in Spiral2 LEBT experiments.

Spiral2 LBET line is very long (10 m), compared to MYRRHA (2 m), but beam distribution damage occurs in the last 2 solenoids.

Even with 7 pumps regularly distributed, its not possible to really well control pressure in the last line section, unacceptable beam losses occur along the rest of the line. In a shorter line that becomes clearly impossible.

Gas injection to better control the pressure seems an option to consider seriously.

Injector : space-charge compensation issue : Spiral2 example



• In Spiral2, despite a current reserve of the source,10 mA, below 2.10-5 mbar, it is not possible to achieve 5 mA at the output of the injector (electron capture).

• For MYRRHA, the plan to get a beam current reserve at the source is a good idea, to be able to increase the pressure & lower the SC transient time.Krypton

• From this, a pessimistic assumption for the SC transient time for MYRRHA injector would be in the order of 10-20 µs, hopefully even less (tb measured during the injector test)

• As a back-up solution, a possible additional magnetic kicker could be used in the 17 MeV MEBT to clean the transients if necessary.er )



Space-charge transient time

Pressure

300 µs 3.5 10-7 mbar

150 µs 7.5 10-7 mbar

30 µs 5.5 10-6 mbar

15 µs 1.25 10-5 mbar

Injector : space-charge compensation issue : Spiral2 example



Very preliminary commissioning & operation strategy

Considering the linac commissioning, once injector is operational and output beam properties is well known.

First stage: pencil beam (a few 10 Watt) to perform alignment and cavity tuning (phase and field), running the beam to the target. As cavities will be progressively turned on, a specific procedure has to be developed in order to manage the energy range at the linac end (20 MeV to 600 MeV), typically we’ll have to match quadrupole fields to energy.

Second stage: the beam current will be increased to nominal value (4 mA), with a few 10-5 repetition rate (around100 Watt at 600MeV). Working directly with nominal beam current seems to be a good choice and clearly the fastest way, avoiding new tuning of the entire machine for each new pulse current.

Last stage: Increase the duty cycle to reach the nominal beam power.

Considering the ADS operation, the strategy for reactor power control (& ramping up) is presently under evaluation; pilotage of the mean beam power could be made:

Keeping the nominal 4mA max. pulse current & adapting the duty cycle (“commissioning-like”, preferred solution from the linac point of view)

Keeping a CW beam and adapting the pulse current (back-up solution, probably more complex to implement)


Beam time structures to be produced: preliminary view

Pencil beam(12W @ 600MeV)

200µs, 0.1mA pulses, 1Hz

First nominal beam(120W @ 600MeV)

50µs, 4mA pulses, 1Hz

Final nominal beam incl. ISOL production(~2.4MW @ 600MeV)

3.8ms, 4mA pulses, 250Hz (keeping 200µs holes for subcriticality monitoring & sending ~190µs pulses to the ISOL facility)

Reactor (& beam) power ramping up & control strategy under definition


Characterization of the produced beams using the test stand injector at UCL

Several beams have to be to produced by the injector. At least:• Pencil beam: low current (0.1 mA), low duty cycle• Full current pulsed beam, 4 mA and variable duty cycle from 10-5 to 1.0• Full power beam: 4 mA CW with short beam holes at the adequate frequency→ strong impact on diagnostic requirements.

For each of these operating points, the prototype injector to be installed at UCL must achieve:• Reproducibility• Reliability• Full characterization of the output beams• A good knowledge of the transient times The main goal of the injector test stand should be to reach these objectives. Diagnostics are essential, first to characterize beams and secondly obviously to tune the injector.

Chopper deviceEmittancemeter in both planesVery long-term work(Krypton)

See Drik’s talk « MYRRHA injector at UCL »


Beam current stability requirement

For the ADS operation, the beam current stability has to be better than +- 2% Example of the current control of the Spiral2 injector

This will also have to be implemented & demonstrated in Louvain-La-Neuve


Linac tuningo The periodic-structure tuning is based on the principle of finding the beam matching to the periodic

channel, rather than adapting the channel to the beam. In other words, the linac is fixed in terms of transverse and longitudinal focusing.

o The beam is properly matched when the beam sizes (trans. & long.) along the structure show similar values.

o This type of adjustment is based on iterative minimization procedure. This can need a lot of iterations and this can be very time consuming.

o Measure the longitudinal beam size is not trivial. For example, in the Spiral2 project, R&D for longitudinal diagnostics has not succeed yet today.



Transverse sizesBunch lengths

Adjustable elements

Requirements:

• R&D about diagnostics is mandatory

• Intercept or not the beam ?• Measurements and controls must

be developed in order to that they are as fast as possible.

Advantages:

• Always valid, this method is completely independent of the beam current or emittances.


Machine protection system

o Obviously, diagnostics are one of the utmost important component of the machine protection system

o But, they are probably one of the most likely causes of spurious MPS signals that would shut off the beam un-necessary (wrong information, noise, failures…)o We must limit the number of diagnostics linked to the MPS. o Control halo, sizes, energy and current of the beam as close as possible to the reactor

could probably protect against most of the failures, whatever its position in the machine. In other terms, is it possible to get unacceptable losses along the machine detecting nothing on the beam control at the reactor entrance?

o We have to perform simulations and error studies to answer clearly for each diagnostics able to be implied in the MPS. Is it really needed.

o Obviously these simulations are only starting points to be checked in commissioning operation.

o Despite the potential damage on the machine, by-pass the MPS is sometime mandatory during commissioning.

o Another important question is: below the beam stability requirements (<2% in current, <1% in energy), what should be the accuracy of the beam power measurement injected into the reactor? Today, this is a show-stopper in the Spiral2 project safety file.


Machine protection system

A simple example to illustrate this notion

In both cases, no loss is observed (simulation with 107 particles).

But rms beam size close to target is increased by 20% and beam halo by 15%

We have to check with beam, who sees first the failure:- Beam monitoring close to

target.- Beam loss monitor along

the structure.

Reference case

Failed case: quad is failing (10% of the field) at beginning of the spoke section



Impact on diagnostic requirements

• Diagnostic is key point in all aspects of high power linac developments.

• Diagnostic reliability is fundamental in machine reliability. It’s particularly true in ADS applications like MYRRHA machine.

• Even, if MYRRHA is designed for high power CW beam, diagnostics have to be also studied for low current, short pulse and low repetition rate. In other terms, they have to cover a large dynamic range of beam operation.

• Write very accurate requirements or specifications are necessary

• Diagnostic performances will impact on tuning procedures, commissioning and MPS

• A good communication between beam physicists and diagnostics and command-control groups is essential in all the stages of the machine conception and commissioning.



Preliminary part counts for beam diagnostic needs

Table from the preliminary work performed within the CDT project

Within the MAX project in 2013: - the linac tuning method will be precisely defined → thorough definition of the diagnostic needs - extensive Monte Carlo error studies will be performed → definition of diagnostics specifications

Beyond this, intense R&D will be needed for beam diagnostics development



High level application programs

Tuning procedures and associated tools should be developed directly by beam physicists. Because these applications are very specific, debugging them could become very complex for a non-specialist.

We must be confident into our simulation code (at least after the injector), because :- RF cavity tuning is partly based on models from this code- All tuning procedures have been checked by this code- It is presently planned that the fault cavity recovery algorithm is performed on-line by a

simulation code, during the 3sec interval necessary to recovery a cavity failure.

→ High power ADS machine must be designed, commissioned and operated in close association with the development of a predictive code

We have to develop and implement the “Virtual Machine” concept.On-line machine simulation allows to:- Compare measured beam to beam simulations .- Have an idea of the beam characteristics everywhere in the machine.- Check the tuning before implementing it into the machine.- Use code tuning flexibility directly to the real machine.


High level application programs: « Virtual Accelerator » concept

This concept is under development at CEA Saclay for IPHI, Spiral2 projects, and could be proposed for MYRRHA.

Three modes of operation are proposed, more or less ambitious.

Control tower Flight simulator Automatic pilot



“Control tower“ modeIn this mode, action on the control system of the machine, acts on the real accelerator and the virtual accelerator. The virtual accelerator therefore is directly linked to the data base of the real machine and simulates it. This simulation can be started automatically every time you change a setting or periodically. The machine is under the constant supervision of the virtual machine. It’s monitored by the simulation code.

Virtual accelerator code only needs read-only access to the virtual database.

Control system Interface

Data base machine

equipment

Simulation code

Equipement

Tuning algorithms

Visualization

Diagnostics

Machine

Interest for the beam physicists: Direct comparison between the real diagnostics and simulate accelerators. Improving simulation codes, a better understanding of the accelerator are expected.

Interest for machine operation: Direct full beam knowledge and visualization (envelopes, phase space, diagnostics, etc..) at all accelerator location

Close to be integrated in the TraceWin CEA code with EPICs



“Flight simulator“ modeAction on the system command control is applied either on real or on virtual accelerator. There is actually two separate machines. This mode obviously implies that there are two separate database representing the two accelerators. Software must allow to easily and securely switch from one to the other, and optionally copy the settings of a database to the other. Virtual accelerator code only needs read-only access to the virtual database.

Interest for the beam physicists: Check tuning points or procedures of tuning before to test on the real machine without affecting its operation.

Interest for machine operation: preview what you're doing before to set to the machine and avoids serious errors. Operator training.

Control system Interface

Data base equipment of the virtual machine

Data base equipment of the

real machine

Simulation codeEquipementTuning

algorithms

VisualizationDiagnostics

Machine

Very complex in term of architecture and no development is planned today




Some developments have been done on the TraceWin code to implemented this mode. And today, a few parts have been already checked on the IPHI injector and used to tune the Spiral2 injectors. But, clearly, plug it into a MW machine is not for tomorrow. It’s a huge challenge.

“Automatic pilot“ modeModern codes have abilities to tune simulated accelerators only taking into account diagnostic information. In this mode, we use the automatic features of the code to tune directly also the real machine. Here, any simulation is performed. Code uses diagnostic measurements from the real machine as input of its algorithms to control the real machine equipment.The simulation code have read and write access to the database.

Interest for the beam physicists: Develop and check tuning procedures directly in our own simulation codes and directly apply them on the machine.

Control system Interface Data base equipment of the

real machine

Simulation code Equipement

Tuning Algorithms

Visualization

Diagnostics

Machine



High level application programs: « Virtual Accelerator », IPHI example

TraceWin

Front view of the beam at the output of the RFQ injection cone.



Conclusions

Beam diagnostic is the key topic in high power accelerator, especially ADS. And is closely linked with the commissioning & operation aspects.

The reliability of diagnostic associated with the control system is one of the MYRRHA challenges.

We are aware of all these topics

Virtual accelerator is a nice concept but we are also aware that reality is not a dream world and like an air plane, an ADS accelerator doesn’t support any approximation.

instrumentation & operation aspects uriot didier

Documents

myrrha injector

pulsed beam

beam emittance

beam current reserve

mbar injector

injector test

output beam properties

spiral2 lbet line