ESS-Bilbao Initiative Workshop. Pulsed Source Requirements from the User’s Point of View

Pulsed Source Requirementsfrom the

User’s Point of View

Helmut Schober, Bilbao 2009

No es una tarea fácil

User’s Goal “Wissen-schaffen”

We have to contribute to the text books of our children


ESS is the camera

Instruments have to provide better view

Dynamic rangeResolution

Speed Sensitivity


Paradox

As it is difficult to anticipate the instrument suite of the years beyond 2020

we should reason as independently as possible from any concrete instrument design.


Philosopy of a phycicist

Try to stay as general as possible by working out the main principles

Danger: There is always the odd case that contradicts the principle


What is in the most general terms

the added value of a time-structured source?


Theorem I

In the linear regime and

at equal integrated intensity

time modulation is always advantageous


ArgumentIn the linear regime the output signal is proportional to the

input signal(we have in particular no radiation damage of the sample and

no saturation effects in the detector)

Thus, if we just ignore the time structure, we get the same results as with a steady state source

Time structure allows, in addition, for filteringand thus increases the sensitivity of the measurement

This is true for any experimental probe

Neutrons fluxes are weak and even with short-pulsed intensities we stay in nearly all cases within the linear regime


The Question

What time structure is optimal?


The main Purpose of time structure

Selecting wavelength via time-of-flight


Remember the Principle

Create Time Structure

Select wavelength

At this point we requirethe adequate spectrum I(λ)

At a reactor you can start anywhere along the line at a pulsed source

you start at the target

t= tf- t0 = L/v

Δ (tf)

Δ (t0)


A pedagogic ESS instrument: Double-TOF

Source

Detector

Sample

Pulses

Time-Distance Diagram

Correlationof time and wavelength

as a function of beam propagation

CreateSpread Select=Integrate

Select = CreateSpreadSelect

time of flight

60 ms

What performancecan we expect from the filter?

Theorem II

Compared to a continuous sourceyou cannot build an instrument that performs

better than the ratio of the peak flux

Just create time structure with choppers and build otherwise identical instruments

Argument

≈20-30 Duty cycle = 3%

Data from ESS Project Report

Lemma to Theorem II

You can do considerably worse if you need additional pulse shaping

Reason:

You do have to create the time structure at the right distance from the source as you

strongly correlate Δt and Δλ

The first IN5 was a typical example of sub-optimal design because the white pulse was too short

A closer look at Time-Wavelength Correlation

If the secondary spectrometer is not a time-of-flight filter then we do wavelength sorting.

Shorter pulses are generally an advantage and rarely a problem.


Reason

Just integrate long enough at the moment of wavelength selection


The exceptionAdditional pulse shaping of the

primary pulse

Reason:

You cannot create the time structure arbitrarily close to the source

Long pulse is generally more forgivingThis is the first time pulse length becomes an argument

An example

Reflectometry (or Backscattering)

Reason:

Chopping the beam down to 1 ms (40 µs) at a few meters from the source limits the

wavelength band

Frame multiplication

Possible at a long-pulse source1 ms from the start could be even better

A closer look at Time-Wavelength Correlation

If the secondary spectrometer is again a time-of-flight filter then shorter pulses are

only advantageousif the primary flight time can be adapted.


Reason

Secondary time-of-flight sets integration time of primary beam (= opening time Δt of

monochromating chopper)By selecting the time of chopping T with respect

to the source pulse we can tune Δt to Δλ


Geometry is the limiting factor

To be more concreteTOF-TOF @ ESS-5MW

Configuration 1 (= reference)2 ms pulse at 16.66 Hz with L(p,m) = 100 m and L(s,d) = 4 m

Balanced resolution, wavelength multiplication ([email protected] Å-1)

Configuration 1I1 ms pulse at 16.66 Hz with L(p,m) = 100 m and L(s,d)= 4 m

Slightly better but unbalanced resolution, no increase in flux, (9/0.2 Å-1 at 5 Å)Possibility of high-resolution option by increasing chopper speed

Configuration 1II1 ms pulse at 16.66 Hz with L(p,m) = 50 m and L(s,d)= 4 m

Identical resolution, twice the flux, (9/0.4 Å-1)Possibility of high-flux option by shortening secondary spectrometer

Configuration V11 ms pulse at 33 Hz with L(p,m) = 50 m and L(s,d) = 4 m

Identical resolution, identical overall flux, but twice the flux in the nominal wavelength channel

Lefmann, Schober, and Mezei, MST, 2008

My personal preference

Answer to our question

Highest Peak Flux with Ample Time between Reasonably Short Pulses

What does “ample” and “reasonable” mean?


How to get the bestout of the source?


Theorem III

Always “moderate” all neutronsif you can (Lemma II.I) afford ulterior pulse shaping


Argument

Ulterior pulse shaping offersflexibility that you do not have witha decoupled or poisoned moderator

Full exploitation requires about 350 µs for cold neutronsThis is the lower limit for the pulse lengthIn other words: Moderation and accumulation time sets the scale.

Pulse shape


From this point of viewa pulse length between

300 μs and 1 ms is close to ideal.

Technology and costs may favor longer pulses.

One also has to consider problem of rise time and tails. In this sense a 2 ms real pulse is not far from an ideal 1 ms pulse.

SNS: 23 kJ/pulse @1.4MW/60Hz

ESS: 300 kJ/pulse @5MW/16.6 Hz


There are always contributions to resolution independent of the pulse length that are

setting the scale for Δλ/λ


Flight-path uncertainties Sample size

Detetor depth etc.

Theorem IVIf you want to optimize

your resources then try to match the duty cycle to Δλ/λ


Duty cycle defines intrinsicwavelength resolution capability of

the source. Short intensive pulses have their price.

Reason


F(SNS)

F(ESS LP)

0 1 2 3 4 5 6 7 8

1012

1013

1014

1015

1016

1017

ILL hot source ILL thermal source ILL cold source

SNS 1.4 MW, 60 Hz thermal moderator coupled cold moderator

ESS LPTS 5 MW, 16.7 Hz, 2 ms bispectral thermal - cold

Sou

rce

brill

ianc

e [n

/cm

2/s

/str/

Å]

Wavelength [Å]

F(ILL)

ESS is best for 3% Δλ/λ

F = Φ min(1,c /(Δλ/λ) ), c = τ/TMezei, Schober et al. 2008

Minimalist’s “tour de table”

• Cold time-of-flight is ideal for ESS as Δλ/λ is about 3 %. 1 ms pulses would further increase performance and/or flexibility. 16.6 Hz is preferred but 33 Hz would be equally viable.

• For SANS the time-of-flight resolution is too good. Can we build shorter instruments for smaller samples?

• For reflectometry the resolution could be better at short wavelengths. 1 ms welcome but 16 Hz seems an upper limit for repetition rate.

• For backscattering the resolution is way too poor both for 2 ms and 1 ms. Pulse shaping is required. But higher peak flux would help.

Tentative summary

Pulse length should be longer than the “full moderation and accumulation time”

This requirement sets the scale

Certain instruments would suffer from a repetition rate higher than 20 Hz

Thus, if technically possible and financially affordable reaching 1 ms pulses at 16.6 Hz would be a worth while goal to pursue.

Tails and rise time?

1 ms at 33 Hz versus 2 ms at 16.6 Hz is a delicate choice.


Tentative summary

One should not totally forget about secondary effects

Reduced length of instruments should lead to reduced costs but makes the experimental zones more crowded

Extremely long guides have reduced transmission at shorter wavelengths

Longer instruments allow for better backgroundetc.


In the end a question of€/n@detector

Remember the mission:Wissen-schaffen

A good movie needs a good story, good actors and a good camera

ESS-Bilbao Initiative Workshop. Pulsed Source Requirements from the User’s Point of View

Technology

Transcript of ESS-Bilbao Initiative Workshop. Pulsed Source Requirements from the User’s Point of View