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

Pulsed Source Requirements
from the
User’s Point of View
No es una tarea fácil

Helmut Schober, Bilbao 2009

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 range
Resolution
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


Argument

In 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 ﬁltering
and 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
Δ (t0)
Create Time Structure
At this point we require
the adequate spectrum I(λ)
t= tf- t0 =
At a reactor you can start anywhere along
the line at a pulsed source
L/v
you start at the target

Δ (tf)
Select wavelength

A pedagogic ESS instrument:
Double-TOF

Detector

Sample

Source

Pulses

60 ms
Correlation
of time and
wavelength
as a function of
beam propagation

Create
Spread
Select=

time of flight
Integrate

Select =
Create
Spread
Select

What performance
can we expect from the ﬁlter?

Theorem II
Compared to a continuous source
you cannot build an instrument that performs
better than the ratio of the peak ﬂux

Argument

Just create time structure with choppers and
build otherwise identical instruments

≈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 exception
Additional pulse shaping of the
primary pulse

Reason:

You cannot create the time structure
arbitrarily close to the source

Long pulse is generally more forgiving
This 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 source
1 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 advantageous
if 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 concrete
TOF-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
My personal Balanced resolution, wavelength multiplication (9@0.2 Å-1)
preference
Lefmann, Schober, and Mezei, MST, 2008
Configuration 1I
1 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 1II
1 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 V1
1 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

Answer to our question
Highest Peak Flux with Ample Time
between Reasonably Short Pulses

What does “ample” and “reasonable”
mean?


How to get the best
out of the source?


Theorem III
Always “moderate” all neutrons
if you can (Lemma II.I) afford ulterior pulse shaping

Argument

Ulterior pulse shaping offers
ﬂexibility that you do not have with
a decoupled or poisoned moderator

Pulse shape

Full exploitation requires about 350 µs for cold neutrons
This is the lower limit for the pulse length
In other words:
Moderation and accumulation time sets the scale.

From this point of view
a 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 IV
If you want to optimize
your resources then try to match
the duty cycle to Δλ/λ

Reason

Duty cycle deﬁnes intrinsic
wavelength resolution capability of
the source. Short intensive pulses have their
price.

ESS is best for 3% Δλ/λ
SNS 1.4 MW, 60 Hz
ILL hot source
thermal moderator
ILL thermal source
1017 coupled cold moderator
ILL cold source
ESS LPTS 5 MW, 16.7 Hz, 2 ms
/s/str/Å]

bispectral thermal - cold
1016
Source brilliance [n/cm
2

F(ILL)
1015

F(ESS LP)
1014

1013

F(SNS)
1012

0 1 2 3 4 5 6 7 8
Wavelength [Å]

F = Φ min(1,c /(Δλ/λ) ), c = τ/T
Mezei, 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 background
etc.


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

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