Post on 31-May-2015
description
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
Helmut Schober, Bilbao 2009
ESS is the camera
Instruments have to provide better view
Dynamic rangeResolution
Speed Sensitivity
Helmut Schober, Bilbao 2009
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.
Helmut Schober, Bilbao 2009
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
Helmut Schober, Bilbao 2009
What is in the most general terms
the added value of a time-structured source?
Helmut Schober, Bilbao 2009
Theorem I
In the linear regime and
at equal integrated intensity
time modulation is always advantageous
Helmut Schober, Bilbao 2009
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
Helmut Schober, Bilbao 2009
The Question
What time structure is optimal?
Helmut Schober, Bilbao 2009
The main Purpose of time structure
Selecting wavelength via time-of-flight
Helmut Schober, Bilbao 2009
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)
Helmut Schober, Bilbao 2009
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.
Helmut Schober, Bilbao 2009
Reason
Just integrate long enough at the moment of wavelength selection
Helmut Schober, Bilbao 2009
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.
Helmut Schober, Bilbao 2009
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
Helmut Schober, Bilbao 2009
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 (9@0.2 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?
Helmut Schober, Bilbao 2009
How to get the bestout of the source?
Helmut Schober, Bilbao 2009
Theorem III
Always “moderate” all neutronsif you can (Lemma II.I) afford ulterior pulse shaping
Helmut Schober, Bilbao 2009
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
Helmut Schober, Bilbao 2009
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
Helmut Schober, Bilbao 2009
There are always contributions to resolution independent of the pulse length that are
setting the scale for Δλ/λ
Helmut Schober, Bilbao 2009
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
Helmut Schober, Bilbao 2009
Duty cycle defines intrinsicwavelength resolution capability of
the source. Short intensive pulses have their price.
Reason
Helmut Schober, Bilbao 2009
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.
Helmut Schober, Bilbao 2009
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.
Helmut Schober, Bilbao 2009
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