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

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

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Pulsed Source Requirements from the User’s Point of View

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

    • 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 Helmut Schober, Bilbao 2009
    • ESS is the camera Instruments have to provide better view Dynamic range Resolution 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
    • Argument Helmut Schober, Bilbao 2009 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? Helmut Schober, Bilbao 2009
    • The main Purpose of time structure Selecting wavelength via time-of-flight Helmut Schober, Bilbao 2009
    • 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 Helmut Schober, Bilbao 2009
    • A pedagogic ESS instrument: Double-TOF Detector Sample Source Pulses
    • Time-Distance Diagram
    • 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. Helmut Schober, Bilbao 2009
    • Reason Just integrate long enough at the moment of wavelength selection Helmut Schober, Bilbao 2009
    • 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. 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 Geometry is the limiting factor Helmut Schober, Bilbao 2009
    • 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? Helmut Schober, Bilbao 2009
    • How to get the best out of the source? Helmut Schober, Bilbao 2009
    • 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 Helmut Schober, Bilbao 2009
    • 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. Helmut Schober, Bilbao 2009
    • 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 Helmut Schober, Bilbao 2009
    • 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. Helmut Schober, Bilbao 2009
    • 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. Helmut Schober, Bilbao 2009
    • 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 Helmut Schober, Bilbao 2009
    • 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 background etc. 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