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A time-of-flight neutron spin echo spectrometer
Physica B 174 (1991) 528-531 North-Holland
A time-of-flight neutron spin echo spectrometer A. Kollmar, A. Seeger, W. Schalt and H. Thyssen Forschungszentrum Jiilieh GmbH (KFA), Institut fiir Festk6rperforschung, Zentrallabor fiir Elektronik, Postfach 1913, W-5170 Jiilich, Germany
1. Introduction At present a new high-resolution neutron spin echo spectrometer, IN15, installed at the horizontal Cold Source of the H F R reactor at the Institut Laue-Langevin (ILL) in Grenoble, is tested with neutrons, being in its static mode. For a time-of-flight (TOF) operation of the spectrometer the velocity selector for the static mode will be replaced by an arrangement of four choppers, producing neutron pulses of variable time width [1]. Neutron tests for the TOF mode will start early in 1991.
As can be seen from eq. (1), even for a relatively small wavelength band, )tmax/Amin between 2 and 3, a large dynamic range is covered. Another aspect of this TOF mode is generally to gain experience for pulsed neutron sources. A broad time width ~- of the pulses in the TOF mode corresponds to the broad incident wavelength band AA/A in static spin echo spectroscopy, which can be used because of the decoupling to energy resolution. This means a higher echo signal.
3. Arrangement for INI5 2. Spin echo spectroscopy in time-of-flight mode 3.1. Choppers In spin echo spectroscopy (see, e.g., ref. [2]) the intermediate scattering function S(q, t) is measured as a function of the Fourier time t,
t = CA3HL ,
(1)
where C is constant, A the wavelength of the neutrons, H the magnetic field strength, L the effective length of the magnetic coil. HL represents the field integral. In static spin echo spectroscopy a scan is made by varying the magnetic field H, whereas the wavelength of the neutrons, provided by a velocity selector, is kept constant. In the TOF mode a chopper produces polychromatic neutron pulses and in the spectrometer, a certain flight path behind the chopper, the echo signal is measured time resolved for the neutrons with different wavelengths.
Figure 1 shows this arrangement schematically with the relevant distances [3]. The choppers 1, 2 and 3 behind a neutron guide produce the pulses. The mutual phase of chopper 1 and 2 determines the pulse width 7, between 0.5 and 25 ms. To shorten the flanks of the pulses, they rotate with a multiple, up to 7, of the repetition frequency. The chopper 3, rotating at the repetition frequency, between 5 and 20 Hz, suppresses the supernumerary pulses. Chopper 4, also rotating at the repetition frequency, limits the wavelength band used, which is planned from about 8 to 24 ~ .
3.2. Flippers In the TOF mode the spin flippers of the spectrometer have to be tuned to the velocity of
0921-4526/91/$03.50 I~) 1991- Elsevier Science Publishers B.V. (North-Holland)
A . Kollmar et al. / Time-of-flight neutron spin echo spectrometer
9.5m
7,3rn
r L~
529
T l Ni-Guide Choppers 1,2,3
e 4
.
L,r~,,,o~ c;T2;5
I
AnalyserI DetectorJ
Fig. ]. Schemeof the arrangement. neutrons just passing. This is inversely proportional to the elapsed time from pulse production. To achieve the constant precession angle of 180° in the coil of the rr-flipper for all neutrons, the transverse magnetic field in this coil and thus the current i has to be driven with a hyperbolic time dependence. Since for the two rr/2-flippers the precession angle in the flipper coils is not constant, this hyperbola is somewhat modified by these flippers. Figure 2 shows an example. The programming of the time dependence for all flipper currents must allow for modifications by adjustment, necessary for example due to a possibly wavelength-dependent effective thickness of the flipper coils. Tests will also be made with broadband flippers [5].
'HX [Oe] 9 ¸
81 71 61
___~x,.~°--8~
#\
/\
51 ............. 20
40
t[msl 60
80
100
120
140
Fig. 2. Transverse field H x in a ~r/2 coil.
4. Hardware solution of the beam definition device
The chopper-flipper subsystem (fig. 3) is composed of 3 main parts: - 5 chopper drive systems, - 6 flipper coils with their power units, -overall-control hardware following the VMEbus standard. This subsystem can be handled via its own local PC or directly by the experiment computer by means of a VME link.
4.1. The chopper system The chopper disks are driven by an integrated synchronous 4-pole 3-phase motor with permanent excitation (ferromagnets). The motor torque of about 120 N cm is sufficient to achieve a reasonable time for acceleration and to keep the phase angle in the required stability range. The relatively high torque is needed for the 'axes-inaxes' solution for choppers 1 and 2. (The bearing of one chopper rests on the axis of the other.) With this mechanical solution three disks ('triplet') can be arranged very close together. The possible mechanical mutual influences of the independently running choppers must be equalized by the motors. Chopper electronics main parts [4]: -transistorized frequency converters in switchmode technology, for frequency variable 3-phase power up to 3 kVA. - control electronics guaranteeing a speed stability of better than 10 -4 and an angle stability of
530
A . Kollmar et al. / Time-of-flight neutron spin echo spectrometer
.I,c I 6 Trigger Maintrigger
I ge
/6 Isetp©int
,
I se~point
Out On Status
"i6I actual[sul;
~er
P[-~ er
{su[¢~ I~c,uo,
5 Choppersystems
~
• 6 Flipper Coils. . . . .
-~
Fig.3. Chopper-flippersubsystem.
better than 0.2 ° whereby the resolution is also about 0.1 °. -incremental speed- and phase pick-up systems, giving speed angle and zero position. Thus each chopper can be driven independently at any speed and any preselectable angle in regard to a master. Even at zero speed, positioning of the chopper is possible.
4.2. The flipper system Three flipper coils are independently fed by four-quadrant amplifiers (4-Q) in analogue operation with 0.4 kVA power capability; frequency response is about 5 kHz. These power units follow the time-dependent functions with a current accuracy of about 10 -3. Six function generators are independently programmable and give the control values to the individual power supplies with an accuracy of about 10 -3. Via a delay system each function can be initialized with a time resolution of 10 Ixs. One of the
choppers (chopper 3) acts as a master clock for the periodic repetition. A current measurement checks the accuracy of the time-dependent functions.
4.3. Common control Parallel I/O boards observe technical data of the flipper system as well as of the chopper system and handle switch-on and switch-off procedures. The main items are: - maximum chopper speed 160 Hz, - p u l s e repetition between 5 and 20 Hz, - p u l s e width between 0.5 and 25 ms. The modules used are standardized as: - s t a t i c frequency converters, - c h o p p e r controls (speed, phase), -power supplies (4-Q amplifiers), - function generators, - delay system, - parallel I / O.
A . K o l l m a r et al. / T i m e - o f - f l i g h t n e u t r o n spin e c h o s p e c t r o m e t e r
5. Software for chopper and flipper control The software necessary for controlling the IN15 consists of the assembler programs installed in the VME modules developed in the assembler code 68 000 and an organization program. The organization program is installed on the hard disk of a Macintosh plus computer from Apple and consists of a main program and approximately 30 software modules. There is a userfriendly graphic user interface consisting of menus. The programming language is C. ~1.
Choppe~
The user first supplies all the programs for chopper control with the required parameters, e.g. speeds, angular position of the rotating disks relative to each other, etc. After this the chopper motors can be switched on. The process of acceleration and synchronization of the chopper motors can be observed in a window on the display screen. The user can also call up this window on the display screen during operation in order to check trouble-free functioning. 5.2. Flippers
The flipper function mentioned in section 3.2 and represented in fig. 2 is calculated separately matched to each operating case and each flipper coil. The flipper function is only known in its inverse function for ~r/2 flippers and this inverse
531
function t = f ( i ) must first be converted into a polynomial of the 12th degree. i = f ( t ) = at + bt z + c t 3 • • • etc.
This can be done by a linear system of equations. The mathematically calculated function can now be matched to the existing physical conditions by varying the coefficients a, b, c, etc. The final calculated function is then loaded into the memories of the DAC's and triggered, off-set in time relative to each other by a start signal and periodically output to the power supply units for flipper magnets. The C-programs installed on the Macintosh are largely capable of running on other computers. However, if transferred to a different computer all modules controlling the user level and all driver modules for the VME bus must be matched to the computer in question.
References [1] F. Mezei and D. Richter, Proposal for the construction of an extremely high resolution neutron spin echo spectrometer at the ILL (IN15 contract Annex I). [2] Neutron Spin Echo, Lecture Notes in Physics, 128 (Springer, 1980). [3] A. Kollmar, C. Lartigue and F. Douchin, Proposition for the choppers for the TOF mode of IN15/ILL, ILL Technical Report 87K010T. [4] S. Hautecler, E. Legrand, L. Vansteelandt, P. O'Hooghe, G. Rooms, A. Seeger, W. Schalt and G. Gobert, MIBEMOL, 211, Neutron Scattering in the Nineties (IAEA, Vienna, 1985). [5] D. Dubbers, R. Vlaming and E. Klemt, Nucl. Instr. Methods A 270 (1988) 95-98.
A time-of-flight neutron spin echo spectrometer A. Kollmar, A. Seeger, W. Schalt and H. Thyssen Forschungszentrum Jiilieh GmbH (KFA), Institut fiir Festk6rperforschung, Zentrallabor fiir Elektronik, Postfach 1913, W-5170 Jiilich, Germany
1. Introduction At present a new high-resolution neutron spin echo spectrometer, IN15, installed at the horizontal Cold Source of the H F R reactor at the Institut Laue-Langevin (ILL) in Grenoble, is tested with neutrons, being in its static mode. For a time-of-flight (TOF) operation of the spectrometer the velocity selector for the static mode will be replaced by an arrangement of four choppers, producing neutron pulses of variable time width [1]. Neutron tests for the TOF mode will start early in 1991.
As can be seen from eq. (1), even for a relatively small wavelength band, )tmax/Amin between 2 and 3, a large dynamic range is covered. Another aspect of this TOF mode is generally to gain experience for pulsed neutron sources. A broad time width ~- of the pulses in the TOF mode corresponds to the broad incident wavelength band AA/A in static spin echo spectroscopy, which can be used because of the decoupling to energy resolution. This means a higher echo signal.
3. Arrangement for INI5 2. Spin echo spectroscopy in time-of-flight mode 3.1. Choppers In spin echo spectroscopy (see, e.g., ref. [2]) the intermediate scattering function S(q, t) is measured as a function of the Fourier time t,
t = CA3HL ,
(1)
where C is constant, A the wavelength of the neutrons, H the magnetic field strength, L the effective length of the magnetic coil. HL represents the field integral. In static spin echo spectroscopy a scan is made by varying the magnetic field H, whereas the wavelength of the neutrons, provided by a velocity selector, is kept constant. In the TOF mode a chopper produces polychromatic neutron pulses and in the spectrometer, a certain flight path behind the chopper, the echo signal is measured time resolved for the neutrons with different wavelengths.
Figure 1 shows this arrangement schematically with the relevant distances [3]. The choppers 1, 2 and 3 behind a neutron guide produce the pulses. The mutual phase of chopper 1 and 2 determines the pulse width 7, between 0.5 and 25 ms. To shorten the flanks of the pulses, they rotate with a multiple, up to 7, of the repetition frequency. The chopper 3, rotating at the repetition frequency, between 5 and 20 Hz, suppresses the supernumerary pulses. Chopper 4, also rotating at the repetition frequency, limits the wavelength band used, which is planned from about 8 to 24 ~ .
3.2. Flippers In the TOF mode the spin flippers of the spectrometer have to be tuned to the velocity of
0921-4526/91/$03.50 I~) 1991- Elsevier Science Publishers B.V. (North-Holland)
A . Kollmar et al. / Time-of-flight neutron spin echo spectrometer
9.5m
7,3rn
r L~
529
T l Ni-Guide Choppers 1,2,3
e 4
.
L,r~,,,o~ c;T2;5
I
AnalyserI DetectorJ
Fig. ]. Schemeof the arrangement. neutrons just passing. This is inversely proportional to the elapsed time from pulse production. To achieve the constant precession angle of 180° in the coil of the rr-flipper for all neutrons, the transverse magnetic field in this coil and thus the current i has to be driven with a hyperbolic time dependence. Since for the two rr/2-flippers the precession angle in the flipper coils is not constant, this hyperbola is somewhat modified by these flippers. Figure 2 shows an example. The programming of the time dependence for all flipper currents must allow for modifications by adjustment, necessary for example due to a possibly wavelength-dependent effective thickness of the flipper coils. Tests will also be made with broadband flippers [5].
'HX [Oe] 9 ¸
81 71 61
___~x,.~°--8~
#\
/\
51 ............. 20
40
t[msl 60
80
100
120
140
Fig. 2. Transverse field H x in a ~r/2 coil.
4. Hardware solution of the beam definition device
The chopper-flipper subsystem (fig. 3) is composed of 3 main parts: - 5 chopper drive systems, - 6 flipper coils with their power units, -overall-control hardware following the VMEbus standard. This subsystem can be handled via its own local PC or directly by the experiment computer by means of a VME link.
4.1. The chopper system The chopper disks are driven by an integrated synchronous 4-pole 3-phase motor with permanent excitation (ferromagnets). The motor torque of about 120 N cm is sufficient to achieve a reasonable time for acceleration and to keep the phase angle in the required stability range. The relatively high torque is needed for the 'axes-inaxes' solution for choppers 1 and 2. (The bearing of one chopper rests on the axis of the other.) With this mechanical solution three disks ('triplet') can be arranged very close together. The possible mechanical mutual influences of the independently running choppers must be equalized by the motors. Chopper electronics main parts [4]: -transistorized frequency converters in switchmode technology, for frequency variable 3-phase power up to 3 kVA. - control electronics guaranteeing a speed stability of better than 10 -4 and an angle stability of
530
A . Kollmar et al. / Time-of-flight neutron spin echo spectrometer
.I,c I 6 Trigger Maintrigger
I ge
/6 Isetp©int
,
I se~point
Out On Status
"i6I actual[sul;
~er
P[-~ er
{su[¢~ I~c,uo,
5 Choppersystems
~
• 6 Flipper Coils. . . . .
-~
Fig.3. Chopper-flippersubsystem.
better than 0.2 ° whereby the resolution is also about 0.1 °. -incremental speed- and phase pick-up systems, giving speed angle and zero position. Thus each chopper can be driven independently at any speed and any preselectable angle in regard to a master. Even at zero speed, positioning of the chopper is possible.
4.2. The flipper system Three flipper coils are independently fed by four-quadrant amplifiers (4-Q) in analogue operation with 0.4 kVA power capability; frequency response is about 5 kHz. These power units follow the time-dependent functions with a current accuracy of about 10 -3. Six function generators are independently programmable and give the control values to the individual power supplies with an accuracy of about 10 -3. Via a delay system each function can be initialized with a time resolution of 10 Ixs. One of the
choppers (chopper 3) acts as a master clock for the periodic repetition. A current measurement checks the accuracy of the time-dependent functions.
4.3. Common control Parallel I/O boards observe technical data of the flipper system as well as of the chopper system and handle switch-on and switch-off procedures. The main items are: - maximum chopper speed 160 Hz, - p u l s e repetition between 5 and 20 Hz, - p u l s e width between 0.5 and 25 ms. The modules used are standardized as: - s t a t i c frequency converters, - c h o p p e r controls (speed, phase), -power supplies (4-Q amplifiers), - function generators, - delay system, - parallel I / O.
A . K o l l m a r et al. / T i m e - o f - f l i g h t n e u t r o n spin e c h o s p e c t r o m e t e r
5. Software for chopper and flipper control The software necessary for controlling the IN15 consists of the assembler programs installed in the VME modules developed in the assembler code 68 000 and an organization program. The organization program is installed on the hard disk of a Macintosh plus computer from Apple and consists of a main program and approximately 30 software modules. There is a userfriendly graphic user interface consisting of menus. The programming language is C. ~1.
Choppe~
The user first supplies all the programs for chopper control with the required parameters, e.g. speeds, angular position of the rotating disks relative to each other, etc. After this the chopper motors can be switched on. The process of acceleration and synchronization of the chopper motors can be observed in a window on the display screen. The user can also call up this window on the display screen during operation in order to check trouble-free functioning. 5.2. Flippers
The flipper function mentioned in section 3.2 and represented in fig. 2 is calculated separately matched to each operating case and each flipper coil. The flipper function is only known in its inverse function for ~r/2 flippers and this inverse
531
function t = f ( i ) must first be converted into a polynomial of the 12th degree. i = f ( t ) = at + bt z + c t 3 • • • etc.
This can be done by a linear system of equations. The mathematically calculated function can now be matched to the existing physical conditions by varying the coefficients a, b, c, etc. The final calculated function is then loaded into the memories of the DAC's and triggered, off-set in time relative to each other by a start signal and periodically output to the power supply units for flipper magnets. The C-programs installed on the Macintosh are largely capable of running on other computers. However, if transferred to a different computer all modules controlling the user level and all driver modules for the VME bus must be matched to the computer in question.
References [1] F. Mezei and D. Richter, Proposal for the construction of an extremely high resolution neutron spin echo spectrometer at the ILL (IN15 contract Annex I). [2] Neutron Spin Echo, Lecture Notes in Physics, 128 (Springer, 1980). [3] A. Kollmar, C. Lartigue and F. Douchin, Proposition for the choppers for the TOF mode of IN15/ILL, ILL Technical Report 87K010T. [4] S. Hautecler, E. Legrand, L. Vansteelandt, P. O'Hooghe, G. Rooms, A. Seeger, W. Schalt and G. Gobert, MIBEMOL, 211, Neutron Scattering in the Nineties (IAEA, Vienna, 1985). [5] D. Dubbers, R. Vlaming and E. Klemt, Nucl. Instr. Methods A 270 (1988) 95-98.