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High frequency measurements at a pulsed muon source: beating the pulse width!
Physica B 326 (2003) 275–278
High frequency measurements at a pulsed muon source: beating the pulse width! A.D. Hillier*, S.P. Cottrell, P.J.C. King, G.H. Eaton, M.A. Clarke-Gayther ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
Abstract The pulsed nature of the ISIS beam is advantageous for many types of experiments. Limitations arise, however, because the finite width of the muon pulse restricts the maximum muon precession frequency and relaxation rate that can be measured. This paper considers two methods currently under development at ISIS that can be used to extend the upper frequency limit at a pulsed muon source. Firstly, a technique using a fast switching electrostatic kicker to time slice the incoming muon pulse is described. Data is presented that demonstrates the effective manner in which the mSR usable frequency response at ISIS is increased in inverse proportion to the pulse width. The second technique uses a 901 radio-frequency (RF) pulse to remove the time structure of the pulsed beam; this method is demonstrated by studying the flux penetration in the mixed state in the type II superconductor YNi2B2C. The merits of both techniques are considered and contrasted. r 2002 Elsevier Science B.V. All rights reserved. Keywords: ISIS facility; Electrostatic kicker; RF mSR; Superconductor
1. Introduction The pulsed structure of the ISIS beam is advantageous for many types of experiments. In particular, it enables time differential data to be measured to long times with minimal background and no time penalty, and it allows the efficient synchronisation of external stimuli (such as radiofrequency (RF), electric current, or light) for novel experiments. Limitations arise, however, because the finite width of the muon pulse restricts the maximum muon precession frequency and relaxa*Corresponding author. Tel.: +44-01235-446001; fax: +4401235-445720. E-mail address: [email protected] (A.D. Hillier).
tion rate that can be measured; at ISIS the muon pulse width of 80 ns corresponds to a FWHM band-pass of approximately 5 MHz (or maximum usable frequency of approximately 8 MHz). This paper considers two methods currently under development at ISIS that can be used to extend the upper frequency limit of a pulsed muon source. Firstly, a technique using a fast switching electrostatic kicker, as reported by Eaton et al. [1], to time slice the incoming muon pulse is described, and secondly, a 901 RF pulse is used in the manner proposed by Carne et al. [2] to remove the time structure of the pulsed beam. Results demonstrating both methods are presented in this paper and the relative merits of each technique are considered.
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 2 9 - 0
276
A.D. Hillier et al. / Physica B 326 (2003) 275–278
At ISIS the three spectrometers are fed from the accelerator double-pulse structure (two 80 ns wide muon pulses with a peak-to-peak separation of approximately 340 ns) by a fast electrostatic kicker. In normal running this is timed to slice the first muon pulse in half spatially to feed the two side beamlines (EMU and DEVA), whilst transmitting the whole of the second pulse to the central beamline (mSR). During the commissioning of the European Muon Facility, Eaton et al. [1] noted that by introducing an additional delay into the electrostatic kicker timing, such that the voltage drop occurs in the centre of the second muon pulse, it was possible to reduce the time width of the pulse feeding the mSR beamline. A practical demonstration of this time-slicing method is shown in Fig. 1. The EMU slit counter (monitoring the beam fed to the EMU beamline) shows the whole of the first pulse plus the first half of the second pulse, whilst mSR receives the second half of the second pulse after the kicker voltage has reduced to zero. The portion of the second pulse going into EMU has a width of 35 ns and an area of 6300 mV ns (45% of a full muon pulse), whilst the mSR pulse has a width of 47 ns and an area of 4600 mV ns (33% of a full muon pulse). The
increased width of the mSR pulse as compared with that going to EMU is probably due to the 26 ns pion lifetime in the muon production target, which has the effect of extending the pulse tail. The actual fall-time of the kicker voltage is much faster than shown by the trace in Fig. 1, which is limited by the bandwidth of the measuring device; considering the proportion of the beam lost in splitting, the true fall-time is thought to be less than 25 ns. The improvement in the frequency response was demonstrated by measuring the amplitude of the triplet precession signal as a function of applied transverse field for muonium formed in fused quartz. Curves were measured in the mSR beamline for both the normal, full-width pulse and the timesliced pulse. The results of these measurements are shown in Fig. 2 and clearly demonstrate the extension of the upper frequency limit that is possible by time slicing the muon pulse. For the full pulse, the half-maximum frequency response is B4.9 MHz while for the time-sliced pulse this is extended to B8.6 MHz, an increase of some 76% that is consistent with the 40% reduction in the pulse width. This highly successful demonstration is of particular importance as the technique forms a key part of the facility upgrade proposed in
Fig. 1. EMU and the time-sliced mSR pulses with the kicker voltage falling in the centre of the second pulse. The voltage fall-time of the kicker is artificially extended by the limited bandwidth of the measuring device.
Fig. 2. Frequency response measured in mSR for both the full muon pulse and the time-sliced pulse. The amplitude of the triplet precession for muonium formed in fused quartz was followed in applied transverse fields.
2. Time slicing the ISIS pulse
A.D. Hillier et al. / Physica B 326 (2003) 275–278
Ref. [3]. The upgrade will introduce an additional two-electrode kicker to divert the first muon pulse that at present continues to the EMU and DEVA beamlines into a new beamline. This will leave the existing facility with a time-sliced single pulse, with the EMU and DEVA beamlines receiving one quarter of the full muon pulse and mSR receiving the remaining half pulse. Developments are underway to enhance the E-field transition time of the present electrostatic kicker to minimise beam loss and enhance the frequency response. It is clear that the instruments situated on the side beamlines (at present EMU and DEVA) will benefit from the sharper muon pulses, unaffected by the pion decay in the production target. These beamlines are therefore best suited to those experiments where the best possible frequency response is required.
3. Using a 901 RF pulse In 1984 Carne et al. [2] suggested that a 901 RF pulse could be used to advantage at a pulsed muon source to remove the frequency limitations imposed on conventional transverse mSR experiments by the finite muon pulse width. Although the first demonstration of pulsed RF methods was carried out at a continuous muon source (work reviewed by Kreitzman in Ref. [4]), recent work at ISIS [5] has shown that the time structure of the pulsed beam can indeed be overcome if a short RF pulse is timed such that it follows the accumulation of muons during the ISIS pulse. The free precession signal following the RF pulse contains similar information to a conventional transverse field mSR measurement. The experiments described in this paper demonstrate that the method can be applied to the considerably harder (and topical) problem of studying the flux penetration in the mixed state of a type II superconductor, YNi2B2C. An RF coil was tuned at 13.6 MHz and the Q spoiled to obtain sharp RF pulses; at this frequency resonance for diamagnetic muons occurs at a field of 1034 G. The RF field strength in the rotating reference frame, B1 ; was determined to be 14 G, corresponding to a 901 pulse length of 1.36 ms. However, a pulse length of 0.75 ms was used because of the
277
expected high depolarisation rate, timed to immediately follow the ISIS muon pulse. The penetration of the RF field into the electrically conducting sample is limited by the skin depth, which at 13.6 MHz is estimated to be 5 mm. To overcome this difficulty finely powdered YNi2B2C was used, contained in a Mylar holder mounted in the RF coil. An Oxford Instruments exchange gas flow cryostat was used for the measurements, with a temperature range from 4 to 600 K. In order to establish a flux line lattice in YNi2B2C, a field of 1034 G was applied above the superconducting transition temperature. The sample was field cooled and the muon spectra were collected whilst warming the sample. This ensured that flux creep resulting from strong pinning did not affect the results. Excellent results were obtained with muon precession frequencies well above the usual upper frequency cut-off at ISIS (see Fig. 3). The free precession signal following the RF pulse is well described by Gz ðtÞ ¼ A0 expðs2 t2 Þ cosðot þ fÞ;
ð1Þ
where A0 is the initial asymmetry, s is the muon depolarisation rate, o is the muon precession frequency and f is the phase. As can be clearly seen from Fig. 3, s increases as the temperature decreases, indicating the presence of a flux line lattice. The determined values of s are in close agreement to those obtained from previous conventional transverse field experiments on YNi2B2C measured at ISIS and PSI [6].
4. Conclusion The work described in this paper clearly demonstrates that the upper frequency limit at a pulsed muon facility, such as ISIS, can be significantly extended either by time slicing the muon pulse or by using a 901 RF pulse. In practice, each technique has particular advantages and some disadvantages, and the method ultimately chosen will depend on the type of experiment required. For time slicing, the E-field transition time of the electrostatic kicker will place a limit on the possible extension of the frequency response, while
278
A.D. Hillier et al. / Physica B 326 (2003) 275–278
For the RF technique, skin depth and the muon response during the radio frequency pulse must be considered, and the method always leads to an experiment that is equivalent to a transverse field measurement. However, in general much higher precession frequencies can be studied using RF methods, far exceeding those available by timeslicing techniques. It is also interesting to note that any muons falling outside the RF coil, and therefore not affected by the RF pulse, do not contribute to the precession signal, making this an attractive technique to obtain unambiguous spectra in the frequency domain. Finally, utilising RF techniques opens the way to novel experiments such as charge state conversion measurements, RF decoupling techniques or other standard NMR multiple-pulse methods that have already been demonstrated at ISIS [7].
Acknowledgements We would like to thank Prof. R. Cywinski for the loan of the YNi2B2C sample.
References Fig. 3. Mixed state type II superconductor YNi2B2C measured using a 901 RF pulse in a static field of 1034 G for temperatures (a) 7.5 K, (b) 12 K and (c) 17 K. The corresponding values for the depolarisation rate, s; are 0.91, 0.75 and 0.12 ms1, respectively.
the rate loss associated with the reduced muon pulse width will clearly be an important consideration. However, this technique places no restriction on the method of the mSR measurement and sample characteristics, high frequency precession and fast relaxation rates can be measured with equal ease.
[1] G.H. Eaton, M.A. Clarke-Gayther, C.A. Scott, C.N. Uden, W.G. Williams, Nucl. Instrum. Methods A 342 (1994) 319. [2] A. Carne, S.F.J. Cox, G.H. Eaton, R. De Renzi, C.A. Scott, G.C. Stirling, Hyperfine Interactions 17–19 (1984) 945. [3] P.J.C. King, S.P. Cottrell, S.F.J. Cox, G.H. Eaton, A.D. Hillier, J.S. Lord, F.L. Pratt, T. Lancaster, S.J. Blundell, New science with pulsed muons—development ideas at ISIS, Physica B, these proceedings. [4] S.R. Kreitzman, Hyperfine Interactions 65 (1990) 1055. [5] S.P. Cottrell, C.A. Scott, B. Hitti, Hyperfine Interactions 106 (1997) 251. [6] R. Cywinski, Z.P. Han, R. Bewley, R. Cubitt, M.T. Wylie, E.M. Forgan, S.L. Lee, M. Warden, S.H. Kilcoyne, Physica C 233 (1994) 273. [7] S.P. Cottrell, S.F.J. Cox, J.S. Lord, C.A. Scott, Appl. Magn. Reson. 15 (1998) 469.
High frequency measurements at a pulsed muon source: beating the pulse width! A.D. Hillier*, S.P. Cottrell, P.J.C. King, G.H. Eaton, M.A. Clarke-Gayther ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
Abstract The pulsed nature of the ISIS beam is advantageous for many types of experiments. Limitations arise, however, because the finite width of the muon pulse restricts the maximum muon precession frequency and relaxation rate that can be measured. This paper considers two methods currently under development at ISIS that can be used to extend the upper frequency limit at a pulsed muon source. Firstly, a technique using a fast switching electrostatic kicker to time slice the incoming muon pulse is described. Data is presented that demonstrates the effective manner in which the mSR usable frequency response at ISIS is increased in inverse proportion to the pulse width. The second technique uses a 901 radio-frequency (RF) pulse to remove the time structure of the pulsed beam; this method is demonstrated by studying the flux penetration in the mixed state in the type II superconductor YNi2B2C. The merits of both techniques are considered and contrasted. r 2002 Elsevier Science B.V. All rights reserved. Keywords: ISIS facility; Electrostatic kicker; RF mSR; Superconductor
1. Introduction The pulsed structure of the ISIS beam is advantageous for many types of experiments. In particular, it enables time differential data to be measured to long times with minimal background and no time penalty, and it allows the efficient synchronisation of external stimuli (such as radiofrequency (RF), electric current, or light) for novel experiments. Limitations arise, however, because the finite width of the muon pulse restricts the maximum muon precession frequency and relaxa*Corresponding author. Tel.: +44-01235-446001; fax: +4401235-445720. E-mail address: [email protected] (A.D. Hillier).
tion rate that can be measured; at ISIS the muon pulse width of 80 ns corresponds to a FWHM band-pass of approximately 5 MHz (or maximum usable frequency of approximately 8 MHz). This paper considers two methods currently under development at ISIS that can be used to extend the upper frequency limit of a pulsed muon source. Firstly, a technique using a fast switching electrostatic kicker, as reported by Eaton et al. [1], to time slice the incoming muon pulse is described, and secondly, a 901 RF pulse is used in the manner proposed by Carne et al. [2] to remove the time structure of the pulsed beam. Results demonstrating both methods are presented in this paper and the relative merits of each technique are considered.
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 2 9 - 0
276
A.D. Hillier et al. / Physica B 326 (2003) 275–278
At ISIS the three spectrometers are fed from the accelerator double-pulse structure (two 80 ns wide muon pulses with a peak-to-peak separation of approximately 340 ns) by a fast electrostatic kicker. In normal running this is timed to slice the first muon pulse in half spatially to feed the two side beamlines (EMU and DEVA), whilst transmitting the whole of the second pulse to the central beamline (mSR). During the commissioning of the European Muon Facility, Eaton et al. [1] noted that by introducing an additional delay into the electrostatic kicker timing, such that the voltage drop occurs in the centre of the second muon pulse, it was possible to reduce the time width of the pulse feeding the mSR beamline. A practical demonstration of this time-slicing method is shown in Fig. 1. The EMU slit counter (monitoring the beam fed to the EMU beamline) shows the whole of the first pulse plus the first half of the second pulse, whilst mSR receives the second half of the second pulse after the kicker voltage has reduced to zero. The portion of the second pulse going into EMU has a width of 35 ns and an area of 6300 mV ns (45% of a full muon pulse), whilst the mSR pulse has a width of 47 ns and an area of 4600 mV ns (33% of a full muon pulse). The
increased width of the mSR pulse as compared with that going to EMU is probably due to the 26 ns pion lifetime in the muon production target, which has the effect of extending the pulse tail. The actual fall-time of the kicker voltage is much faster than shown by the trace in Fig. 1, which is limited by the bandwidth of the measuring device; considering the proportion of the beam lost in splitting, the true fall-time is thought to be less than 25 ns. The improvement in the frequency response was demonstrated by measuring the amplitude of the triplet precession signal as a function of applied transverse field for muonium formed in fused quartz. Curves were measured in the mSR beamline for both the normal, full-width pulse and the timesliced pulse. The results of these measurements are shown in Fig. 2 and clearly demonstrate the extension of the upper frequency limit that is possible by time slicing the muon pulse. For the full pulse, the half-maximum frequency response is B4.9 MHz while for the time-sliced pulse this is extended to B8.6 MHz, an increase of some 76% that is consistent with the 40% reduction in the pulse width. This highly successful demonstration is of particular importance as the technique forms a key part of the facility upgrade proposed in
Fig. 1. EMU and the time-sliced mSR pulses with the kicker voltage falling in the centre of the second pulse. The voltage fall-time of the kicker is artificially extended by the limited bandwidth of the measuring device.
Fig. 2. Frequency response measured in mSR for both the full muon pulse and the time-sliced pulse. The amplitude of the triplet precession for muonium formed in fused quartz was followed in applied transverse fields.
2. Time slicing the ISIS pulse
A.D. Hillier et al. / Physica B 326 (2003) 275–278
Ref. [3]. The upgrade will introduce an additional two-electrode kicker to divert the first muon pulse that at present continues to the EMU and DEVA beamlines into a new beamline. This will leave the existing facility with a time-sliced single pulse, with the EMU and DEVA beamlines receiving one quarter of the full muon pulse and mSR receiving the remaining half pulse. Developments are underway to enhance the E-field transition time of the present electrostatic kicker to minimise beam loss and enhance the frequency response. It is clear that the instruments situated on the side beamlines (at present EMU and DEVA) will benefit from the sharper muon pulses, unaffected by the pion decay in the production target. These beamlines are therefore best suited to those experiments where the best possible frequency response is required.
3. Using a 901 RF pulse In 1984 Carne et al. [2] suggested that a 901 RF pulse could be used to advantage at a pulsed muon source to remove the frequency limitations imposed on conventional transverse mSR experiments by the finite muon pulse width. Although the first demonstration of pulsed RF methods was carried out at a continuous muon source (work reviewed by Kreitzman in Ref. [4]), recent work at ISIS [5] has shown that the time structure of the pulsed beam can indeed be overcome if a short RF pulse is timed such that it follows the accumulation of muons during the ISIS pulse. The free precession signal following the RF pulse contains similar information to a conventional transverse field mSR measurement. The experiments described in this paper demonstrate that the method can be applied to the considerably harder (and topical) problem of studying the flux penetration in the mixed state of a type II superconductor, YNi2B2C. An RF coil was tuned at 13.6 MHz and the Q spoiled to obtain sharp RF pulses; at this frequency resonance for diamagnetic muons occurs at a field of 1034 G. The RF field strength in the rotating reference frame, B1 ; was determined to be 14 G, corresponding to a 901 pulse length of 1.36 ms. However, a pulse length of 0.75 ms was used because of the
277
expected high depolarisation rate, timed to immediately follow the ISIS muon pulse. The penetration of the RF field into the electrically conducting sample is limited by the skin depth, which at 13.6 MHz is estimated to be 5 mm. To overcome this difficulty finely powdered YNi2B2C was used, contained in a Mylar holder mounted in the RF coil. An Oxford Instruments exchange gas flow cryostat was used for the measurements, with a temperature range from 4 to 600 K. In order to establish a flux line lattice in YNi2B2C, a field of 1034 G was applied above the superconducting transition temperature. The sample was field cooled and the muon spectra were collected whilst warming the sample. This ensured that flux creep resulting from strong pinning did not affect the results. Excellent results were obtained with muon precession frequencies well above the usual upper frequency cut-off at ISIS (see Fig. 3). The free precession signal following the RF pulse is well described by Gz ðtÞ ¼ A0 expðs2 t2 Þ cosðot þ fÞ;
ð1Þ
where A0 is the initial asymmetry, s is the muon depolarisation rate, o is the muon precession frequency and f is the phase. As can be clearly seen from Fig. 3, s increases as the temperature decreases, indicating the presence of a flux line lattice. The determined values of s are in close agreement to those obtained from previous conventional transverse field experiments on YNi2B2C measured at ISIS and PSI [6].
4. Conclusion The work described in this paper clearly demonstrates that the upper frequency limit at a pulsed muon facility, such as ISIS, can be significantly extended either by time slicing the muon pulse or by using a 901 RF pulse. In practice, each technique has particular advantages and some disadvantages, and the method ultimately chosen will depend on the type of experiment required. For time slicing, the E-field transition time of the electrostatic kicker will place a limit on the possible extension of the frequency response, while
278
A.D. Hillier et al. / Physica B 326 (2003) 275–278
For the RF technique, skin depth and the muon response during the radio frequency pulse must be considered, and the method always leads to an experiment that is equivalent to a transverse field measurement. However, in general much higher precession frequencies can be studied using RF methods, far exceeding those available by timeslicing techniques. It is also interesting to note that any muons falling outside the RF coil, and therefore not affected by the RF pulse, do not contribute to the precession signal, making this an attractive technique to obtain unambiguous spectra in the frequency domain. Finally, utilising RF techniques opens the way to novel experiments such as charge state conversion measurements, RF decoupling techniques or other standard NMR multiple-pulse methods that have already been demonstrated at ISIS [7].
Acknowledgements We would like to thank Prof. R. Cywinski for the loan of the YNi2B2C sample.
References Fig. 3. Mixed state type II superconductor YNi2B2C measured using a 901 RF pulse in a static field of 1034 G for temperatures (a) 7.5 K, (b) 12 K and (c) 17 K. The corresponding values for the depolarisation rate, s; are 0.91, 0.75 and 0.12 ms1, respectively.
the rate loss associated with the reduced muon pulse width will clearly be an important consideration. However, this technique places no restriction on the method of the mSR measurement and sample characteristics, high frequency precession and fast relaxation rates can be measured with equal ease.
[1] G.H. Eaton, M.A. Clarke-Gayther, C.A. Scott, C.N. Uden, W.G. Williams, Nucl. Instrum. Methods A 342 (1994) 319. [2] A. Carne, S.F.J. Cox, G.H. Eaton, R. De Renzi, C.A. Scott, G.C. Stirling, Hyperfine Interactions 17–19 (1984) 945. [3] P.J.C. King, S.P. Cottrell, S.F.J. Cox, G.H. Eaton, A.D. Hillier, J.S. Lord, F.L. Pratt, T. Lancaster, S.J. Blundell, New science with pulsed muons—development ideas at ISIS, Physica B, these proceedings. [4] S.R. Kreitzman, Hyperfine Interactions 65 (1990) 1055. [5] S.P. Cottrell, C.A. Scott, B. Hitti, Hyperfine Interactions 106 (1997) 251. [6] R. Cywinski, Z.P. Han, R. Bewley, R. Cubitt, M.T. Wylie, E.M. Forgan, S.L. Lee, M. Warden, S.H. Kilcoyne, Physica C 233 (1994) 273. [7] S.P. Cottrell, S.F.J. Cox, J.S. Lord, C.A. Scott, Appl. Magn. Reson. 15 (1998) 469.