News and Events
- Signal feedback applications in low-field NMR and MRI. 2019. Vyacheslav Kuzmin, Pierre-Jean Nacher ⟨hal-02069159⟩
AbstractTuned pick-up coils with high quality factors Q are used in NMR and MRI for high-sensitivity and low-noise detection. However, large Q-factors introduce bandwidth issues at low frequency and the associated enhanced currents may cause significant radiation damping effects, especially with hyperpolarised samples. Signal feedback can be used to actively control these currents and adjust the detection bandwidth without resistive losses. Capacitive and inductive coupling methods are compared using detailed models and the operating conditions for efficient feedback with negligible noise penalty are discussed. Several high-impedance commercial preamplifiers have been found to affect the resonance characteristics of tuned coils in a gain-dependent way, or could not be used in low-frequency NMR because of oscillations at large positive gain. This is attributed to an undocumented internal feedback, and could be neutralised using external feedback. The implementation of an inductive coupling scheme to feed a suitably amplified phase-adjusted signal back into the PU coils of low-field NMR systems is described, and three experimental applications are reported. One system is used for NMR studies of distant dipolar field effects in highly polarized liquid 3He without or with radiation damping. The moderate intrinsic Q-factor (≈7) could be reduced (down to 1) or increased (up to 100) to control transient maser oscillations. Another system was used for MRI of water samples around 2~mT with Q≈190 Litz-wire detection coils. The detection bandwidth was increased by actively reducing the Q-factor to obtain uniform sensitivities in images and avoid artifacts introduced by intensity corrections. Finally, parallel acquisition in MRI was performed using two separately tuned detection coils placed above and below the sample. They were actively decoupled using two feedback systems. For an imaging field of view smaller than the sample, artifact-free unfolded images demonstrate the efficiency of this active coil decoupling scheme.
- A fast MOSFET RF switch for low-field NMR and MRI. 2019. Pierre-Jean Nacher, Sashika Kumaragamage, Geneviève Tastevin, Christopher Bidinosti. ⟨hal-02266979⟩
AbstractTRansmit Array Spatial Encoding (TRASE) MRI uses trains of rf pulses alternatively produced by distinct transmit coils. Commonly used coil switching involving PIN diodes is too slow for low-field MRI and would introduce wait times between pulses typically as long as each individual pulse in a few mT. A MOSFET-based rf switch is described and characterised. Up to hundreds of kHz, it allows for sub-μs switching of rf currents from a single amplifier to several coils with sufficient isolation ratio and negligible delay between pulses. Additionally, current switching at null current and maximum voltage can be used to abruptly stop or start pulses in series-tuned rf coils, therefore avoiding the rise and fall times associated with the Q-factors. RF energy can be efficiently stored in tuning capacitors for times as long as several seconds. Besides TRASE MRI, this energy storage approach may find applications in fast repeated spin-echo experiments. Here, a three-fold acceleration of TRASE phase-encoding is demonstrated when MOSFET switches are used instead of fast reed relays.
- A model of quantum collapse induced by gravity. 2019. Franck Laloë <hal-02160619>
AbstractWe discuss a model where a spontaneous quantum collapse is induced by the gravitational interaction, treated classically. Its dynamics couples the standard wave function of a system with the Bohmian positions of its particles, which are considered as the only source of the gravitational attraction. The collapse is obtained by adding a small imaginary component to the gravitational coupling. It predicts extremely small perturbations of microscopic systems, but very fast collapse of QSMDS (quantum superpositions of macroscopically distinct quantum states) of a solid object, varying as the fifth power of its size. The model does not require adding any dimensional constant to those of standard physics.
Maul A., Blümler P., Nacher P.-J., Otten E., Tastevin G., and Heil W.
Phys. Rev. A (2018) 98, 063405 [12 pages]
J. Low Temp. Phys., First Online: 15 July 2019 [18 pages]
Mécanique Quantique, tomes 1 et 2, nouvelle édition. C. Cohen-Tannoudji, B. Diu et F. Laloë (EDP Sciences - Savoirs Actuels - Sept. 2018).Comprenons-nous vraiment la mécanique quantique ?.
Franck Laloë (EDP Sciences - Savoirs Actuels - 2nd ed., 2018).Mécanique Quantique - Tome III - Fermions, bosons, photons, corrélations et intrication. C. Cohen-Tannoudji, B. Diu, F. Laloë (EDP Sciences - Savoirs Actuels, 2017).
The MARGIN project
MARGIN (2020-2023) mainly deals with MAgnetic Resonance studies of Gas diffusion In Nanoporous materials. Investigations focus on of the influence of gas-wall interactions on NMR diffusion measurements.
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Lionel DJADAOJEE (Teaching assistant at ENS Paris – PSL, Agrégé de Physique) has joined the group for PhD work on the metastable phases of solid and liquid 4He (J. Grücker, superviser; Sept. 2019 – …).
Philippe Jacquier, leader of the “Quantum solid and liquid helium” team since 2003, passed away on Jan.24, 2019.
2018-2019 - Master / PhD
Polarised helium - Low field MRI
Skilled and motivated students are welcome to join our investigations on low field MRI with laser polarised ³He gas or thermally polarised water. Work may focus on the impact of concomitant field gradients on image quality, or investigate the short-pulse limit and revisit the Bloch-Siegert effect in NMR. This topic is open for PhD.
Polarised helium - Liquid NMR
Skilled and motivated students are welcome to join our investigations on liquid NMR in laser polarised ³He-⁴He dilute solutions. Work will deal with recently observed non linear NMR dynamics and focus on the comparison between experimental and numerical data. This topic is not (yet) open for PhD.
2018-2019 - M1 level internships
Skilled and motivated students are welcome to join on-going investigations. M1-level students may contribute to:
- Investigation of the impact of concomitant field gradients on image quality in low field MRI
- Investigation of collisional processes in helium discharges using laser spectrocopy
- Implementation of annular beam shaping for helium laser optical pumping
We are always looking for brillant motivated scientists to join our group.
Check the open positions (see above) or send us your CV for application.
Département de Physique Ecole Normale Supérieure, 24 rue Lhomond, F-75231 Paris Cedex 05
- Jules Grucker : jules.grucker at lkb.ens.fr
- Philippe Jacquier : philippe.jacquier at lkb.ens.fr
- An Qu : an.qu at lkb.ens.fr
- Franck Laloë : laloe at lkb.ens.fr
- Pierre-Jean Nacher : nacher at lkb.ens.fr
- Geneviève Tastevin : genevieve.tastevin at lkb.ens.fr