Generation of Quadrupole Magnetic Field for Trapping Atoms in Cs Fountain being Developed at NPL India

Transcription

1 MAPAN - Journal of Metrology Society of India, Vol. 26, No. 4, 2011; pp ORIGINAL ARTICLE Generation of Quadrupole Magnetic Field for Trapping Atoms in Cs Fountain being Developed at NPL India K. PANT*, P. ARORA, S. YADAV and A. SENGUPTA National Physical Laboratory India (NPLI) Council for Scientific and Industrial Research (CSIR), Dr. K.S. Krishnan Marg, New Delhi * pantk@mail.nplindia.ernet.in [Received: ; Revised: ; Accepted: ] Abstract We describe the development of a constant current source for magneto-optical trap (MOT) coils. The system is designed to supply maximum 5 A current in continuous or pulsed mode to the anti- Helmholtz (AH) coils which produce quadrupole magnetic field inside the chamber for trapping of Cs atoms. To obtain a stable field gradient the coils are operated at low current to minimize current fluctuations. With AH coils having 100 turns and 5 Ù resistances, we achieved rise and fall times of current as 5 ms and 7 ms, respectively. With a field gradient of 6 G/cm at the center of the MOT, 7 number of atoms trapped is about 10. To prevent burning of coils due to failure of current source, a thermal protection circuit has been incorporated. Measurements made for switching times of current profile in coils is also reported. 1. Introduction the laser (by the magnetic field the Zeeman effect), will absorb a photon, and the momentum received Cesium (Cs) fountain frequency standard is one from the photon will give the atom a kick towards of the highly accurate measurement devices the center of the trap [4]. currently available. Such atomic fountains are being developed at several laboratories around the world Figure 2 shows anti-helmholtz coil and the [1-3] and a few of them operate as primary frequency octagonal MOT chamber which is a part of the standards. In a Cs fountain, atoms are first cooled physics package of Cs fountain being developed at and trapped in a MOT. The MOT consists of two NPL India [3]. Generation of quadrupole magnetic coils with currents flowing in opposite directions field is crucial for trapping the atoms. Quadrupole that produce a quadrupole magnetic field as shown magnetic field means the magnetic field at the center in Fig. 1. There are three sets of counter-propagating, between the two coils is zero and there is a uniform circularly polarized laser beams that are at a field gradient as one moves away from the center. frequency slightly less than that of the atomic The axial gradient, B z, is twice as compared to the resonance (red-detuned). Atoms that drift towards radial gradient, B ñ, of the field as estimated with the one of the lasers will be brought into resonance with following equations: Metrology Society of India, All rights reserved

2 K. Pant, P. Arora, S. Yadav and A. Sengupta Fig. 1. Schematic of a MOT dr B NI3 z, B z 3 NI d r 0 5/2 dr d r 0 5/2. where is 0 the magnetic permeability, I is the current flowing through the coils, N is the number of turns in each coil, is any vector in the x-y plane, r is the radius of the coils and 2d is the distance between the centers of the two coils. All the electronic modules required for the development of Cs fountain-cs source temperature controller, acousto-optic modulator (AOM) driver, sequence control circuits; various current sources etc. are indigenously developed in our laboratory. In (1) this paper, we report the development of constant current source for generating quadrupole magnetic field for trapping atoms. It is designed to operate in pulsed current mode as well as in continuous current mode when power dissipation in power devices is very high. For this reason power devices are mounted on a large heat sink. Operation in continuous current mode is required during initial testing of MOT. The design details of current source are discussed in detail in the next section. 2. Constant Current Source - Design Details The current source is composed of seven circuit elements viz. anti-helmholtz (AH) coils, DC supply, power circuit for driving the coils, control circuit, optically isolated switching interface, current display circuit and thermal protection circuit. 286

3 Generation of Quadrupole Magnetic Field for Trapping Atoms in Cs Fountain being Developed at NPL India Fig. 2. One of the anti-helmholtz coils fixed on the top of octagonal chamber which is a part of physics package of Cs fountain The block diagram and schematic diagram of field gradient at the cost of higher switching times complete circuit are shown in Figs. 3 and 4, because lower currents and power are easier to respectively. handle [ 5]. 2.1 Anti-Helmholtz Coils (MOT Coils) 2.2 Power Supply These coils act as load for the current source. For There are three power supplies viz. 50 V/6 A magneto optical trapping of cold neutral atoms, two unregulated for supplying power to anti Helmholtz coils are arranged in anti-helmholtz configuration (AH) coils, ±12 V/1 A regulated for control and (current passing in opposite directions) and fixed on thermal protection circuits and +5 V/1 A for current the top and bottom of the octagonal cooling chamber display module (Fig. 3). These supplies are derived (Fig. 2) of the physics package of the Cs fountain. from the secondary windings of the same These coils are wound on a non magnetic aluminum transformer. Input to the primary winding is 230 frame of 150 mm diameter, have 100 turns each and V/50 Hz mains supply. We have chosen create a magnetic field gradient of 6 G/cm (for a unregulated supply to lower the weight of supply current of 3.5 A) at the centre of the MOT chamber. and moreover it does not impair the magnetic field DC resistance of both coils in series is 5. Instead of gradient inside the chamber. Bleeder resistors are operating the coils at higher currents with few turns connected after LC filter of unregulated supply to we have used many turns to operate the coils at low stabilize it to some level. At no load output voltage currents. This ensures a well stabilized magnetic of supply is about 50 V and is chosen to be the 287

4 K. Pant, P. Arora, S. Yadav and A. Sengupta DPM: Digital Panel Meter D : Schottky Diode (SB2060CT) AH Coils: Anti-Helmholtz Coils F Fig. 3. Block diagram of Constant Current Source for Anti-Helmholtz Coils minimum by selecting proper secondary winding of transformer for minimum power dissipation in the power devices. Table 1 Power dissipation in MOSFETs when operated in continuous current mode 2.3 Coil Driving Power Circuit S. No Vs IL VDS PD (Volts) (Amps) (Volts) (Watts) The circuit diagram is shown in Fig.4. For operating the circuit in continuous current mode a constant DC voltage proportional to the set-current (adjusted by potentiometer P) is applied from the control circuit to the junction A of gate resistances The MOSFETs (IRF-250) turn-on and a steady DC current flows in the AH coils to produce a magnetic field gradient for trapping the Cs atoms. VS Unregulated DC supply voltage I L Load current through anti Helmholtz coils During this mode the Schottky diodes (also VDS Voltage across MOSFETs called flyback or freewheeling diodes) connected PD Power dissipation in MOSFETs across the coils do not play any role and remain reverse biased for the entire period. Table 1 shows power dissipation in MOSFETs at different load currents. The maximum power dissipation is at 3 A. The current source is designed for maximum current rating of 5 A and a single MOSFET (I D=30 A 288

5 Generation of Quadrupole Magnetic Field for Trapping Atoms in Cs Fountain being Developed at NPL India Fig. 4. Schematic diagram of Constant Current Source for anti-helmholtz coils for IRF-250) is sufficient for carrying this current but the current and voltage waveforms across the AH to share the large power dissipation we have used 3 coils during this mode. MOSFETs in parallel which are mounted on a large heat sink.to ensure that power devices operate The fountain is in initial testing phase, hence the within their safe operating areas the current MOT current period is kept little more ( 500 ms) unbalance in the parallel devices should be than what is normally used in the working fountains minimised. Unbalance of current is generated due to in order to increase the number of trapped atoms mismatch of i) drain source resistance- R DS (on); ii) and hence improve the return signal and signal to gate threshold voltage V T ; iii) gate decoupling noise ratio. resistance- R G ; iv) gate drain and gate source capacitances-c GD, C GS. Except R DS (on), unbalance We are using unregulated supply for powering due to other factors is very small. Difference in R DS the coils. Therefore during turn-on initially the (on) causes current unbalance and extra conduction current in the coils reaches a peak value of 5.6 A in 5 losses but these are limited due to positive ms and falls smoothly to a 3.8 A in about 150 ms. This temperature coefficient of the MOSFETs [6]. We is shown in Figs. 6 and 7 respectively. measured current in each device and found improvement in current sharing by connecting When the MOSFETs are switched-off, the resistances r 1, r 2, r 3 (0.2 /2 w) in the source circuit. magnetic field energy stored in the inductance of the coils resists the sudden drop of current by reversing When the circuit is operated in pulsed current the voltage across its ends. At this instant the mode, a steady current flows in the coils for a very Schottky diode becomes forward biased, clamps small period (about a few milliseconds) during voltage across coils to about 1 V and conducts which the AH coils behave like a simple resistor and current until the energy stored in the inductor is get fully energized by the DC current. Fig. 5 shows dissipated in the resistance of the wire and diode. 289

6 K. Pant, P. Arora, S. Yadav and A. Sengupta Fig. 5. V A voltage across AH coils, I A current in AH coils Fig. 6. Rise time of current in AH coils. Due to unregulated supply voltage, current (I ) in coils rises to A 5.6 A in 5 ms and reaches a stable value after some time (shown in Fig. 7) 290

7 Generation of Quadrupole Magnetic Field for Trapping Atoms in Cs Fountain being Developed at NPL India Fig. 7. Current in AH coils takes 150 ms to reach the stable value of 3.8 A As shown in Fig. 8 the freewheeling diode DF reference formed by precision voltage reference ensures smooth fall of current in about 7 ms. LM431 (IC-3), 1 kù current setting potentiometer and associated resistances constitute the control When the devices are switched-off high dv/dt circuit [8]. Current in the MOT coils is sensed by R S and di/dt values in the drain source circuits inject and amplified by IC-1. The gain of this IC is adjusted currents into the gate circuit through reverse such that at maximum load current output voltage at transfer drain-gate capacitance and induce high pin-6 of IC1 is 2.5V which is the maximum reference frequency parasitic oscillations in the gate circuits. voltage that can be set by the potentiometer. When These undesirable oscillations cause over voltage some reference is given to non-inverting pin-3 of ICtransients on the gates which may lead to 2 its output goes high and turns-on the MOSFETs. destruction of one or more devices. We have used 56 The current is sensed by R S, amplified by IC-1 and (ferrite beads are also used) resistances connected fed back to the inverting input of IC-2. IC-2 adjusts to gates of each MOSFET for damping these its output voltage and hence the drain current (I D) undesirable oscillations. 15 V zeners across gate such that the voltage drop across R S is very close to source of each MOSFET reduce low frequency reference voltage after amplification by IC 1. This oscillations and protects gate source from high ensures a constant current in R S which is the current voltage transients when the devices are switched off in MOT coils. [7]. 2.5 Optically Isolated Switching Interface 2.4 Control Circuit As shown in Fig. 4, two operational amplifiers IC1 and IC2, current sensing resistor R S, current Switching cycle and on-off time of current source is decided by timing and sequence control circuit. Opto-coupler 4N35 (IC-4 in Fig. 4) isolates 291

8 K. Pant, P. Arora, S. Yadav and A. Sengupta Fig. 8. Fall time of current in AH coils is 7ms and interfaces these two circuits and a BNC The temperature is sensed by a thermistor (T h) mounted on the front panel is used for feeding the glued on the MOT coil fixed on the top of the input signal. The current source is operated in octagonal chamber (Fig. 3). Voltage reference point continuous mode by grounding the BNC input and P connected to threshold pin 6 of IC555 sets the cutin pulsed mode by giving signal to the BNC input. off temperature. One of the secondary windings of the high current transformer is directly connected to 2.6 Current Display Circuit the 25 A bridge and another winding is connected through a relay (RL). When switch SW1 is pressed The circuit is shown in Fig. 4. For displaying the and released, output of timer (IC 555) goes high and current in the MOT coils, voltage across current transistor T1 turns on. This energizes relay RL and sense resistor R S is filtered by a low pass R-C filter, the supply to current source is switched-on. With amplified and given to the digital panel meter rise in temperature resistance of thermistor (DPM) mounted on the front panel. decreases resulting in increase of Vref or the voltage at threshold pin of IC 555. When this voltage reaches 2.7 Thermal Protection Circuit rd 2/3 of supply voltage, output of this IC goes low, switching-off the supply to the current source. The circuit is shown in Fig. 9. It protects MOT coils from overheating when they are subjected to 3. Results maximum current (5 A) continuously for long duration or in the event of burning of the MOSFETs The magnetic field gradient inside the chamber which causes the current to exceed 5 A and only follows the current in the AH coils; hence limited by the DC resistance of the coil. observations were taken for rise and fall times of 292

9 Generation of Quadrupole Magnetic Field for Trapping Atoms in Cs Fountain being Developed at NPL India Fig. 9. Thermal Protection Circuit current in the coils. Voltage and current in the coils and period of the fountain cycle. Our fountain is in are measured with Tektronix TDS2012C the initial stage of testing.. Therefore to increase the oscilloscope as shown in Fig. 3. Voltage across 1 Ù/2 number of atoms in the return signal and improve W resistance gives directly current in amperes. the signal to noise ratio the loading time is kept little Summary of the results corresponding to Figs. 5 to 8 more about 500ms. As shown in figures 6 and 8 rise is given in Table 2. and fall of current in the coils is smooth and without any transients. Before starting the next sequence Table 2 after the mot current is switched-off a dead time of Summary of measurements taken for current about 20ms when all the lasers are off has been profile in anti-helmholtz coils corresponding to included to take care of 7ms fall time of the current. Figs. 5 to 8 4. Conclusion S. No. Parameter ms Fig. 1. Mot current duration T h e c u r r e n t s o u r c e w a s t e s t e d f o r 2. Current rise time 5 6 characterization of atomic cloud in the magneto- 3. Stabilisation time of current optical trap. The effect of changing the magnetic 4. Current fall time 7 8 field gradient was also investigated by changing current from 0 A to 5 A. The effect of changing the The relation of magnetic field gradient and current and hence the magnetic field gradient on number of trapped atoms is shown in Fig. 10. total number of trapped atoms is shown in Fig. 10. Duration of mot current and switching times of mot As the field gradient increases, the restoring force current pulse influence number of atoms trapped becomes stronger and helps in trapping more 293

10 K. Pant, P. Arora, S. Yadav and A. Sengupta Fig. 10. Number of trapped atoms as a function of magnetic field gradient produced by the anti-helmholtz coils number of atoms. This confirms that the current [3] A. Sen Gupta, A. Agarwal, P. Arora and K. source for the AH coils works as per the Pant, Development of Cesium Fountain requirement. The module has been working Frequency Standard at NPL, India, Current satisfactorily since last few months. Science, 100 (2011) [4] C.J. Foot, in Atomic Physics, Oxford Acknowledgement University Press, Oxford, (2005), The authors are highly thankful for the sustained support and encouragement to the NPLI fountain project by its present and previous Directors - Dr. R.C. Budhani, Dr. V. Kumar, Dr. K. Lal and Dr. A.K. Raychoudhury. Financial support from CSIR is gratefully acknowledged. [5] T.P. Meyrath, Electromagnetic Design Basics for Cold Atom Experiments, (2004) www. george.ph.utexas.edu/ meyrath/informal/ electromagnets.pdf. [6] J.B. Forsythe, Paralleling of Power MOSFETs For Higher Power Output. technical-info/ appnotes/para.pdf References [7] J.Dodge, Eliminating Parasitic Oscillation between Parallel MOSFETs, Application [1] D.M. Meekhof, S.R. Jefferts, M. Stepanovic Note-APT-0402, Rev. A, Advanced Power and T.E. Parker, Accuracy Evaluation of a Technology, (2004). Cesium Fountain Frequency Standard at [8] AN-968 Current Sources: Options and NIST, IEEE Trans. Instrum. Meas., 50 (2001) Circuits Application Note, /static/imported-files/application_notes/ [2] S. Weyers, U. Huebner, R. Schroeder, C. AN-968.pdf Tamm and A. Bauch, Uncertainty Evaluation of the Atomic Cesium Fountain of the PTB, Metrologia, 38 (2001)