VARIABLE INDUCTOR AS DOWNHOLE TUNER

BACKGROUND

1. Field

Disclosed are methods and circuits for adjusting the time constant of an LC circuit to maintain stable electron energy and minimize power losses in an accelerator device, such as a Betatron.

2. Description of the Related Art

Betatrons are magnetic devices used to accelerate electrons to relativistic energies. A high energy electron beam is extracted and directed on a suitable target, generating high energy x-rays. One application of the high energy x-rays is for logging oilfield boreholes, such as to map subsurface density and lithology.

Effective operation of a Betatron requires high, pulsed, currents and voltages to generate the magnetic field necessary for acceleration and confinement of the electrons. The Betatron device is controlled and run by several power supplies, which form the Betatron modulator. A conventional Betatron driving circuit utilizes a high voltage D.C. power supply, coupled to a pulse generating modulator circuit, which in turn drives the Betatron coils. U.S. Pat. No. 5,077,530 to Chen discloses a Betatron driving circuit having a combination of a low voltage D.C. power supply and a high voltage excitation capacitor to drive the Betatron. U.S. Pat. No. 5,077,530 is incorporated by reference herein in its entirety.

Proper timing is needed to maintain stable electron energy and to minimize power losses. The time constant, τ_LC, is the time required for the current to rise from a base value to a peak value during a single duty cycle:

τ_LC=t₁−t₃   (1)

As illustrated in FIG. 3, t₁ is the time when current begins to rise and t₃ is the time peak current is reached. A typical τ_LC for a Betatron modulator on the order of 20-40 μseconds. τ_LC is mainly determined by the time constant combination L and C (reference numerals 10 and 12 in FIG. 1).

τ_LC=√{square root over (LC)}  (2)

Varying either L or C, controls the length of this time interval. However, the LC time constant is affected by manufacturing tolerances and by component variations as a function of temperature. Temperature variations in a borehole are particularly extreme and may differ by on the order of 250° C. from the surface to the bottom of the borehole. Because of this, existing Betatron modular circuits cannot be accurately tuned to the precise time at which the peak of the current is reached and even if tuned at the surface, go off-peak due to temperature fluctuations. An additional source of heat that could increase the temperature variation is electrical resistance of the circuit components.

One way to tune an LC circuit is by using a capacitor bank with different value capacitors and a switch to connect the appropriate capacitor (or combination thereof) in series with or parallel to inductor L. Another way to tune LC circuits is disclosed in U.S. Pat. No. 6,121,850 to Ghosal that discloses a digitally adjustable inductive element utilized to provide a tunable oscillator.

A tunable inductor is disclosed in U.S. Pat. No. 5,426,409 to Johnson, which is incorporated by reference herein in its entirety. The tunable inductor has a magnetically saturable core with a pair of outer limbs joined to a center limb by connecting limbs. Signal winding are formed about each outer limb and a signal source connected to the signal windings induces a signal flux in the core through the outer limbs. A bias winding formed around the center limb induces a variable bias flux into the core. The flux induced by the signal windings is maintained below saturation. Adding flux via the bias winding controllably changes the inductance of the variable inductor. By applying a varying control current, we can move to a different section of the B-H curve, thereby obtaining a different amount of change in flux density provided by the same amount of variation of magnetization force.

BRIEF SUMMARY

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages will be apparent from the description and drawings, and from the claims.

Disclosed below is tunable LC circuit that may be used obtain precise timing for electron beam injection and extraction from an accelerator device, such as a Betatron. The circuit includes a first inductor having a first inductance electrically coupled in series with a capacitor. A second inductor having a variable inductance is electrically coupled, either in series or parallel, to the first inductor. Recognizing that the time to capacitor discharge is governed by:

τ_LC=√{square root over ((L+L_TUNE)C)}

adjusting the inductance of the variable inductor (L_TUNE) facilitates continuous adjustment of the discharge time. This is particularly useful when the LC values change in response to external stimuli, such as borehole temperature when a Betatron is used to log borehole features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first embodiment of a modulator circuit for Betatron.

FIG. 2 illustrates an exemplary saturable inductor as known from the prior art.

FIG. 3 graphically illustrates how the saturable inductor adjusts the time constant, τ_LC.

FIG. 4 graphically illustrates how a saturable inductor facilitates movement along a B-H curve (Flux Density, B, as a function of Magnetizing Force, H).

FIG. 5 is a second embodiment of a modulator circuit for Betatron.

Like reference numbers and designations in the various drawings indicated like elements.

DETAILED DESCRIPTION

FIG. 1 illustrates a modulator circuit for a Betatron. Components of this modulator circuit include low voltage D.C. power supply 44, a low voltage capacitor 50, a high voltage capacitor 12 and a Betatron coil 10. The capacitance of the low voltage capacitor 50 is much greater than the capacitance of the high voltage capacitor 12, nominally on a scale of 100 times higher. The Betatron coils 10 having a first inductance, are coupled in an LC oscillating relationship between the low voltage capacitor 50 and the high voltage capacitor 12. The Betatron coils form a portion of the L component of an LC circuit. High voltage storage capacitor 12 forms the C component. When switches 14, 16 are closed, a current starts flowing through the Betatron coils 10. A variable inductor 18 in series with Betatron coils 10 controls the timing by adjusting the inductance L of the coils and thus the time constant of the LC circuit. The time constant is now given by:

τ_LC=√{square root over ((L+L_TUNE)C)}  (3)

The time constant of the LC circuit can now be continuously adjusted to compensate for manufacturing tolerances and temperature effects of the modulation frequency of the Betatron device by constantly measuring the time interval t₁-t₃ and assuring that this interval remains constant.

Any suitable variable inductor that is continuously variable over a range of desired inductances may be utilized. An exemplary variable inductor 18 is illustrated in FIG. 2 and includes a first core 20 and second core 22 both formed from a highly permeable material. The desired range of inductance preferably includes the saturation point of the variable inductor 18. A first signal winding 24 around first core 20 and a second signal winding 26 around a second core 22 are connected inducing a signal flux 30. A first control winding 32 around the first core 20 and a second control winding 34 around the second core 22 are connected to a DC power source 36 inducing a control flux 38. Connection points A and B align with connection points A and B in FIG. 1 to illustrate an embodiment of how the variable inductor 18 may be fit into the modulator circuit.

FIG. 3 illustrates how changing the rate of current rise slope 42 changes the time to peak current from peak 60 (t₃) to peak 62 (t₃′). Line 64 represents a modulator circuit without a variable inductor, or with the variable inductor at saturation. The time constant of the rise of current is governed by Equation (2) above. Line 66 represents a modulator circuit including an unsaturated variable inductor. The time constant of the rise of the current is governed by Equation (3) above from ti until t₂, when the variable inductor reaches saturation At t₂, the saturable inductor saturates and its inductance becomes effectively 0. When this happens, Equation (3) reduces to Equation (2) and the rise of current proceeds from t₂ until t₃ as if the saturable inductor were not present. Likewise, the time constant of the current fall, from t₃ to t₄ is governed for both line 64 and line 66 by Equation (2).

The inflection point 68 at time, t₂, where the rate of rise of the current changes, can be adjusted by adjusting the bias current or voltage level of DC power supply 36 (FIG. 2) of the variable inductor. At the point of saturation, the inductance collapses and the saturable inductor acts as a short circuit. Consequently, when a variable inductor is in the circuit, for time interval t₁-t₂ the time constant is governed by Equation (3) and for time intervals t₂-t₃ and t₃-t₄ the time constant is governed by Equation (2). It is noted that the shift in time until peak current, i.e. from 60 to 62 in FIG. 3, is due to the change in rate of current rise and may be achieved even if the variable inductor does not saturate, i.e. there is no inflection point 68.

A feedback loop may monitor t₃ and the desired time to peak current. If these two values diverge, the inductance of the variable inductor may be varied to return the values to convergence. A sensor may monitor external data that would influence the timing, such as Betatron temperature, and cause the variable inductance to be varied in response to this external data.

FIG. 4 illustrates how a change in magnetizing force, H, influences the flux density, B, in a saturable inductor. When relatively removed from saturation, a change in magnetizing force has a more significant impact on flux density, than close to saturation. Curves C-D and C′-D′ show that when removed from saturation, a 0.1 oersted change in magnetizing force results in approximately a 1.3 kilogauss change in flux density. Curves E-F and E′-F′ show that close to saturation, a 0.1 oersted change in magnetizing force results in approximately a 0.4 kilogauss change in flux density. This indicates that finer tuning of the inductance is possible when close to saturation. The distance between the pre-bias points (G or H) and the saturation point I determines the t₁-t₃ interval.

In an alternative embodiment that is illustrated in FIG. 5, the variable inductor 18 is in parallel with the Betatron coil 10 that requires tuning. The time constant for this implementation is given by:

τ

LC

=

(

L

·

L

TUNE

L

+

L

TUNE

)



C

(

4

)

In this embodiment, Equation 4 governs the time constant for t₁-t₂ and Equation 2 governs the time constant for t₂-t₃.

The circuits of FIGS. 1 and 5 are useful with components having a wide range of values. Following are exemplary ranges:

44

Power Supply

40 volts

46

Isolation Choke

small inductance,

optional

48, 52, 54, 56

Diode

1200 V, 200 A

50

Low Voltage Storage Capacitor

600 μf, 60 V

10

Betatron Coils

100-135 μH

18

Variable Inductor

variable, 0-10 μH

12

High Voltage Storage Capacitor

1200 V, 5 μF

36

D.C. Source

0-10 A

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the disclosed circuits could also be used for tuning antennas for nuclear magnetic resonance, coils for induction tools or antennas for propagation tools. Accordingly, other embodiments are within the scope of the following claims.