GB2213633A - Temperature control of microchannel plate electron multiplier - Google Patents

Abstract

A constant current is passed through the microchannel plate so that the voltage at an electrode 88 thereon is dependent on the plate temperature, this voltage being compared 74 with a reference in order to control a plate cooling system 80, whereby the plate is maintained at a predetermined operating temperature. The microchannel plate may have a first array 22 following by a second array 24 which is one fifth the thickness of array 22. Power supplies 70, 72 provide respective constant currents Ii Io to arrays 22, 24. Io is five times Ii so that the heat dissipation is the same in both arrays. The cooling system 80 may be thermoelectric resistance use of the two arrays 22, 24 of different thickness and resistance, in association with manufacture of the arrays from a high temperature glass and the provision of the temperature control system, allows operation at a frequency greater than 100 kHz, which is 500 times as great as prior arrangements. The microchannel plate may have three successive arrays (Fig 5). <IMAGE>

Description

MICHOCHANEL PLATES AND APPARATUS UTILISIG SAME.

This invention relates to microchannel plates ("MCP"'s) and to apparatus employing MCP's.

MCP's have now been long known in the art; early patents were Goodrich et al. U. S. Patent No. 3,128,408, "Electron Multiplier", granted April 7, 1964 and Goodrich et al. U. S. Patent No. 3,341,730, "Electron Multiplier with Multiplying Path Wall Means Having a Reduced Reducible Metal Compound Constituent", granted September 12, 1967.

An early patent disclosing chevron-paired MCP's was Goodrich U. S. Patent No. 3,374,380, "Apparatus for the Suppression of Ion Feedback in Electron Multipliers", granted March 19, 1968.

In typical prior art MCP's, recovery time (owing to slowness of movement of electrons in channel walls to replenish electrons previously emitted from the walls) has been in general several milliseconds. This has limited the frequency (herein, in proper context, "frequency") of use of the device to about the order of 200 Hz.

A single section (total of two electrodes) MCP with lower resistance in amplified-end channel surface zone material has previously been suggested.

We have now discovered that recovery time may be considerably shortened in MCP's--indeed, to frequencies great than 100 kHz--in practical embodiments of MCP.

The present invention is divided out of our British Patent Application 8722922 (Serial No. 2197120) which is concerned with microchannel plates and to which the reader should refer for further details.

In accordance with the present invention, there is provided electrical apparatus comprising: a microchannel plate; a constant-current power supply adapted for imposing voltage to cause current flow in said microchannel plate; cooling means adapted for cooling said microchannel plate; and control means adapted to regulate said cooling means to maintain operation of said microchannel plate at a predetermined temperature.

We describe circuitry which prevents thermal runaway and makes possible controlled higher temperature of operation.

Preferably the microchannel comprises a plurality of arrays, a first array in an electronamplifying direction from a second array having a surface zone resistance lower than that of said second array.

In preferred embodiments there are two sections, in contact, chevron-related, and with a common electrode between them; each section is driven by a constant current power supply, resistances in the sections being controlled by cooling means in turn controlled through a voltage comparator; and the sections are fabricated from high-temperature glass.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a side elevation of a preferred embodiment of MCP; Fig. 2 is a sectional view, taken at 2-2 in Fig.

1, and somewhat diagrammatic; Fig. 3 is a corresponding sectional view through one of the channel members of each section of the MCP of Fig. 2; Fig. 4 is an enlarged view of a section of the MCP of Figs. 1 to 3, showing the electric field; Fig. 5 is a modified embodiment with three sections; and Fig. 6 is a schematic diagram of an embodiment of electrical apparatus in accordance with the present invention showing the control system.

In Figs. 1 and 2 is seen a two-section t1CP 20 (detail only shown in upper left-hand corner) with an input array 22 and an output array 24, each including a multiplicity of channel portions 23, 25 with identical channel inside diameters and channel center-to-center spacings. The inside diameter of channels 31, 33 in channel members 23, 25 of arrays 22, 24 is 25 microns.

The glass from which arrays 22, 24 are formed has the following formulation: % by Weight Si02 34.8 A1203 0.2 Rb20 3.5 Cs20 2.4 PbO 54.9 BaO 4.0 As 205 0.2 This glass is capable of continuous operation at 125"C.

Different resistivities are achieved by different processing, in manners well known in the art, of this same glass.

Energy is provided through circuitry hereinafter described and including lines 28, 30, and 32 to provide increasing potential across array 22 and array 24. Array 22 has conductive coatings 36 and 38 on the input and output surfaces respectively, and array 24 has such coatings 40, 42 respectively. Preferably the facing coatings 38 and 40 are provided by ion implantation of nichrome, and are spaced apart-by a thin layer of glass 34 deposited by transverse flow so as not to block channel passages 31, 33 in channel members 23, 25, which layer 34 secures together arrays 22, 24. Bonding is by techniques as in Pomerantz U.S. Patent No. 3,397,278, August 13, 1968, "Anodic Bonding", and Pomerantz U.S. Patent No. 3,417,459, December 24, 1968, "Bonding Electrically Conductive Metals to Insulators".

A ring of nichrome is placed around glass layer 34 to short between layers 38 and 40 so that those layers form in effect a common electrode 84. Layers 36 and 42 provide electrodes 86 and 88 respectively.

Although shown diagrammatically as of equal thickness (in, i.e., an electron'flow direction) with array 22, array 24 is in fact much thinner, and is assembled to array 22 and then ground down to final desired thickness. In this preferred embodiment, array 22 has a thickness of 1000 microns, and array 24 a thickness of 200 microns.

The electric field existing in an array is shown in Figure 4 where field lines 44 are shown parallel to the walls of the channel in the array but bend upon leaving the array channels to assume a direction that is substantially perpendicular to the unipotential surfaces 36 and 38 in the case of array 22.

The control circuitry is shown in Fig. 6. Ri and R refer to the resistances of the sections or arrays 22 0 and 24. A power supply 70 supplies a constant current (not voltage) Ii of.50 microampercs per square centimeter (of array 22 cross-sectional--i.e., in a direction perpendicular to net electron flow directions--area), while a power supply.

72 supplies a constant current 10 of 250 micoamperes per square centimeter (of array 24 cross-sectional area), across the two arrays or sections respectively. Voltage comparator 74 through line 76 monitors the voltage there, and through control loop 78 varies the amount of cooling done by thertuo- electric cooling system 80, which operates to cool both arrays 22, 24; arrows 82 indicate heat leaving the arrays.

The set point voltage in comparator 74 is chosen so that the voltage drops across the arrays 22 and 24 are respectively 1000 volts and 200 volts. (Resistances in the two arrays are respectively 20 megohms per square centimeter and 0.8 megohms per square centimeter.) There is shown in Fig. 5 a modification embodying three sections or arrays 62, 64, and 66 and two lines from common electrodes.

Because the conductivity in array 24 is five times as great as that in array 22, current is five times as great.

Since thickness of array 24 is only one-fifth that of array 22, heat dissipation is the same in both arrays. Heat dissipation through the entire MCP is thus a fraction of what it would be if both section 22 and section 24 had the lower resistance of section 24.

Because increasing quantities of electrons are removed from channel walls the farther along the channel one goes in an amplifying direction, so is wall electron depletion increasingly severe in that direction. (In fact, in this preferred embodiment array thicknesses are chosen such that the total number of electrons lost by each channel wall, net, is the same in each channel 31, 33.) Accordingly, resistance may be larger in the array 22 without unduly affecting recovery time, inflow-of-electrön requirements for recovery in that array being less demanding.

Using constant current power supplies in conjunction with output current of both arrays leads to thermal stability, for rising MCP temperature causes thermal dissipation to fall (because of wall zone resistivity negative temperature coefficient) and radiation losses to rise until a balance is reached.

Use of the two-array approach thus described makes possible a five-fold frequency increase, for wider MCP applicability.

The provision of a glass that can operate at high temperature and of a control circuit to prevent runaway permits a further frequency increase of 100 times, so that our preferred embodiment allows a useful operation frequency about 500 tirnes as great as disclosed in the prior art.

Instead of a centre electrode between the arrays, a separate electrode could be used at the adjacent ends of the two (or more) arrays; or, they could be spaced apart; both as in the chevron patent above mentioned.

The channels of the arrays might have channel axes parallel rather than at an obtuse angle to each other.

Claims (11)

CLAIMS:

1. Electrical apparatus comprising: a microchannel plate; a constant-current power supply adapted for imposing voltage to cause current flow in said microchannel plate; cooling means adapted for cooling said microchannel plate; and control means adapted to regulate said cooling means to maintain operation of said microchannel plate at a predetermined temperature.

2. Apparatus according to Claim 1, in which said control means is a voltage comparator.

3. Apparatus according to Claims 1 or 2, in which said cooling means is thermoelectric.

4. Apparatus according to any precediny claim, wherein said microchannel plate comprises a plurality of arrays, a first array in an electron-amplifying direction from a second array having a surface zone resistance lower than that of said second array.

5. Apparatus according -to Claim 4, in which said second array is thicker than said first array.

6. Apparatus according to Claims 4 or Claim 5, in which the product of conductivity and thickness is the same for said first array and said second array.

7. Apparatus according to any of Claims 4, 5 or 6, in which said first array abuts said second array.

8. Apparatus according to Claim 7, in which said first array and said second array share a common electrode.

9. Apparatus according to any of Claims 4 to 8, in which said arrays are formed of glass permitting continuous operation at temperatures in excess of 100 C.

10. Apparatus according to Claim 9, in which said arrays are formed of the glass set forth in the description of the preferred embodiment herein.

11. Electrical apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.