NL1004822C2 - Material for a photo-electrode in a free electron laser - Google Patents

Abstract

A caesium-potassium-tellurium composite is used as a photoelectric material.

Description

Title: Photoelectrode

The invention relates to a photoelectrode, and more particularly to a photoelectrode suitable for use in a free electron laser.

The invention further relates to a method for manufacturing such a photoelectrode.

A free electron laser is known per se, and the nature and construction thereof is not the subject of the present application. Suffice it to note that in such a laser electrons are forced to travel a curved path, whereby radiation is generated. Preferably, a photoelectrode is used, that is to say an element designed to generate electrons (photo emission) upon irradiation with light of a predetermined wavelength, which electrons are accelerated under the influence of an electric field to a predetermined energy. Such an electrode will hereinafter also be referred to by the term "photo-cathode".

A photo-cathode must meet, inter alia, the following criteria. First, the photo-cathode must be compatible with the ultra-high vacuum prevailing in a free electron laser. Secondly, a high quantum efficiency (QE) is desired, which means the ratio between the number of electrons released from the photo-cathode and the number of photons irradiated on the photo-cathode. Furthermore, a long service life is desired, which in practice amounts to the time that the quantum efficiency during operation is higher than a predetermined value.

A material that has been found suitable for use as a photo-cathode is Cs2Te. However, this material has, among other things, the important drawback that it is not stable enough.

During use, Cs can evaporate from the photo-cathode, which means that 1004822 2 not only reduces quantum efficiency, but can also cause sparks in the accelerator of the free electron laser.

It is therefore an object of the present invention to provide an alternative material which can be used as a photo-emitting material in a photo-cathode for, for example, a free electron laser. More particularly, the present invention aims to provide a photo-cathode which has a good life and whose photo-emissive material is chemically more stable than Cs2Te. Even more particularly, the present invention aims to provide a photo-cathode whose quantum efficiency is better than achieved with Cs2Te.

According to an important aspect of the present invention, a photo-cathode contains the elements Cs, K and Te in combination.

The above and other aspects, features and advantages of the present invention will be elucidated by the following description of an embodiment of a photo-cathode according to the invention, with reference to the drawing, in which: figure 1 schematically shows a part of a free electron laser shows; figure 2 shows for a given experiment the measured quantum-25 efficiency as a function of the precipitation time; and Figure 3 shows the spectral response of a Cs-K-Te photocathode prepared according to the present invention.

Figure 1 schematically shows a part of a free 30 electron laser 1, comprising an accelerator space 2 with a photo-cathode 10 disposed therein. Upon irradiation with light 3, from a laser source 4, the photo-cathode 10 emits electrons 5, which are for the sake of simplicity, accelerating means not shown are accelerated and leave the accelerating space 2 via its output 6 to reach the laser space 7 of the free electron laser 1.

1004822 3

The free electron laser 1 is provided with a preparation chamber 8. The photo-cathode 10 is mounted on an elongated support 11, which is axially movable between an extended position, the photo-cathode 10 being in the operating position in the accelerator space 2, as shown in Figure 1, and a retracted position where the photo-cathode 10 is in a preparation position in the preparation chamber 8.

The usual procedure for operating the 10 free electron laser 1 is as follows. First, a substrate for the photo-cathode 10 is mounted on the support 11, and it is brought into the preparation position. Then, the preparation chamber 8 is closed and evacuated to ultra-high vacuum, after which manufacturing steps are carried out to produce a photo-emission layer on the substrate.

When this layer is ready, the preparation chamber 8 is brought into communication with the accelerator space 2, the carrier 11 is brought into the accelerator space 2 in the operating position, and the free electron laser 1 can be put into operation until "wear" causes the quantum efficiency of the photo-emission layer has become too small.

The above-described process is then repeated. In a practical situation where the photo-emission layer consists of Cs2Te, a preparation / operating cycle with a period of about one day has been found, whereby the preparation takes several hours, and the life of the photo-emission layer is also several hours. .

In the following, the photo-emissive properties of the photo-emissive material proposed according to the present invention will be demonstrated by discussing test results of a photo-cathode manufactured according to the present proposals.

A substrate for a photo-cathode 35 was mounted on the support 11. The substrate was made of molybdenum, but other suitable materials are also possible.

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Subsequently, a thin Te layer was applied to the substrate by means of a deposition method known per se, which method will be discussed briefly, in summary, below. A container with Te mounted on an actuator 5 was positioned in the vicinity of the Mo substrate and heated to a temperature of about 300 ° C. The temperature of the substrate was kept at a lower value in the example experiment discussed here at about 120 ° C. The pressure in the preparation chamber 8 was about 10-10-10-9 Torr.

The Te evaporates, and settles on the colder substrate. The precipitation process was continued for about 30 minutes. Although this has not been experimentally verified, it is believed that the thickness of the Te layer 15 thus deposited is about a few tens of nanometers.

Then, in a similar manner, K was deposited on the Te layer, however, the temperature of a container with K should be chosen higher than the previously mentioned temperature for the Te container, because the evaporation point of K is higher than that of Te, as will be clear to an expert. A suitable temperature for the K container has been found to be about 500 ° C, and was used in the present experiment. The substrate temperature TSUb was again maintained at about 120 eC.

Then, Cs was similarly deposited on the Te-K composition, keeping the temperature of a container with Cs at about 565 ° C. The temperature T8Ub of the substrate was now kept at about 150 eC.

As mentioned earlier, the quantum efficiency QE is an important parameter with regard to the quality of a photo-cathode, which parameter in the context of the present invention is defined as the number of electrons produced divided by the number of striking photons, and are expressed in percentages. This parameter was measured at different wavelengths during the precipitation process of Cs and K. To this end, the substrate was irradiated with light from a mercury lamp, different r ''; Band-pass interference filters were used to select desired wavelength bands, as will be apparent to one skilled in the art. The photocurrent of the cathode was measured using a pico-ammeter, the cathode being biased using a battery to a value of -90 V.

Figure 2 shows an illustrative graph of the measured QE at 259 nm as a function of the precipitation time t, for an experiment in which the temperature Teub of the substrate 10 was initially kept at about 120 ° C during the precipitation of Te and K, and at about 150 ° C was maintained during the precipitation of Cs. The zero point of the horizontal axis corresponds to the beginning of the precipitation of K.

Six stages can be identified in this graph, as follows: 1) In the first stage, QE is zero, at least very low; in the example shown, the first stage takes about 10 minutes.

2) In the second stage, QE increases relatively quickly as a function of time? in the example shown, the second stage takes about 3 minutes.

3) In the third stage, QE is almost constant; in the example shown, the third stage is quite short, less than a minute. The value of QE reached at this stage will also be referred to hereinafter as maximum value QEm, k *, and the precipitation time at which this maximum value Qem, k is reached will also be referred to below as tMfK. Thus, in the example shown, tM, K is about 13 minutes, and QEm, k is about 7.5%.

4) In the fourth stage does QE decrease relatively quickly as a function of time? in the example shown, the fourth stage takes about 1-2 minutes. At the end of the fourth stage, QE is about 6%.

5) In the fifth stage, QE remains almost constant. The value of QE reached in this stage will also be referred to hereinafter as saturation value QEs, r, and the precipitation time at which this saturation value QEs, k is reached will also be referred to below as "O488 6 ts, K · in the example shown is tS / K about 27 minutes, and QEs, k about 6%.

6) At t = 27 min switch was made to the precipitation of Cs, marking the beginning of the sixth stage. In this fifth sixth stage, QE increases relatively quickly as a function of time. As time progresses, the rate of ascent of QE gradually decreases, until QE eventually remains almost constant. The value of QE achieved in this stage will also be referred to hereinafter as 10 saturation value QEs, kcs / and the precipitation time at which this saturation value QEs, kcs is measured, measured from the beginning of the Cs precipitation, will also be described below. referred to as ts, KCs · In the example shown, the sixth stage takes about 15 minutes (ts, KCs ** 15 min), and 15 QEs, kcs is about 20.8%.

In the figure, a measuring circle at 23.4% is still indicated with a solid circle, which value was measured after removing the Cs holder. It is believed that the Cs holder during the sixth stage partially shielded the light from the mercury lamp, so that the QE occurring during the sixth stage is in fact higher than the measured QE, about 12.5%. This means that QEs, kcs is in reality about 23.4%.

The said six stages appear to be characteristic of the behavior of a Cs-K-Te photocathode as a function of time during the deposition process of K and Cs, although the values mentioned may depend on the process parameters. For example, in general, QEm, k is lower for higher values of tSUb · 30. Presumably, said characteristic behavior can be explained by assuming that the composition of the surface layer of the cathode changes during the manufacturing process. Initially, the surface layer mainly consists of Te (first stage). During the K precipitation process, the K concentration will increase, with the composition of the surface layer presumably changing to K2Te via Κ2Τβ3 and KTe. It seems reasonable to assume that the photo. .. 482 2 7 emissive properties depend on the composition of the K-Te material.

During the sixth stage, when Cs is added to the K-Te composition, QE increases with increasing Cs-5 concentration, up to a certain saturation value.

The precipitated Cs atoms are believed to diffuse into the previously applied K-Te composition.

The measured QE value of 23.4% is very high. Until now, CS2Te was believed to be the best material for use in the soft ultraviolet region, but the QE value of that material is lower than the QE value currently offered by the material of the present invention. For example, reference is made to an article by S.H. Kong 15 et al in J.Appl.Phys., Vol.77 (1995), p. 6031, where for CS2Te a QE value between 15 and 18%, measured at 251 nm, is reported.

In a comparative experiment, a photo-cathode was prepared five times according to the above-described preferred procedure. QEs, kcs of these photo-cathodes were always measured at 259 nm; the measured values were in the range from 21.9% to 23.4%. In the same preparation chamber, under otherwise comparable conditions, a photo-cathode was made of Cs2Te five times, and QE was measured; of these 25 photo-cathodes, the measured values of QE were found to be in the range of 10.3% to 12.2%. This shows that the QE value currently offered by the material of the present invention is on average a factor of 2 higher than the QE of the known material.

30

Figure 3 illustrates the spectral response of QEs, kcs of the Cs-K-Te photocathode of the invention. For comparison, Figure 3 also shows the spectral response of QEs, k of a K-Te photocathode of which Tsub was 120 eC, ie a photocathode in which the manufacturing process was stopped after the fifth step. Furthermore, Figure 3 shows for comparison the spectral response of QEs, cs of a Cs-Te photocathode of which,. . -rÓ 2¾ 8 TSub was 150 ° C, that is, a photocathode in which steps 1 to 5 were skipped during the manufacturing process. It has been found that the Cs-K-Te-5 photocathode according to the invention offers a better QE value than the conventional Cs2Te photocathode both at relatively large wavelengths and at relatively small wavelengths. In all cases, the best QE value is achieved at relatively small wavelengths.

The Cs-K-Te-10 emission material proposed by the present invention appears to be quite stable. A copy of a photocathode prepared by the described method was stored under a pressure of about 10 ~ 10 Torr for 100 hours. No significant change of QE (259 nm) was measured, nor of the spectral response. This is seen as an indication that the service life during operation is relatively long.

Evaporation of Cs from the cathode material is considered to be an important cause for the reduction of the QE during operation. It is suspected that the process described achieves that Cs diffuses into the previously formed composition of Te and K, and it is more difficult for Cs to evaporate therefrom. This presumption is supported by an experiment in which the temperature was chosen to be lower than 150 eC, namely 120 ° C, during the sixth step. After the precipitation process was completed, the temperature was raised to 150 eC, which was found to yield an increase in QE. A possible explanation for this observed phenomenon is that at the lower temperature Cs diffuses poorly in the previously formed composition of Te and K, and thus, as it were, a layer of excess Cs is formed on the previously formed composition of Te and K. By raising the temperature to 150 ° C, the diffusion of those excess Cs was stimulated.

An important aspect of the present invention is that it avoids the presence of a layer of excess Cs on the surface of the photocathode, since it allows Cs to be easily released to contaminate the accelerator.

It has been investigated whether QEs, kcs depends on the duration of the 5 K evaporation / precipitation process. This was done by repeating the process described with reference to Figure 2 several times, varying the starting time of the sixth step (27 min in Figure 2). The duration of the sixth step was always chosen so long that it was ensured that the saturation value QEs, kcs was reached.

It was found that ts, KCs were always approximately equal to half the duration of the K-evaporation / precipitation process. Surprisingly, it appeared that the saturation value QEs, kcs does not change significantly if the duration of the K-evaporation / precipitation process is varied in the range from tM> K to ts, K * A possible explanation for this result is that already at tM> K the maximum achievable result is achieved, and that with a further continuation of the K-evaporation / precipitation process and a correspondingly longer Cs-evaporation / precipitation process, only a thicker Cs-K-Te layer is produced , or perhaps a thicker Cs-K layer on Te. For this reason, it is preferable to terminate the K-evaporation / precipitation process at the time tM, K / or shortly thereafter and switch to the Cs-evaporation / precipitation process, because the total process time is thus reduced. kept as short as possible, and material and energy costs are limited.

It will be clear that the properties of the Cs-K-Te emission material discussed above are particularly promising for use in a free electron laser. This expectation is further reinforced by a test in which a cathode manufactured according to the present invention, which was manufactured according to the example described above, was used in the accelerator space 2, and the free electron laser 1 was put into use. After approx.

For 3 hours of operation, QE had fallen to a "resf" value of about 2–3%, and this "resf" value remained for 1 C'4? 2.2.

10 stable for a longer time (more than 25 hours). Although the stated value of 2 to 3% is considerably lower than the initial value of QE, it is still still higher than the "residual" values achieved by Cs-Te emission material.

5

It will be apparent to a person skilled in the art that it is possible to change or modify the illustrated embodiment of the Cs-K-Te photocathode according to the invention, without departing from the inventive idea or the scope of protection as defined in the claims.

For example, it is possible that the photo-emissive material proposed according to the present invention may be used for applications other than a photo-cathode of a free electron laser.

It is also within the scope of the present inventive idea to change the order of deposition of the components Te, K and Cs in the manufacturing procedure, or to deposit intermittently different layers of those components, or to make those components simultaneously. time down, in an experiment a photo-cathode was made according to a procedure as described above, it being understood that the order of precipitation of Cs and K was reversed, so that first Te, then Cs and then K knocked down. Again, a useful QE of about 15% was achieved, which value is much higher than the QE of a Cs2Te material, but not as high as the value achieved in the more detailed process sequence, which is therefore preferred enjoy.

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Claims (18)

1. Use of a Cs-K-Te composition as a photoelectric material. Photo-cathode (10) for emitting electrons (5) under the influence of electro-magnetic radiation (3), comprising a substrate and an active layer applied to it, consisting essentially of a Cs-K-Te composition . Free electron laser (1), comprising a photocathode (10) according to claim 2, which can be positioned in an accelerator space (2). Method for manufacturing a photocathode (10) according to claim 2, comprising the following steps: in a first step, a layer of Te is applied to a substrate of a suitable material, preferably molybdenum; in a second step K is precipitated; and in a third step, Cs is precipitated. The method of claim 4, wherein in the first step the Te is applied by a vapor deposition process, wherein a container of Te is heated for a first predetermined time to a first predetermined temperature higher than the temperature (TSUb) of the substrate. 25 6. Method according to claim 4 or 5, wherein in the second step the K is applied by means of a vapor deposition process, wherein a container with K is heated for a second predetermined time to a second predetermined temperature higher than the temperature (T8Ub ) of the substrate. The method of claim 6, wherein the temperature of the K container is maintained at about 500 ° C. 1004822 A method according to any one of claims 6-7, wherein at least during the second step TSUb is kept at a value of about 120 ° C. 5 9. A method according to any one of claims 6-8, wherein during the second step the quantum efficiency QE of the photocathode is measured, and wherein the deposition process of K is terminated when or after QE has reached a maximum value QEM, K. The method of any one of claims 4-9, wherein the second step is performed after the first step. A method according to any one of the preceding claims, wherein in the third step the Cs is applied by means of a vapor deposition process, wherein a container with Cs is heated for a third predetermined time to a third predetermined temperature higher than the temperature (TSUb) of the 20 substrate. The method of claim 11, wherein the temperature of the Cs container is maintained at about 565 ° C. The method of any one of claims 11-12, wherein at least during the third step Tgub is kept at a value of about 150 ° C. 14. A method according to any one of claims 11-13, wherein during the third step the quantum efficiency QE of the photocathode is measured, and wherein the deposition process of Cs is ended when or after QE has reached a saturation value QEm, kcs. The method of any preceding claim, wherein the third step is performed after the first step. R? The method of any preceding claim, wherein the third step is performed after the second step. 17. A method according to any one of the preceding claims, wherein the second step and the third step are performed alternately. The method of any preceding claim, wherein the second step and the third step are performed simultaneously. *: o 9