Nuclear Instruments and Methods in Physics Research A 331 (1993) 809-816 North-Holland

NUCLEAR INSTRUMENTS

& METHODS IN PHYSICS RESEARCH

Section A

Diagnostics of an L band FEL high current injector

X.Z. Shi, Y.L. Pan, J. Yao, J.Y. Chen, X.F. Guan, Y.Q. Gao and Q. Sun China Institute of Atomic Energy, P.O. Box 275/17, Beijing 102413, China

This paper describes the diagnostics of an L band FEL high current injector at CIAE. The single slit and probe method has been used for measuring the emittance of an electron gun. Thepepper pot method is adopted to measure the true emittance, hyperemittance and brightness at the exits of the subharmonic prebuncher (SHB) and the buncher respectively, with a precise optical system and image processing system (IPS).

1. Introduction

The main parameters to be measured for the injec- tor are: electron energy: 2 MeV; current: 25-50 A; pulse width (gun, buncher): 3 ns, 20 ps; cross section (FWHM): 5-20 ram; and normalized emittance: 0.01- 0.02 cm rad.

In order to measure the above parameters, it is necessary to provide a reliable and useful scheme. Several kinds of detectors have been designed, and some of them have been installed and tested. The principal detectors for an injector are shown in fig. 1.

The energy measurement uses an absorbing method with aluminum. The beam current transformer (BCT) [1] and Faraday cup (CUP) [2] are used to measure the pulse width (from 3 to 1 ns)~ whose rise time is less than 0.7 ns. The pulse width of the SHB is shown in fig. 2. A streak camera will be used to measure the pulse width of 20 ps. Beam profile monitors, whose maximum error of repeating location is less than 0.02 mm, are used to measure the cross-section, the posi- tion, and the density distribution of the electron beam.

Emittance measurement is important to us. It is highly interesting and important to use the technique of image recognition to measure the hyperemittance and the brightness. Analysis and processing of the data can be done automatically within 2 min.

2. Profile and position measurement

There are three profile monitors to be used. They are called PM1, PM2 and PM3. PM1 and PM2 are installed behind the SHB and PM3 behind the buncher. The profile monitors are used to measure the profile and position of the electron beam with a fluorescent plate which is fixed at an angle of 45 ° to the axis of the

injector. The image on the fluorescent plate passes through the vaccum window, the reflecting mirrors, the focussing lenses, and reaches the CCD camera. The information is inputted into the image processing sys- tem (IPS) [3].

The system error of the mechanical center is less than 0.036 mm and the maximum error of repeating accuracy is less than 0.02 mm. For a quantitative mea- surement, three sets of precise light path systems are designed (as shown in fig. 3). Their parameters are as follows: linearity is better than 0.5%, transmissivity is greater than 75%. When the optical transfer function (MTF) is greater than 50%, the resolution is about 20-40 line pairs /mm. The diffuse spot caused by the depth of field in the imaging region is less than 0.02 mm, which overcomes the imaging error caused by 45 ° inclination of fluorescent plate.

The IPS is shown in fig. 4. We have developed some programs for the IPS. They have the main following functions: (a) Measurement of the profile of beams, assuming a Gaussian distribution of the cross-section. (b) False color dying on beam cross-section with differ- ent colors for different beam intensity. It benefits us, as the human eye cannot distinguish brightness varia- tions in a small range. (c) It can find the centre of the beam automatically and it gives the deviation of the beam from the axis of the injector. (d) With different intensity, it draws 3-dimensional color distribution pictures.

Fig. 5 shows some results of the processing.

3. Electron beam emittance measurement

Emittance has been studied in three aspects, as discussed in the following subsections.

0168-9002/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved XIII. BEAM DIAGNOSTICS

810

PP 1 -t PM I

!3e:TI BCT:~

Col N,S

BCT1-BCT4 PM1 -PM2

cuP PP1-PP2

4Q B.M. Col -0o2 VA1-VA2 M.S.

X.Z. Shi et al. / Diagnostics of an L band FEL

B C IT3

- - " 3 8 ~ -

PPg

Beam Current Transformer Profile Monitor

Faraday Cup

Pepper Pot Plate

QuadrupoIe Magnet

Energy Monitor

Collimator

Valve

Magnetic Shield

E . N

P~t3

RF.T÷ ,at)

C02 fused quarlz

Target Chamber Optical System

Streak Camera

Fig. 1. Diagram of electron beam diagnostics for the injector.

3.1. Emit tance measurement f o r a high current grid-con- trol pulsed electron gun

A single slit and probe method has been used for measuring the beam emittance of an electron gun. The article has been published [4]. The measuring device is

Fig. 2. Pulse waveform of BCT2 ( I= 3.3 A). Vertical: 1 V/div. Horizontal: 2 ns/div.

shown in fig. 6. A movable slit with 0.1 mm width is used for sampling. A probe, 0.1 mm in diameter, parallel to the slit and moving at a constant velocity, is used to catch the beam through the slit. The distance between the slit and the probe is 59 ram, the angle resolution is 3.4 mrad, and the maximum system accep- tance is 0.64 cm rad. With a fine shielding and an adequate second electron suppressor in the measure- ment system, current on the probe of the order of 10-10 A has been clearly measured in strong distur- bance fields. The measurement repeatability (relative standard deviation) is about 6%. The relative error of the measured phase plots is about 8%.

The system is useful to study the effects on the emittance of electron energy, the grid pulse voltage, cathode temperature, and pulse current intensity. One of the measured phase plots is shown in fig. 7 (data shown in table 1), which is a result of changing the grid pulse voltage. The phase area is greatly changed by different grid pulse voltages. The results agree with those of a physical analysis and analogue computation.

X.Z. Shi et a L I Diagnostics of an L band FEL 811

/

Beam - @ - SHB-- - -

SC1 Wall

PM 1 11 PM2 M4

I / I / - - I / I z SC2

M1 M3

ld5 ~ SC3

- - 7 - - I I / [ / I I / I /

PM3

Fig. 3. Diagram of the optical system for IPS. M1-M5: reflecting mirrors; PM1-PM3: profile monitors; SC1-SC3; shielding chambers.

3.2. Electron beam emittance measurement at the exit o f a subharmonic prebuncher

A pepper pot emittance measuring device has been installed after the subharmonic prebuncher. The de- vice consists of two parts. One is the pepper pot, simply called PP1, and the other is PM2. The beam going through PPI's little holes drifts freely a given distance and forms a group of light spots on PM2 with different angles. We can calculate the emittance in the horizontal and the vertical direction.

The target material is tantalum. The seperation of holes is 4.0 mm and 6.0 mm with an error less than 0.02 mm. All the holes have a diameter of 0.5 mm.

The system of PP1 and PM1 (shown in fig. 8) is in the same plane, which is vertical to each other. It is complex in design. The use of a point contacting struc- ture decreases much the strict demand on the concen- tricity.

The resolution of the whole measuring system can reach the level of x: 12.35 pixels/mm; y: 18.20 pixels/mm. A measurement software code called EBDS (electron beam diagnostics system) has been developed to measure the emittance. The result of an analogue test is shown in fig. 9.

3.3. Injector emit tance measurement

A quadrupole magnet method and a pepper pot method (as mentioned above) is used. Because the energy is 2 MeV, the current is higher and the pulse width is smaller, the space charge effect must be paid

attention to. That effect after PP2 can be neglected. Thus, it is necessary to use this method.

4. M e a s u r e m e n t m e t h o d of the hyperemi t tance and br ightness wi th IPS

4.1. Principle

The hyperemittance is related to the hypervolume V4 enclosing all particle points in the 4-dimension

PMI PM~ %-

c T 7 .....

PM3

\

Fig. 4. Diagram of the image processing system.

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812 X.Z. Shi et al. / Diagnostics of an L band FEL

Fig. 5. Results of profile processing.

Fig. 6. Principle of the emittance measuring setup. 1, 2, 3: cathode grid anode of the electron gun; 4: BCT; 5, 9: drift pipes; 6, 10: steering coils; 7, 11: magnetic lenses; 8: valve; 12: magnetic shield; 13: slit; 14: probe N; 15: second electron suppressor; 16: emittance measurement chamber; 17, 18: vacuum pipes; M: stepping motor; P: precision multiring potentiometer; Re: X - Y

function recorder; F: ns Faraday cup.

X.Z. Shi et al. / Diagnostics o f an L band FEL 813

/ ' P ° ~ ' ~ " 4 d ~ f

/ , / 20

r

- 8 (

x '/mard (1)

, / (2)

' ~ . , _ _ / ' , / j 3>

+ : ,.4K,., / :

/ /

Fig. 7. Variation of phase area with grid control voltage. For the meaning of (1), (2) and (3) see table 1.

Table 1 Emittance measurements for an electron gun

1 2 3

Grid control voltage Vg [kV] 1.6 2.1 2.5 Pulse current intensity I [A] 1.75 2.4 3.6 Beam energy E 0 [keV] 80 80 80 Cathode temperature T [°C] 880 880 880 Phase area including 80%

of particles A x [10 - 2 c m rad] 12.8 5.6 4.7 Normalized phase area including

80% of particles A x [10 -2 cm rad] 7.4 3.2 2.7

phase space T4, namely [6]:

v+ 1 6 Ti.2 ffffdx d x ' d y d y ' . ,IT 2

The br igh tness is the average value of the density in T4 phase space:

B = P4 = I / V 4 = I / ~ 2 e 4 •

The normal ized hyperemi t t ance is def ined as e4n = /32Tze4 and the normal ized br ightness is B n = B / [ 3 2 7 2.

6 7

U..L/

EU

rFh i

U

Fig. 8. Structure diagram of PP1 and PM1. 1, 8: 10Y-2K pneumatic executor; 2, 6: corrugated flexible metal tube; 3: column of limited position; 4: fluorescent screen; 5: pepper pot target; 7: micromoved regulating system; 9: vacuum viewing window; 10:

supporting system.

XIII. BEAM DIAGNOSTICS

814 X.Z. Shi et aL / Diagnostics of an L band FEL

The principle of the method is: The pepper pot stops most of the beam and lets only the rest passing through the holes go on. The sampling beamlets hit the fluorescent plate through the free drift distance L. An electron from the hole (x0, Y0), which hits the point (x[, y£) On the fluorescent plate, has an angular distri- but ion (xi, Yk). It satisfies the following formula:

rx l=[,JL 0][xi I E x0JL y, + . L k] 0 1 /L Yk -Yo/LJ

The flux incident per uni t area of the fluorescent plate at a point (xi, Yk) of the spot corresponding to the beamlet from .the hole (x0, Y0) is given by: Js = p(xo, Yo, x~, y~)ds/L 2, in which ds denotes the area of the hole, and L the drift distance. From the two formulas above, one can conclude: if the radiant brightness n(xi, Yk) of a point (xi, Yk) on the fluores- cent plate is l inear for the incident flux, then for each hole (x0, Y0) its corresponding spot on the plate with a brightness distribution will represent the current den- sity distribution p(Xo, Yo, x', y ') in gradient space. In real measurements, at the first step, we measure the area f (x , y) enclosed by isodensity curve p = pf in the gradient plane (corresponding to the isobrightness curve n = nf on fluorescent plate). At the second step,

integrate f (x , y) on the x-y plane. Then we can calculate the phase volume, namely:

1 f(x,y)=f£dx'dy'= ffdxdy, d,

= ffffdx dx ' dy dy' .

Assume that the current density distribution in 4-di- mensional phase space is a Gaussian distribution, then we can derive that for a threshold density Pf=fPo (where 0 < f < 1), the ratio of the current considered to the total current I 0 is [5]:

If Pf Po Ps - - = I - in Io Po Pf Po "

4.2. Method of data processing and software realization [5]

When the pepper pot method is used to measure the hyperemittance, one must consider every light spot on the fluorescent plate and calculate the area of every spot from different holes. Therefore the following problems have to be solved in image processing.

(a) Light spot recognition. Divide the whole image into a lot of small rectangular regions and see if every

Fig. 9. Diagrams of analogue computed emittance with the pepper pot. (top left) Phase area. (top righ0 Three-dimensional space distribution with various thresholds. (bottom left) Original data plot. (bottom right) Threshold plot.

X.Z. Shi et aL / Diagnostics of an L band FEL 815

Fig. 10. Analogue test to measure the hyperemittance. (a) Analogue light spots plot. (b) Boundary plot. (c) f f dx' d y ' - xy plot. (d) Hyperemittance plot.

spot is in the expected region corresponding to a certain hole on the pepper pot. Using this method we can easily recognize every spot originating from differ- ent holes.

(b) Calculation of the area and perimeter of the light spot. The holes on the pepper pot, on the one hand, define the system solid angle resolution: AQ = (nvrg + S ) / L 2, (where S is the area resolution of the fluorescent plate and r 0 is the radius of the hole on the pepper pot). On the other hand, the hole makes the light spot grow along the boundary of the ideal spot. If the perimeter of the spot is P, then the area increase is Pro, so that it is necessary to calculate the perimeter of the spot. We have adopted the image recognition technique to solve the problem. After find- ing the edge of the spots, we cannot only calculate the perimeter but also the area, which is precise and fast.

4.3. Hypervolume calculation by dualistic interpolation

In order to solve the problem of a limited number of holes on a pepper pot (to avoid the overlap of spots), we adopt a dualistic interpolation to calculate the gradient space integration f ( x , y) in the x-y plane. Then we have:

g 4 = f f f ( x , y ) d x dy.

We now have already done the analogue test; the result is shown in fig. 10.

5. Conclus ion

Now the injector is being tested. Most of the mea- suring detectors have been used and are very effective. With the IPS it is easy to clearly display the profile and

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816 X.Z. Shi et al. / Diagnostics o f an L band FEL

density distribution. It will be much more convenient for us to measure the emittance and brightness, which is very important for diagnostics.

References

[1] D.K. Liu, 1.1 GeV electron Linac measuring system, Insti- tute of High Energy Physics, China, Internal report (1988).

[2] N.J. Norris and R.K. Hanst, IEEE Trans. Nucl. Sci. NS-16 (3) (1969) 927.

[3] D.K. Liu and K. Nakahara, Image processing system for electron Linac beam diagnosis, contributed to the 1986 Linear Accelerator Conf., 1986, SLAC, USA.

[4] X.F. Guan, X.Z. Shi and Y.H. Zhang, High Power Laser Part. Beams 4 (2) (1992) 215, in Chinese.

[5] Y.L. Pan, X.Z. Shi and X.F. Guan, A method to measure the hyperemittance and brightness for accelerator bunch, to be published in High Power Laser Part. Beams.

[6] A. Septier (ed.), Applied Charged Particle Optics (Academic Press, New York, 1980).

[7] P.E. Oettinger et al., Appl. Phys. Lett. 50 (1987) 1867.