Extraction of an Intense Electron Beam from the Acceleration Chamber of a High-Current Betatron by an Electrostatic Method
Abstract Generated abstract
This paper describes an electrostatic method for extracting an intense electron beam from the accelerating chamber of a high-current 25 MeV betatron, motivated by the need to obtain longer extracted-beam pulses than those produced by pulsed single-turn extraction. The authors analyze orbit expansion using the existing central displacement winding combined with a transverse electrostatic field, and propose a special capacitor design in which an elongated deflecting plate both increases the orbital step and serves as part of the extraction device. Calculations of electron trajectories are used to select the device geometry and installation position, yielding an estimated extraction efficiency of about 40 percent under specified assumptions. The implemented system produced pulses containing about 8 x 10^10 electrons, with a radiation pulse duration of approximately 1 microsecond and a pulse beam current of 12.8 mA.
Full Text
EXTRACTION OF AN INTENSE ELECTRON BEAM FROM THE ACCELERATING CHAMBER OF A HIGH-CURRENT BETATRON BY AN ELECTROSTATIC METHOD
V. A. MOSKALEV, V. V. SHASHOV, A. M. SLUTSKII
In [1] a system is described for extracting an electron beam from a high-current betatron by a pulsed single-turn method. The application of this method made it possible to obtain from the betatron a pulsed current of the extracted beam whose amplitude exceeds by hundreds of thousands of times the current in a pulse obtained from betatrons up to the present. The duration of the extracted-current pulse is
Fig. 1. Diagram of extraction of an electron beam from the accelerating chamber: 1 — accelerating chamber; 2 — plate intended to increase the orbital step; 3 — deflecting plate; 4 — trajectory of the extracted beam; 5 — equilibrium orbit; 6 — expanded orbit; 7 — injector; 8 — inflector
5–6 nsec. Such short pulses have found application in various areas of science and technology. However, for a number of studies, especially for investigating the effect of radiation on
various materials, it was necessary to increase the duration of the extracted-beam pulse. The requirement of increasing the duration of the pulse of electron current extracted from a high-current betatron was satisfied as a result of implementing an electrostatic extraction method by means of an extraction device of special design. It is known [2] that, in order to dump onto a target electrons accelerated in a high-current betatron to 25 MeV, by means of the central winding, a current generator with an average power of 30 kW is required. In this case, in the working region of the accelerator a perturbing magnetic field is produced whose energy
Fig. 2. Circuit of the generator supplying the expansion winding
is equal to 360 joules. The radial velocity of the orbit motion [3] when the electron beam is displaced onto the target in the region where the inflector is located is only \(2.9 \cdot 10^4\) cm/sec, and the orbit step is of the order of microns. A significant increase in the perturbing magnetic field leads to a very small increase in the orbit step.
Fig. 3. Circuit of the generator supplying the extraction device
To solve the problem of extracting electrons from a high-current betatron by the electrostatic method, it was necessary to find such a method of bringing the beam to the slit of the deflecting capacitor that, already at radii smaller than the boundary of the inflector location, the step of the electron orbit during its expansion would be much larger than the intercepting plate of the capacitor. This requirement follows from the fact that the capacitor must be installed at a radius somewhat smaller than the radius of placement of the
inflector, since otherwise the entire beam, before entering the capacitor, will fall onto the inflector.
The authors proposed a capacitor design with an elongated deflecting plate.
In [4] it is shown that if, during the process of symmetrical expansion of the equilibrium orbit by means of the central winding, a perturbing electric field \(E\) is created in a certain azimuthal sector in the working region of the accelerating chamber, then a strong radial deflection of the particles can be obtained in this sector.
The betatron under investigation has a central displacement winding and a ready-made current generator for supplying it [2]. Therefore it appears possible, for expanding the orbit, to use a combination of the existing displacement scheme and an azimuthal plate, to which, at a certain moment after the central winding is switched on, a pulse of positive polarity is applied. From the calculation carried out in [4] it is seen that, with such a combined method of orbit expansion, it is possible to achieve an orbit step that permits the use of a capacitor as the extraction device.
Fig. 4. Photograph of the cross section of the extracted beam
Thus, for example, using the existing central winding for orbit expansion and applying to a plate 50 cm long a pulsed voltage with an amplitude of 100 kV, the orbit step at the entrance to the capacitor is equal to 0.17 mm. The electron-extraction efficiency \(\eta\), neglecting losses inside the capacitor and with a cutting-plate thickness of 0.1 mm, will be about 40% [4].
The calculation and construction of electron trajectories during extraction by the electrostatic method made it possible to choose the optimum design of the extraction device and the place of its installation in the accelerating chamber.
We proposed a capacitor design that differs from those used up to now. This is due to the fact that, in order to increase the orbit step in the betatron under investigation, a transverse electrostatic field is used. Plate 2, to which a pulse of positive polarity is applied, is made as a single unit with the deflecting plate of the capacitor (Fig. 1) (in the following exposition—the deflecting plate).
Thus, the device intended to increase the orbit step during its expansion and the extraction device are combined into one structure.
The deflecting plate and the main part of the cutting plate are made of nonmagnetic stainless steel 1.2 mm thick. The input end of the cutting plate is made of tantalum foil 100 μm thick. The mounting of the extraction device is designed so that radial displacement of the capacitor is possible both with the input and with the
on the exit side without disturbing the vacuum in the chamber. The exit end of the capacitor is separated in azimuth from the extraction window by 30–35 degrees.
The basic circuits of the generators intended for feeding the orbit pre-expansion winding and the extraction device are shown in Figs. 2 and 3, and their parameters are given in Tables 1 and 2. Table 2 also gives the parameters of the extraction device and of the extracted beam.
Table 1
Parameters of the orbit pre-expansion system [2]
| Number of turns of the central winding \(W\), turns | Voltage amplitude on the winding \(U_m\), kV | Amplitude value of the current in the winding \(I_m\), A | Capacitance in the discharge circuit \(C\), \(\mu\)F | Winding inductance \(L\), H | Duration of the current pulse in the winding \(\tau\), \(\mu\)s |
|---|---|---|---|---|---|
| 10 | 5 | 2000 | 50 | \(360 \cdot 10^{-6}\) | 280 |
Table 2
Parameters of the extraction device, its power-supply circuit, and the extracted beam
| Parameters | Designation | Value |
|---|---|---|
| Capacitor length, cm | \(y\) | 60 |
| Length of the deflecting plate, cm | \(l\) | 110 |
| Distance between the capacitor plates, cm | \(d\) | 1.0 |
| Height of the capacitor plates, cm | \(z\) | 4.0 |
| Voltage amplitude on the deflecting plate, kV | \(U_m\) | 100 |
| Duration of the current pulse on the deflecting plate, \(\mu\)s | \(\tau_{\mathrm{i}}\) | 3.0 |
| Energy of extracted electrons, MeV | \(\varepsilon\) | 25 |
| Number of extracted electrons in the pulse | \(n\) | \(8 \cdot 10^{10}\) |
| Duration of the radiation pulse, \(\mu\)s | \(\tau_{\mathrm{z}}\) | 1.0 |
| Average beam current, \(\mu\)A | \(i_{\mathrm{av}}\) | 0.64 |
| Beam current in the pulse, mA | \(I_{Bm}\) | 12.8 |
| Horizontal beam size, cm | \(\Delta R_e\) | 3.5 |
| Vertical beam size, cm | \(\Delta Z_e\) | 1.5 |
| Beam divergence, deg | \(\gamma\) | 12–14° |
REFERENCES
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V. A. Moskalev, V. V. Shashov. A system for extracting an electron beam from the accelerating chamber of a high-current 25-MeV betatron. Proceedings of the All-Union Scientific and Technical Conference on the Use of Accelerators in the National Economy and Medicine. Leningrad, 1971.
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V. G. Shestakov. Dissertation. Tomsk, TPI, 1969.
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V. V. Shashov. Selection of a method for extracting a beam of accelerated electrons from the “Luch” high-current betatron. Scientific Report No. 03592, Research Institute of Nuclear Physics, Electronics, and Automation at TPI, 1968.
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V. V. Shashov. Dissertation. Tomsk, TPI, 1970.