Beijing Proton Linac (BPL)   

The 35 MeV Beijing Proton Linac (BPL) was completed and put in operation in 1987. It is the first accelerator of its kind ever designed and built by our country. It is used for basic and application research in many fields. In 1989 it passed the technical appraisal organized by the Chinese Academy of Sciences, which demonstrated that it had met or exceeded the designed parameters and reached advanced international standard of the same kind of accelerators. In 1991, it was awarded the First National Prize for Science and Technology Progress.

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Beijing Proton Linac

BPL belongs to a new generation of proton linear accelerators, containing the injector, the low energy beam transport line, the accelerating cavity, the RF power source, the water-cooling system, the vacuum system, the magnet power supply system, the computer control system, the beam measuring system and the intermediate energy beam transport line. Figure 1 shows the general layout of BPL.

I. The Injector

It is a Cockcroft-Walton with the output energy of 750 keV and the current intensity of 200 mA. It is composed of the high voltage generator, the high gradient accelerating tube and the high current ion source. With the help of the low stability system, the high voltage can be stabilized to £ ± 0.1%. The accelerating tube has an inner diameter of 600 mm and is 2 m long. It is very difficult to fabricate and seal such large accelerating tubes. By controlling the hydrogen to be filled up in the accelerating tubes, they can maintain long stable operation. The ion source is of a double plasma type. Using oxidized cathode, its lifetime can be prolonged to 600 hours. The normalized emittance at the exit of the accelerating tube is £ 3.0 pmm-mrad. The local control of the ion source is realized through the optic fiber and computer using infrared.

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Cockcroft-Walton

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Beam transport line 1

II. The low energy beam transport line

It constitutes the beam path between the injector and the linear accelerating cavity, which is 6.5m long. It consists of 17 quadruple magnets, 1 bunching cavity, 2 pairs of steering coils and many kinds of beam measuring probes. They make the beams to match the acceptance at the entry of the accelerator vertically and horizontally and ensure that the capture rate can reach 60%.

III. The Accelerating Cavity

It is made of copper clad steel. It is fabricated with the copper clad steel rolled, welded and fine machined. The outer layer is of 16 mm thick steel and the inner layer 4 mm thick copper. 

The smooth finish of the inner wall has reached the 9th degree (Chinese standard). The Q value of the empty cavity reaches 86,000, its diameter is about 160 – 180 mm and the total length about 22m. In the cavity, there are 105 accelerating electrodes (drift tubes) with their outer diameter being 160 – 180 mm and the length 48 – 274 mm. Each electrode contains a small focusing magnet with its outer diameter being about 110 mm, its inner diameter 22 - 34 mm, its  length 25-100 mm and its magnetic gradient 9.2-2.0KG/cm. There is a water-cooling sheath between the electrode and the magnet. The accelerating electrode is composed of several work pieces of oxygen-free copper welded with electron beam. Their location tolerance in the accelerating cavity is controlled within ±0.1 mm. In addition, in the accelerating cavity, there are 50 post couplers, 13 frequency tuners, several water-cooling pipes, 12 1000 l/s ion pumps, 4 450 l/s molecular pumps and two RF power couplers, etc. So it is no exaggeration to say that the accelerating cavity has a very complicated structure and involves great technical difficulty.  

the accelerating cavity

IV. The RF Power

It is a RF transmitter with the working frequency of 200 MHz and the output power of 5MW – the RF transmitter with the highest output power at this frequency band in our country. The RF signal coupled from the crystal oscillator – the reference line, is sent to the final amplifier after 4-step amplification. The final amplifier is an imported TH 116 triode. The RF transmitter feeds power symmetrically to the accelerating cavity from two paths. It has 3 servo control loops – frequency, amplitude and phase. In recent years, modulation of the final amplification of the RF power, in accordance with the characteristics of TH 116 triode, was changed to grid modulation from the joint hard tube plus grid modulation with the hard tube modulation eliminated. Thus the complex high voltage-floating platform was eliminated. And as a consequence, the operation stability and reliability are further improved and the operation cost is cut by a wide margin.  

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the RF power source

V.  The Water-cooling System

It is used to cool all the heated components. For the accelerating cavities and drift tubes, constant temperature water-cooling system is adopted to control the harmonic frequency of the accelerating cavities. The temperature of the inner cooling water loop can be controlled within ±0.10c. The working temperature of the accelerating cavity is about 18.90c – 19.50c.

VI.  The Vacuum System

The vacuum system of BPL consists of twelve 1000 l/s sputtering ion pumps, twelve 450 l/s molecular pumps and a number of small ion pumps. The vacuum of the accelerating cavity, with a diameter of 1m and a length of 22m, is better than 5×10-7 torr. At the joint of beam pipes, fast detachable flanges are used for easy installation and repair.

VII.  The Magnet Power Supply system

This magnet power supply system consists of 100 odd quadruple magnets power supplies, 20 steering coils power supplies, 8 bending and analyzing magnets power supplies. The output of quadruple magnets power supplies is 200 – 500A, the top flat width of pulse is greater than 500 ms and the stability reaches ±0.3%. The steering coils adopt DC power supplies with the maximum output of 10A and the stability being ±0.1%. The bending and analyzing magnets also adopt DC power supplies with the maximum output of 100A and the stability reaching ±0.01%.

VIII.  The Computer Control System

With the connection of the interface systems and the local stations of some sub-systems, a number of the main components can be tuned, monitored and 

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the central console

interlocked by the central console and the CAMAC system.The three terminals in the central console are used respectively to tune 100 odd power supplies, monitor the parameters of the RF power and adjust the parameters of the ion source. The adjustment of parameters of the ion source is realized through the optic fiber and computer using infrared laser. The ion source, the RF power, the magnet power supply system and the beam measuring circuits were made to operate synchronously by the timing system. Computer PDP11/34A had retired. Recently more

compact PC system was adopted.

IX. The Beam Measuring System

It consists of 16 beam current measuring meters, 2 interrupt type emittance measuring meters, 10 beam cross-section measuring meters and 1 energy spectrum measuring meter. 

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the beam measuring system

The beam current is measured with the beam transformer whose magnetic core is made of Bo – Mo alloy with high permeability. The average current at the end of the target is measured with the current integrator. The emittance of the 750 keV beam is measured with the stepping slit and multi-layer target. The beam cross-section and the emittance of the 35 MeV beam are measured with the multi-wire target. The energy and energy spread are 

measured with the slit, analyzing magnet and multi-wire target. With the PC and the developed software, the beam measurement probes are driven and the beam measurement data collected, analyzed and treated. This set of beam measuring system can measure all the beam performances. It is appropriately distributed and its measurement results  are of high quality.

X. The Intermediate Energy Beam Transport Line

It is 60m long, containing the common transport line, the transport line for the isotope production target, the transport line for the neutron production target and the energy spread  the energy spread measurement line with the transport rate of 90%. 

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Beam transport line 2 

To sum up, BPL designed and developed by our own country has synthesized many high technologies. It is a large scientific tool with sophisticated technologies. Only a few developed countries in the world have built such kind of accelerators. The completion of this accelerator is made possible with the concern of our country and the painstaking efforts made by the State Science and Technology Commission (now called the Ministry of Science and Technology), the Chinese Academy  of Sciences, the departments concerned and dozens of factories and research institutions in tackling the technical difficulties. More than 90% of the components are new products developed by our country. Its completion has made important contributions to the development of accelerator technologies in our country.  

BPL was intended to operate as the injector of the proposed 50 GeV Proton Synchrotron. The first R & D project for BPL was a 10 MeV linac. In order for this facility to produce more benefits in scientific research and economic construction, it was decided later to extend it to 35 MeV.

The 10 MeV linac was awarded the First CAS Prize for Science and Technology Progress in 1986. The RF power system was awarded the Second CAS Prize for the Science and Technology Progress in 1987. The accelerator physics design and the research result were awarded respectively the Second CAS Prize for Science and Technology Progress in 1986 and the Third National Prize for Science and Technology Progress in 1987.

In May 1989, the 35 MeV BPL passed the technical appraisal organized by CAS and was awarded the First CAS Prize for Science and Technology Progress in 1991.

Concerning the basic and application research, the Institute of Modern Physics succeeded in synthesizing the new nucleon Ma235 for the first time in the world using the 35 MeV proton beam, and this was awarded the Second CAS Prize for Science and Technology Progress in 1997.

Institute of High Energy Physics 17/07/02