A Multi-Purpose Manipulator system for W7-X as user facility for plasma edge investigation

The investigation of edge plasmas at the stellarator W7-X requires a flexible tool for integration of a variety of different diagnostics as e. g. electrical probes, probing magnetic coils, material collection, or material exposition probes, and gas injection. A multi-purpose manipulator (MPM) system has been developed and attached to the W7-X vessel before the operational phase 1.1. The system was designed as user facility for many diagnostics, which can be mounted on a unique interface without breaking the W7-X vacuum. The manipulator system, located in the equatorial plane, transports the inserted diagnostic probe to the edge of the inner vacuum vessel. From there the probe can be moved over a maximum distance of 350 mm to different positions inside the plasma with a maximum acceleration and deceleration of 30 m/s. Acceleration, speed and stroke depth are individually adjustable and programmable by a PLC system within predetermined limits. The MPM system can be equipped with multifunctional probes and is prepared for cooling/heating of the probe head, gas injection, and a flexible setting of the electrical diagnostic. In the paper the MPM components, their functionalities and the interface to the probe head are described. The scope of physical parameters is presented for the development of diagnostic probes applicable in the MPM user facility. This also includes the operational parameters for the movements and the electrical environment for measurements. The operation of the MPM during the OP1.1 is demonstrated on the application of a multipurpose probe head, including electric, magnetic and thermal probes.


Introduction
Within the operational planning for the Wendelstein7-X (W7-X) stellarator at the Max Planck Institute for Plasma Physics in Greifswald it was decided to include a system for investigation of edge plasmas within the early operation phase OP1.1 [1] as one of few non-machine diagnostics. For later operation W7-X requires a flexible tool for integration of a variety of different diagnostics as e. g. electrical probes, probing magnetic coils, deposition or erosion probes, and gas injection [2]. The early operation of the multi-purpose manipulator (MPM) system gave the opportunity to prepare the system as user facility for many diagnostics, which can be mounted on a unique electro/mechanical interface. Adapter for this interface are available, supporting the design of additional diagnostic probes. The manipulator system is equipped with a probe exchange chamber, which is decoupled from the W7-X main vessel vacuum by two serial gate valves. Conditioning of the probe (probe heating system is possible) is done in the autonomous vacuum system of the MPM. Falling below the W7-X safety level of 1 10 -5 mbar allows the gate valves to be opened. The electrical driven low speed axis transports the inserted diagnostic probe to the edge of the inner vacuum vessel. From there the probe can be moved over a maximum distance of 350 mm to different positions inside the plasma by the high speed drive. Acceleration, speed and stroke depth are individually adjustable and programmable by a PLC system within predetermined limits [3]. Operation is inhibited if the temperature of the inserted probe, which is permanently controlled, is above a given threshold. The

Mechanical setting /components
The multi-purpose manipulator system consists of two independently pumped UHV chambers, which are arranged in line and separated by a gate valve. For safety reasons an intermediate chamber was integrated and is connected to the W7-X chamber via a gate valve. This chamber also includes three adjustable bearings as support for the outer tube. The second gate valve at the end of the intermediate chamber is followed by the probe exchange chamber. Due to multiple ports the manually exchange of the probe head can be done here. In addition the chamber is equipped with 4 windows for observation of the probe by cameras if access to the torus hall is not possible. To fix and center the retracted outer tube, the probe chamber is followed by a second set of three adjustable bearings. For operation this set is retractable by pneumatic pistons to limit the amount of supports of the tube to two positions during operation. The main vacuum vessel is continued by the first edge welded bellow, containing the central carrier for the probes which consists of two coaxially arranged tubes. The outer tube driven by the slow linear motion system transports the probe through the duct in the cryostat to the inner vessel wall. A second linear motion drive, carried on the slow system, then does the controlled fast movement for the inner tube. This central carrier tube contains a unique interface, which is provided to the user, and which allows fast exchange of the probes. The system finally is covered by the second edge welded bellow.
Both vacuum systems are separately equipped with a rotary vane pump in combination with a turbo-molecular pump. The pumping systems are retracted from the magnetic field of W7X by additional pipes in order to keep the magnetic stress below 10 mT. Pumping capacity of the systems is 65l/s (TURBOVAC SL 80 DN 63 CF ) for the intermediate chamber and 400l/s (TURBOVAC 361; DN 160CF) for the main vacuum.

Main operation positions of the manipulator motion
The MPM system is equipped with two mechanical coupled, electrical driven, linear drives. For OP1.1 three different modes of operation have been defined: Both linear drives are completely retracted. The slow and the fast motion systems are mechanically blocked by massive pistons in this position. Both gate valves to the vessel are closed. The probe head is accessible within the service chamber for maintenance or exchange.

b. Parking
Both gate valves to W7-X vessel are open. The probe head is moved 2412 mm through the duct of port AEK40 by the slow motion system and stays close to the virtual inner vessel surface within this port (0.2m/s). The low speed axis again is locked in this position by a piston.

c. Operation
All positions from parking position to maximum stroke depth into the inner vessel are handled by the fast linear drive. The maximum penetration depth is 350 mm. A fixed position, multiple steps or up to three (more in preparation) repeated strokes to certain positions are programmable by the PLC-system [3]. For OP1.1 a maximum acceleration of 30m/s² was used. The design value of the maximum speed is set to 3.5 m/s. For the whole cycle of one 350 mm stroke a time of 432 ms was achieved.

MPM location at W7-X
The MPM system is located at W7X-port AEK40 near the equatorial plane. The absolute Cartesian coordinates of the parking position and the maximum stroke position, using a probe head of 150 mm length, are given in Tab. 1.
The access to the same magnetic flux tubes, which simultaneously are observed by means of spectroscopic methods at the lower (section 3.0) and the upper divertor regions (section 5.1) in the neighboring sections of W7-X is shown in fig. 2. In all sections short gas pulses can be injected into the flux tubes to investigate the toroidal and radial transport of plasma particles [4]. Due to the large stroke depth of 350 mm for all magnetic configurations in W7-X the LCFS is accessible, only limited by the toroidal heat load.

Probe head interface
To obtain an easy use and exchange of different probe systems, a new probe interface was designed [ fig. 3]. The probe head is mounted on a base adapter with an external 70 mm thread. This adapter has to be fixed by a knurled nut on the manipulator system. For the probe itself the dimensions are restricted to a maximum length of The interface includes 32 electrical contacts (max values: 3A, 500V). Four pins are reserved for thermal measurement, 28 are free for application. One gas inlet system with an inner diameter of 3 mm is also available. The carrier of the probe head allows three different modes of electrical operation (floating, grounded, biased). The maximum allowed isolation voltage is defined to 1 kV.

Electrical measurement
In order to reduce measurement noise even for very small signals the electrical measurement system was divided in three parts [ fig.4]. Close to the probe head (cable length 12 m) a first amplification stage with optional isolation could be used. Inside the cubical at the manipulator rack two 19'', 3U racks are placed. Those racks are designed for quick exchange to give the opportunity of preparing a dedicated sensor electronic for every unique probe head. A second and a third cubicle are placed inside the W7X torus hall (SWG-OG1-E2/E3). One cabinet is equipped with power electronics e.g. for biasing, the other one contains additional isolating amplifier and the MDS plus based data acquisition systems. With the example the configuration of the electronics is shown during use of a combined electrical/magnetical probe head [5]. Most of the electrical standard signals (e.g. motor current, encoder drive position) are logged by the PLC-System. In addition some diagnostics are used to generate important basic signals for each stroke: -Two Pt100 sensors to measure the flange temperature of the probe head. These sensors are placed inside the plug of the probe interface. They are also used as cold-junction compensation for thermo-couples of the probe head. In addition operation of the manipulator is blocked if the interface temperature rises above 100°C.
-Two thermocouple sensors within the probe head are mandatory. As standard the K-type is used, other types are possible.
-Two independent optical distance measurement (laser triangulation). One signal gives the position of the MPM low speed axis in relation to the W7X cryostat to verify the distance of those systems which are decoupled by a bellow. The second sensor measures the stroke depth of the fast system in relation to the MPM low speed axis. With those measurements the position of the probe head in W7x is well defined within a range of ±0.2 mm. Fig. 4: scheme of sensor electronics -To get some information of mechanical oscillation or vibration of the probe head, which may influence the measured data, a three dimensional acceleration measurement is placed inside the inner carrier tube, right behind the probe head interface. The sensors can measure acceleration up to 100 m/s2 with 1ms time resolution.

First operation, Measured Data
Operation of the MPM system started with commissioning of the drives at FZ-Jülich. Fig. 5 shows a good accordance between the theoretical progress of the movement and the real measurement. After transportation of the complete and preset system to Greifswald it was installed at W7-X. First Operation during plasma campaign OP1.1 was started. A video snapshot shows the probe head in contact with the outer plasma region.

Fig. 6: plasma contact at 275mm stroke depth
During the experiments several shots with identical parameter sets have been performed. Fig.7 shows the maximum deviation of the positioning signal in comparison with the mean value for 6 discharges. During the dynamic phases of acceleration and deceleration a deviation up to 5 mm could be measured between a single signal and a calculated mean value out of 6 experiments with identical parameters. Therefore the position signal is important for all calculations during the acceleration or deceleration phase. Another relevant result of this measurement is the good accuracy of the maximum stroke depth, which is essential for the constant heat load of the probe head during similar experiments. The maximum difference in that point is below 0,2 mm .First results of probe data have been shown in [5].

Conclusion
With the MPM System a new, powerful diagnostic is operational at W7X. During the development one of the main goals was to build a highly flexible tool which could be accessed by many users and allows a fast change of different probe systems and their electronics. For the positioning system this flexibility was already demonstrated in OP1.1, but development of new drive cycles is still going on for additional applications. Within OP1.2 of W7X the MPM increasingly will be used with different probe heads, also for this challenge the system is well prepared.