Description

Description of MST HIBP
Research Plan
Research Team
1. Contact People...

1. Description of MST HIBP
1.1 Reversed Field Pinch and MST
Unlike tokamaks and stellarators the RFP magnetic field structure is not simply dominated by the toroidal magnetic field. In fact, the toroidal and poloidal field components are roughly the same order of magniude. The name RFP arises because the toroidal magnetic field at the edge reverses direction relative to the interior. This configuration is produced by a self relaxation process of the plasma. Although the reason for this relaxation process is not exactly known, the process itself has been theoretically described by Taylor and is attributed to the attainment of a minimum energy state by the plasma, subject to the constraint that the total helicity be conserved. Another interesting feature of the RFP is the dynamo mechanism. While this mechanism is the chief reason behind RFP field sustainment, it is also the cause of turbulent magnetic fluctuations in this type of
configuration.

The MST is a reversed field pinch toroidal conguration. Its minor radius of 52 cm and major radius of 150 cm makes it one of the largest RFPs in the world. See figure below.

1.2 Heavy Ion Beam Probe

The heavy ion beam probe (HIBP) has been a subject of study in fusion plasma diagnostics for about thirty years. HIBP diagnostics have been an important tool in understanding magnetically confined plasmas, especially those of interest to fusion research. This tool has been the only diagnostic to measure:

1) the radial electric field profiles in tokamaks, stellarators, mirrors, and bumpy tori,

2) electric potential fluctuations in tokamals and stellarators,

3) particle flux arising from electrostatic fluctuations in tokamaks,

4) magnetic potential fluctuations in tokamaks.

These diagnostics have also been used to measure the plasma electron density, localized density fluctuations, and plasma electron temperature in several devices.

The HIBP on MST uses the accelerator, power supplies and some vacuum chambers from the HIBP that was on ATF (some of those pieces came from the HIBP that was on ISX-B). The analyzer for the HIBP on MST and some chambers are from the HIBP on TEXT, and some of the bellows, gate valves and pumps are from the HIBP on TEXT-U. Therefore this is the ISX-B/ATF/TEXT/TEXT-U/MST HIBP.

1.3 200 keV Accelerator

The accelerator and power supplies are rated for 170kV if SF6 insulating gas is used. For MST only 120kV is required and the accelerator has been tested to this voltage with room air as the insulating gas

1.4 Primary Sweep Plates

The primary sweep system is a crossover sweep. In each plane, (the local toroidal and poloidal directions), the beam is first bent away from the center line by one set of plates, and then bent back by a second set of plates. The net effect mimics steering plates located in the MST vessel wall, with the remarkable ability to steer ¡À20 degrees in the poloidal direction and ¡À5 degrees in the toroidal direction. This is done with a 2" diameter hole in the 5cm thick MST vessel wall.

1.5 Primary Ion Beam Detectors

Primary beam detectors are used to measure the ion beam's shape and size, as well as to calibrate the primary sweep plates. They are consisted of: (1) Beam profile monitor (BPM) in the primary beam line, and (2) primary beam detecting rods at the MST pumping holes.

1.6 Secondary Sweep Plates

3 sets of steering plates allow ions that leave the 4.5" output port to hit the entrance slits of the energy analyzer. The range of exit angles of the ions is similar to the range of entrance angles, though the steering on the secondary is not as extensive.

1.7 Proca-Green Energy Analyzer

The analyzer has a geometric gain of 3 and the detected ions have a charge state of 2, which results in an analyzer anode voltage that is 1/6 of the accelerator voltage, or 20kV maximum. This is well below its original design limit of 100kV. The analyzer has been modified since it was used on TEXT. It now uses a shaped anode and a shaped grounded boundary condition plate to eliminate the need for intermediate electrodes. This concept was proven on the TEXT-U HIBP and has several advantages. The lack of intermediate electrodes eliminates the need for very accurate high voltage dividers. More importantly, the boundary condition plate produces a more uniform electric field. Calibration runs of the analyzer at both RPI and the University of Wisconsin, showed nearly perfect agreement with the predicted response based on the analyzer model. This is particularly true when the ion beam is not centered in the analyzer or when the ion beam enters the analyzer away from the ideal angle. The net effect will be more accurate measurements of the plasma potential. Careful modification and testing has resulted in what should be the best behaved analyzer used on any HIBP to date.

1.8 Current to Voltage Detectors: I/Vs

An I/V is an electronic circuit that translates input ion signal into voltage output with a high gain and certain bandwidth. We are currently using two kinds of I/Vs: one has gain = 10^7 V/A, which is used to measure secondary signals; the other has gain = -10^6 V/A, which is used to detect primary or secondary ion signals on the sweep plates, BPM, and primary beam detectors. For an example of gain=-10^6 V/A current to voltage convertor, click here.

1.9 High Voltage Systems

The high voltage systems include two parts: one is the 170keV accelerator which provides high energy diagnoctic ion beam and its controller. The other consists of forteen 4kV~20kV trek voltage amplifiers (power supplies) which provide necessary sweep of the beam.

1.10 Vacuum Systems

This is including the primary system and the secondary system are two independent vacuum systems which are isolated from the MST system. The primary vacuum system has one rough pump and one turbo pump. The secondary system has two rough pumps and two turbo pumps. The background vacuum level is < 10^-6 torr.

1.11 Control and Data Acquisition Systems

Manual controlling is using at present during the initial operation. Eventually, we'd like to develop an automatic control system by using LabView running on the SUN Ultra1 workstation. LabView is a virtual instrumentation environment which enable us to virtually adjust knobs on the screen of a computer, and the physical controlling is done by communicating between the computer and the CAMAC crate through GPIB interface. Data acquisition is performed by the MST data acquisition system.

2 Research Progresses

2.1 Trajectory Calculations

Trajectory was initially calculated on the CRAY machine by Ying Dong and Uday Shah to get preliminary results before 1997. The calculation has been further developed by J. Lei after that. Here are some trajectories and sample volumes examples.

2.2 System Designs and Fabrications

The total system designs have been done during 1997 - 1999.
The sweep systems were designed in 1998, and fabricated before the summer of 1999.
The energy analyzer was modified in the summer of 1998, and tested in the fall of 1998.
The required entrance hole on the MST chamber was drilled in the summer of 1999.
The vacuum vessels were designed in 1998, and new pieces were fabricated before the fall of 1999.
The vauum interlocks were designed in 1998, and wired before the fall of 1999.
The manual control system was designed in 1998, and modified in 1999.

2.3 Installation

The primary beam line (including the accelerator) was installed during the summer of 1999.
The secondary beam line (including the analyzer) was installed during the fall of 1999.

2.4 Initial Experiments

Vacuum leak check was performed right after the installation, and any leaks have been removed.
The primary ion beam was formed and measured in the summer of 1999. This includes detecting the beam signal on the beam profile monitor, on the sweep plates, on the shutter, and on the primary beam detectors at the bottom of the MST chamber.
The energy analyzer was tested separately with the ion gun from the accelerator in Oct. 1999. The gain curve measured fits the result from our theoretical model.
We are now working on the focusing issue of the primary ion beam.

3 Research Team

Research group for MST HIBP is a small group, which has 3 professors, 1 engineer, 1 postdoc, and 4 graduate students.
3.1 Contact peoples...

For engineering questions, please contact John Schatz or Prof. P. Schoch

For project progress, please contact Diane Dermers.

For simulation issues, please contact Jianxin Lei.

For any physics issues, please contact Uday Shah.

For any other research issues, please contact Prof. K. Connor