Neutral Beam
A Russian-manufactured beam acquired from TAE (Tri-Alpha Energy), rated for 700kW peak, with beam voltage and current ratings up to 20kV and 45A independently, or 20kV and 35A simultaneously.
As of Jan. 25th 2019, the beam has been tested to a 35A peak current and 20kV peak voltage. Since the current and voltage peaks do not generally overlay, the peak beam power is estimated at 630kW.
The 2019 lab notable for LTX requires 500kW of injected beam power. Since neutralizer efficiency is estimated at 86(?)%, this requires 625kW of pre-neutralizer beam power.
Documentation
- Shared Google Drive Folder
- English language user manual (>100MB). Another user manual exists (TAE? Wisconsin?)
- 2004 RSI , 2011 RSI
NBI Setup
Beam setup is detailed in the neutral beam operating procedure.
Gas Lines
Anode and cathode lines share a pump/purge connection and are electrically connected to each other but isolated from ground. The neutralizer has its own connection to the pump/purge line and is grounded to the neutralizer tank.
NBI Operation
The NBI is controlled by LTX-Fry-NB7, located in the NBI power supply cage in L111. It is typically operated remotely from the Nibbler workstation.
To access the NBI control interface:
- Log into Nibbler
- Open a remote desktop to LTX-FRY-NB7\ltxuser
- Launch "nbiGUI"
Note that nbiGUI may not save sessions (typically if it exits improperly) so all timings and other control parameters must be verified manually.
nbiGUI saves digitized data in Labview .TDMS files in a local folder, but the files overwrite each other if the program is restarted. A Labview VI that runs at startup on LTX-Fry copies new files to archive folders (both local and on /p/ltxdata/NBI) with a shot number filename, and writes the data to the tree. Locally initiated NBI-only shots are written to the LTX_NBI tree with shot numbers beginning with 500000. NBI shots triggered by a shot cycle are written to the LTX_NBI subtree accessible from the main LTX_B tree with the same shot number beginning with 100000.
An AutoHotKey script that runs at startup on LTX-Fry copies and saves the timing and control parameters to a text file and archives them.
The internal calorimeter thermocouples should be recorded by nbiGUI, but don't presently work. C++ source code and .ini files on Fry could be used to modify the GUI. Eventually, an NBI LabView VI should be developed as the main operating interface, allowing greater interface optimization as well as recording data to the tree directly.
nbiGUI
The script currently run from Fry to control the NBI system. May be replaced with a LabView VI, in the future. Saves .TDMS files to ddc directory, which can be opened in Labview or in Excel with an plugin from NI. Source files are in 'C:users\ltxuser\Documents\pavel'.
When opening nbiGUI from a fresh login, there will typically be a DataSocket error, and you will have to relaunch the program. Then there will be a second DataSocket session, which you can close.
Some parameters for nbiGUI are controlled by .INI files in the 'current' folder. The nbicore.ini file seems to control internal settings of nbiGUI that are not otherwise accessible, including the maximum allowable grid voltage. The nbi.ini file seems to define initial values for the user-editable parameters in nbiGUI when it starts up. Upon a "good" shutdown of nbiGUI, the values in use at shutdown should be saved in nbi.ini and restored as the initial values when reopened.
Timing panel
Labeled "Timing1" (Fig. 1), this panel allows enabling/disabling of various subsystems including gas puff valves and electrostatic grids, and control over the relative timing according to which various subsystems are energized. All nbiGUI timings are relative to an internal t_NBI=0, which will be separately referenced to the general LTX t=0. As nbiGUI is a legacy script used elsewhere, some of its features currently remain unclear.
nbiGUI is capable of triggering manually, automatically at a set interval, or on the rising or falling slope of an external trigger, as selected by the user (Fig. 2).
- Delay: Additional time between t_NBI=0 and all other timings. Has a 0.1ms minimum value.
- FW: Function unknown, has been kept set at 5.
- Period: Time between beam discarges when running in "Periodical" trigger mode (see below).
- Trigger Mode pulldown menu (Fig. 2)
- Single: t_NBI=0 is at press of "Start Single" button.
- Periodical: t_NBI=0 is (Period) seconds after last discharge.
- Ext(rising): t_NBI=0 is at the rising edge of an incoming (TTL?) trigger pulse.
- Ext(falling): t_NBI=0 is at the falling edge of an incoming (TTL?) trigger pulse.
- Timings: All of the following have an enable/disable checkbox, a start time, and an end time:
- GVA: The anode gas valve is triggered.
- GVC: The cathode gas valve is triggered.
- GVN: The neutralizer gas valve is triggered
- MIPS: The magnetic insulation coil begins to energize. Roughly 1ms rise time.
- ARCPS: The arc chamber is energized, applying a voltage between the anode and cathode, but breakdown is not initiated.
- PREARC: An optional setting the provides a small, early gas puff. In theory, may help to get breakdown at the cost of arc performance. May or may not be useful.
- IGNITION: The arc ignition electrode is energized, and initiates the arc. The arc takes about 500μs to form.
- HVPS: Beam acceleration grid begins to energize. Roughly 300μs rise time.
- North and South: Not clear what these numbers mean. They don't seem to affect beam performance for our purposes.
- Shot: Shot number since nbiGUI was started.
- Elapsed: Seconds since last discharge.
- ADC start: Beginning of nbiGUI diagnostic record timebase.
- ADC stop: End of nbiGUI diagnostic record timebase.
System parameter panel
This panel, titled "Dac" in nbiGUI (Fig. 3), provides control over various system output parameters, specifically target voltages and currents, and current droop offsets.
- U(HVPS): Target ion-acceleration potential. Initially reaches about 90% of this value within 500μs, then rises to near 100% over ~5ms.
- U(2GR): Target electron suppression potential. Rises to full setting within ~100μs.
- I(ARC): Target arc discharge current. Mechanism for this setting is not well understood at this time. How this setting affects arc breakdown is not completely clear. Not targeted very precisely, commonly overshooting.
- I(ARCCORR): An arbitrary-scaling correction factor to counteract droop of the arc current. Higher values increase dI/dt of I_ARC.
- I(MIPS): Target magnetic insulation coil current. At least 30% difference possible between setting and measured result (compare Fig. 3 and Fig. 6)
- I(MICORR): An arbitrary-scaling correction factor to counteract droop of the magnetic insulation current. Higher values increase dI/dt of I_MIPS.
- dac6: unused channel
- dac7: unused channel
State control panel
The nbiGUI panel titled "DigO" (Fig. 4) provides Digital Outputs controlling various system states, such as voltage inversion and setting the system enable state.
Beam operation requires that "ENABLE" is set high.
- DISCH_OPEN: Opens/closes HVPS discharge IGBT.
- ARCINVERT ON: Activates arc power supply
- HVPS INVERT ON: Activates high-voltage power supply
- ENABLE: Closes/opens charging contactor (may be bypassed)
- do4: unused channel
- CALORIMETER IN: Moves calorimeter in
- CALORIMETER OUT: Moves calorimeter out
- do7: unused channel
The state control panel has 8 additional unused digital output channels.
State display panel
The panel titled "DigIM" (Fig. 5) displays system status flags, chiefly regarding interlock status.
Beam operation requires that "ENABLE" and "NBI READY" are high.
- TEST MODE ON: Key is set to CAL - Beam cannot be fired with calorimeter out
- FRC MODE ON: Key is set to INJ - Gate valve to LTX must be open, calorimeter must be moved out
- PushButton: Unknown as of 9/10/2019
- WATER OK: Bypassed, air cooling used
- DOOR1: Power supply cabinet door interlock (bypassed)
- DOOR2: Another power supply cabinet door interlock (bypassed)
- ENABLE: Beam can be fired
- MC IN: Calorimeter in
- MC OUT: Calorimeter out
- NBI READY: Beam can be fired
Diagnostic panel
Several diagnostic signals are digitized to monitor system and beam output performance.
- I_ARC(A): Current between arc chamber anode and cathode.
- U_ARC(V): Voltage across arc chamber anode and cathode.
- I_BEAM(A): Beam emission current.
- U_HVPS(V): Beam emission potential.
- I_2Grd(A): Current to electron deceleration ("decel") grid.
- U_2Grd(V): Potential relative to ground of electron deceleration ("decel") grid.
- I_MIPS(A): Magnetic insulation current.
- I_HVPS(A): Beam acceleration power supply current.
Beam Sourcing and Injection Systems
Beam Source
The beam source is composed of an arc chamber with two gas injectors, three electrostatic grids, and a neutralizer volume, and is mounted to a neutralizer tank. The arc chamber generates an arc discharge across a roughly 200V potential between anode and cathode, initiated by a 6kV ignition electrode. Axial-field magnetic insulation prevents the arc from shorting to the walls of the chamber, and together with the pressure gradient from the differential timing and back-pressure of the anode and cathode gas injectors, arc plasma flows from the arc chamber into the grid region. An accelerator grid charges strongly positive, driving the free electrons(ions?) to approximately 20kV and taking on an equivalent electron current to maintain plasma quasi-neutrality. The ion beam passes two grids before entering the conical neutralizer volume: (1) a secondary-electron suppression grid, or deceleration grid, which prevents any secondary electrons liberated in the neutralizer or from the third grid from flowing back onto the acceleration grid, and (2) a grounding grid which shields ions in the neutralizer from the potentials of the electrostatic grids. Ions passing through the neutralizer tank strip electrons from neutral gas, forming the fast neutral beam and leaving behind cold ions easily deflected onto the tank walls by ambient fields. The neutralizer volume is relatively conformal to the size and shape of the beam, allowing a fairly high neutral pressure (order of mtorr) with the minimum gas puff. The neutralizer tank is much larger, with significant turbopumping, planned titanium gettering, and minimal gas conductance to LTX that still allows entry of the beam.
Injection
The neutral beam will enter the vessel from section A-B (φ~22.5 degrees), aimed toward TF magnet K (φ~235 degrees), where it will strike the dump plate.
Scrapers
TZM (??) plates mounted above and below the beam path at port A-B, approximately 4 inches inside the 70cm boundary of the LTX vacuum vessel wall. These serve to define the upper and lower extrema of the neutral beam, as well as diagnosing whether the beam center is off the midplane at the point of injection. A cryopump is positioned just above the scrapers on the beam side in order to pump incident gas before it enters the plasma volume, while an additional turbopump and titanium gettering are also planned on the beam side of the scrapers.
Dump Plate
A 0.25" TZM plate mounted to the inner face of the vacuum vessel at φ~235 degrees (under magnet K). Including an extension plate at the -φ edge, the plate is ~15° wide in the toroidal direction (an arc length of ~7.25") with the appropriate curvature, and 5.7" in the vertical direction, so that the shells shadow the edges of the plate.
The dump plate (1) provides a robust target for beam energy deposition, and (2) is diagnosed with RTDs to serve as an injected beam calorimeter and measure of beam shine-through.
Diagnostics
Source and Grid Diagnostics
Several diagnostic signals are digitized to monitor system and beam output performance.
Arc: Measures the current and voltage of the ion-sourcing arc discharge (I_ARC and U_ARC in nbiGUI).
Accel grid (?): Measures electron current deposited onto the positively-charged grid (???), used as representation of accelerated ion current (I_BEAM in nbiGUI).
Decel grid: Measures the bias voltage and deposited current on the secondary-electron deceleration grid (I_2Grid and U_2Grid in nbiGUI).
Magnetic insulation: A magnetic field applied to prevent the source arc from shorting to the sides of the arc chamber (I_MIPS in nbiGUI).
High-voltage power supply: Measures the output of the accel grid power supply (I_HVPS and U_HVPS in nbiGUI).
Calorimetry
Calorimetry is key for gauging beam power after neutralization, as well as beam trajectory.
The in-tank calorimeter is a (how thick?) copper plate diagnosed by (how many?) thermocouples (where?).
Beam Scraper RTDs
Mounted to the back of the beam scrapers, the scraper RTDs are used to diagnose whether the beam centroid is above or below the midplane at the point of injection (i.e. upper or lower scraper sees more deposited energy), and to estimate the toroidal angle at injection (position along scraper length at which energy deposition is highest). Details on the mounting, calibration, and acquisition of the beam scraper RTDs can be found here.
Dump Plate RTDs
Mounted to the back of the dump plate, the dump plate RTDs are used to measure total energy deposited, ideally representing either total power injected into the plasma volume (in a calibration shot), or total shine-through power (in a plasma shot), as well as a measure of beam energy profile and beam centroid position. Details on the mounting, calibration, and acquisition of the dump plate RTDs can be found here.
Related Pages
RTDs on the beam scrapers and beam dump
LTX_NBI subtree
