HelCat: The Helicon-Cathode Device

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The HelCat device is a laboratory plasma physics experiment at the University of New Mexico. HelCat stands for Helicon-Cathode, which are the two type of sources used on the device for generating plasma. HelCat is unique in using two very different types of sources simultaneously, allowing us to study plasma regimes typically unavailable to most basic plasma laboratory experiments. The helicon source uses radio frequency waves to ionize and heat the plasma, sort of like microwave can heat your food in a microwave oven. The cathode source ionizes the gas with generating a current of high-energy electrons. HelCat was originally called alpha before moving to the University of New Mexico, and focused on research on multiple helicon sources to create a large-area plasma. Check out the Photo Gallery

Helcat is used to study basic plasma physics phenomena in the laboratory important for understanding fusion energy and plasma astrophysics. This research includes:

• Alfvén waves, waves that travel along magnetic field lines like waves on a string, are important in heating the solar atmosphere, Earth's magnetosphere and the formation of Earth's aurora.
  
• Expanding plasmas, using the Plasma Bubble Experiment (PBX), where a dense magnetized plasma expands into a less dense region, and magnetic reconnection, the tearing and reconnection of the magnetic field. Here we are trying to understand the formation of astrophysical jets.
  
• Plasma turbulence is a major problem in fusion energy research, since it causes particles and heat to be transported out of the plasma. However, some transport is good. Thus, we are researching ways to control the turbulence and transport related to fusion devices.
  
• Helicon plasma sources are a very efficient way to create high-density, highly-ionized plasma. How they work is not well understood. This work includes looking at multiple source helicons on the alpha device.
  

HelCat isn't large enough to be considered "fusion relevant". But, the research we do is very relevant to fusion energy. We can study basic plasma processes, in particular how particles and heat are confined in the plasma, that impact research on large fusion devices. Because you can't simulate space in the lab, astrophysical experiments in Helcat look at individual processes thought to contribute to the complex interactions in space. One of the research topics of Helcat is the study Alfvén waves. There are typically very difficult to study in the laboratory. Recently, we have investigated neutral particle effects, which are important for understanding Earth’s upper ionosphere and the solar photosphere.

   
The HelCat experiment at the University of New Mexico

Design Overview

The design of HelCat was motivated by several considerations with regards to the range of intended research planned in the device. HelCat incorporates two plasma sources (one at each end of the device) – An RF helicon and a thermionic cathode. Two sources allow operation over a wide range of densities (n ~ 10¹⁸ – 10²⁰ m^-3), and collisionalities (base pressures P₀ ~ 5x10^-5 – 5x10^-3 Torr). It is hoped that by operating both sources simultaneously, high density (n > 10¹⁹ m^-3), nearly fully ionized plasmas can be obtained with relatively low collisionality (P₀ ~ 10^-4 Torr), by pre-ionizing the helicon discharge.

Helicon: The helicon source is used to generate a high-density, highly ionized, steady state, quiescent plasma discharge. The radio frequency source (RF) is tunable over 2-30MHz, with a maximum power of up to 5 kW. The high density helicon source is particularly useful for Alfvén wave studies, which constrain laboratory plasmas to be large and/or dense, where the nominal Alfvén wavelength, v_A / w_ci scales as 1/√n, the density. in contrast all previous Alfvén wave studies, the discharge is inherently current-free. A prototype of the helicon part of the facility, ALESPI, is in operation at Auburn University.

Cathode: For plasma turbulence studies, HelCat also has a cathode source. This thermionic cathode is BaO coated nickel backed by a set of tungsten heating filaments. Cathodes create lower density plasma with a flatter density and temperature profile. In addition, cathodes generate plasma by driving an axial current along the device. The two different plasma sources may have different turbulence characteristics, and part of our research aims at comparing the two.

Approximate plasma parameters are as follows.

Magnetic field, B₀: < 2.2 kG

Helicon:

Cathode:

Details of much of the HelCat design are provided in a paper describing the construction of alpha, HelCat's predecessor. You can learn about the construction of alpha. The device consists of a 4.0 m long cylindrical vacuum chamber 50 cm in diameter. A large number of both large and small ports allows for easy diagnostic access. The chamber is surrounded by a magnetic solenoid of 13 magnets, each with a nominal diameter of 82 cm and consisting of 140 turns. These create a maximum field on axis of 0.22 T. The field strength and large diameter are sufficient to insure that the ions are magnetized; at a magnetic field of 0.1 T the ion gyroradius is ~2 mm for He in our plasmas. The measured magnetic field ripple is less than 1% along the machine axis, and the uniformity is better than 3% across the plasma cross-section. At one end of the chamber is located a 1000 l/s turbo/molecular drag pump which maintains a nominal base pressure of 2x10^-7 torr.

The helicon plasma source is located at one end of the chamber, while the cathode source is located at the other. Either can be operated separately, and we have plans to operate the two simultaneously. Possible advantages of both sources include pre-ionization to help in helicon plasma breakdown, and enhanced electron temperature over that of the helicon source, while maintaining a density much higher than that typical of the cathode.

Current diagnostics include an array of RF-compensated radial Langmuir probes, double and triple probes. These measure T_e and n_e and V_f. A single chord microwave interferometer operating at 40 GHz and one at 94 GHz is used to corroborate probe density measurements. Magnetic loops are used to measure the fluctuating magnetic field of the Alfvén waves. A 0.5 m Ebert spectrometer has been used to measure the ion temperature via Doppler broadening. However, the ions are relatively cold, with a Doppler width the order of the spectrometer width, and results were inconclusive. An LIF system has just been installed for ion velocity distribution measurements in both argon (Ar II) and helium (He I) plasmas.

  
Plasma operation in HelCat: Argon helicon left, Argon cathode right.

  
Typical plasma parameters (ion saturation current) for the separate sources in HelCat: Helium helicon left, Argon cathode right.

Dual Source Plasma Operation

HelCat first demonstrated successful dual helicon-cathode source operation in October of 2007. There were/are several motivation for simultaneous dual source plasma:

• Heating and/or density increase of the long-pulse helicon discharge by the cathode pulse
• Reduce break-down power of the helicon discharge when fired into and existing cathode plasma
• Variable collisionality from the merging plasmas
• Modification of the turbulence due to different ionozation mechanisms
• It's cool!

For the initial tests the cathode electrode was grounded while the anode floated. During these experiments we noted the following:

1. The cathode heater assembly significantly reduced the start-up power required to break down the plasma during helicon operation. Others have seem similar behavior from pre-heating the gas.
2. Pre-ionization of the gas using the cathode immediately prior to firing the helicon showed no change over normal operation.
3. Firing the cathode into the helicon discharge resulted in varying effects, depending on measurement location.
   1. Helicon end :
      • an lowering of the density during the cathode pulse, followed by a density increase once the cathode shuts off
      • a slight temperature rise with a broader profile, followed by a decrease with cathode off
   2. Cathode end:
   • an initial density decrease followed by a density increase during the cathode pusle, then lowering of the density once the cathode shuts off
   • no temperature mesurements at the cathode end
4. There was a significant recoverly period, lasting up to 50 ms after the cathode turns off
5. During the recovery period the fluctuation level rises, with chaotic like behavior.

Below is a plot of the ion saturation current during the discharge where the cathode was fired into the helicon plasma. The cathode was pulsed for 10 ms at t=50 ms during the 150 ms argon helicon discharge. The magnetic field was 1 kGauss, 800 W of helicon power, and 500 A of cathode current. The gas fill pressure was 0.5 mTorr. Below this are links to several QuickTIme movies of the evolution of the plasma profiles during the experiments.

  
Discharge ion saturation current during the helicon-cathode dual source experiments

    
Links to movies of the evolution of the plasma profile during dual source operation. Left to Right: ne and Te helicon end, ne cathode end.

Plasma Characterization of Helicon Source

To characterize the plasma, an number of diagnostics measure various quantities of the discharge. Density measurements are made with tungsten double probes, which also measures the electron temperature. Profiles of the density and temperature across the discharge are shown below in the plot on the left. The plot on the right shows the density and temperature along the axis. These measurements were made with 500 W of radio frequency power and a fill pressure of ~4 mTorr in argon. Density measurements are confirmed using a 4 mm microwave interferometer, which indicates a chord averaged density of ~9x10¹⁸ m^-3 across the plasma. The interferometer can be see in the above photo just above the blue core. Helium discharges require significantly higher power and fill pressure to yield an average density of ~6x10¹⁸ m^-3, though with higher temperatures. This is due to the greater ionization energy of helium.

   
Graphs of the density and temperature in ALESPI.

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